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9.1: Basidiomycota- The Club Fungi - Biology

9.1: Basidiomycota- The Club Fungi - Biology


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The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cell of a hypha. The best-known fairy ring fungus has the scientific name Marasmius oreades. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen.

These mushroom-producing basidiomyces are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The “gills” are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungus, which cling to the bark of trees like small shelves. In addition, the basidiomycota includes smuts and rusts, which are important plant pathogens, and toadstools. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness.

The lifecycle of basidiomycetes includes alternation of generations (Figure 2). Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.

Practice Question

Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

[reveal-answer q=”197947″]Show Answer[/reveal-answer]
[hidden-answer a=”197947″]Statement d is true.[/hidden-answer]


Top 11 Features of Basidiomycetes| Club Fungi

1. The somatic phase consists of a well-developed, septate, filamentous mycelium which passes chiefly through two stages.

It is formed by the germination of a basidiospore and contains a single haploid (n) nucleus in each cell. It bears neither sex organs nor any basidia and basidiospores. It is short-lived.

(b) Secondary or dikaryotic mycelium:

It constitutes the main food absorbing phase and consists of cells each containing two haploid nuclei (n+n). It is long-lived and plays prominent role in the life cycle.

In the Homobasidiomycetidae it may continue to grow for years producing fructifications every year by the interweaving of hyphae. The fructifications bear basidia and basidiospores.

In the Heterobasidiomycetidae it forms teleutospores or brand spores which germinate to produce basidia bearing basidiospores.

2. Except in rusts and smuts the septal pore in the Basidiomycetes is complex. It is dolipore parenthesome type.

3. The motile cells are absent in the life cycle.

4. The clamp connections on the dikaryotic hyphae are of universal occurrence.

5. Asexual reproduction by spores plays an insignificant role in the life cycle. The Homobasidiomycetidae do not form any asexual spores. The Heterobasidiomycetidae form them in the dikaryotic mycelium. The latter produces uredospore’s and aeciospores in the rusts.

6. The sex organs are lacking in the Basidiomycetes. The sexual process is represented by plasmogamy and karyogamy. Karyogamy is immediately followed by meiosis.

7. Basidium is the characteristic reproductive organ of Basidiomycetes in which both karyogamy and meiosis take place.

8. Typically the basidium bears four basidiospores exogenously. The number, however, varies from one to many depending on the species.

9. The basidiospore germinates to produce the primary mycelium.

The Basidiomycetes and Ascomycetes resemble each other in the following respects:

1. The Basidiomycetes and Ascomycetes are similar in their habit as both include parastitic as well as saprophytic species.

2. The purely terrestrial mycelium consists of septate, filamentous hyphae both in the Basidiomycetes and Ascomycetes.

3. The septa have each a central pore in both.

4. The motile cells are completely lacking in the life cycle of both the classes.

5. The sex organs which are completely absent in the Basidiomycetes have gradually been eliminated from the life cycle in the advanced Ascomycetes.

6. The sexual process in both the classes comprises plasmogamy and karyogamy. The latter is immediately followed by meiosis.

7. The delayed fusion of nuclei of opposite strains after plasmogamy has resulted in the origin and establishment of a binucleate or dikaryophase in the life cycle of both Basidiomycetes and Ascomycetes.

8. The characteristic reproductive organ, basidium of Basidiomycetes and ascus of Ascomycetes resemble each other in development and cytology till the initiation of spores.

Both arise from the binucleate cells, the basidium from the dikaryotic hyphae and ascus from the ascogenous hyphae. Karyogamy and meiosis both occur in the basidium as in the ascus.

9. The basidiospores are usually uninucleate and haploid as are the ascospores in most of the Ascomycetes.

10. The dikaryotic mycelium in the Basidiomycetes is comparable to the ascogenous hyphae of the Ascomycetes.

11. The basidiocarps of Basidiomycetes are comparable to the ascocarps of Ascomycetes but the two are not homologous structures.

12. A clamp connection of the Basidiomycetes is considered homologous in structure and analogous in function to the hook of the Ascomycetes.

The Basidiomycetes differ from the Ascomycetes in the following respects:

1. The septal pore in most of the Basidiomycetes is not a simple hole as in Ascomycetes but is a complex structure known as a dolipore.

The actual pore is barrel-shaped. It is surrounded by a swollen rim which is a part of the annular septum. The opening of the pore on either side is guarded by a curved pore cup called parenthesome.

2. The primary mycelium consisting of cells, each with a haploid (n) nucleus is short-lived. On the other hand the primary mycelium in the Ascomycetes is dominant and long-lived.

3. The primary mycelium in the Basidiomycetes bears neither the sex organs nor basidia or basidiospores.

4. The conidia (uredospores and aeciospores) are borne on the secondary mycelium whereas in the Ascomycetes they are borne on haplomycelium.

5. Excepting the Uredinales all traces of sexual apparatus have been lost throughout the class.

6. Presence of clamp connections is a characteristic feature of the secondary mycelium.

7. The dikaryotic (secondary) mycelium is a long-lived, independent structure whereas the ascogenous hyphae of the Ascomycetes which are homologous to the former are short-lived, dependent and occur only inside the fruit body.

8. The dikaryotic mycelium which is the result of a single mating of compatible hyphae may produce a large number of fructifications instead of only one as in the Ascomycetes.

9. The fruit bodies in the Basidiomycetes consist entirely of dikaryotic hyphae whereas in the Ascomycetes the basal hyphae, peridia and paraphyses are haploid.

10. The basidium bears a definite number of basidiospores, which is usually four whereas the ascus of the Ascomycetes usually produces eight ascospores.

11. The basidiospores are produced externally or exogenously on the basidium whereas the ascospores are produced inside or endogenously in the ascus of the Ascomycetes.


Life Cycle of Basidiomycetes (With Diagram) | Club Fungi

The well developed, filamentous mycelium consists of a mass of branched, septate hyphae generally spreading in a fan-shaped manner. The cell wall is chitinous in nature. Within the cell wall is the plasma membrane.

The cytoplast contains a complement of usual cell organelles except the chloroplasts. The septum, as in the Ascomycetes, originates as an annular outgrowth on the inside of the tubular wall.

It grows inwards like a narrow shelf reducing diameter of the pore. The hyphae ramify in the substratum and absorb food. The mycelium generally is a weft of interlacing and anastomosing hyphae.

In a few genera, however, the mycelial hyphae run parallel to one another and get bundled together to form definite and conspicuous thick cords of macroscopic size. These are the rhizomorphs.

The rhizomorph is covered by a sheath (cortex) and behaves like a unit. The mycelium in different species varies in colour and may be white, yellow or orange. It is generally perennial.

The mycelium of Basidiomycetes passes through three distinct stages namely, the primary, the secondary and the tertiary before the fungus completes its life cycle.

The first stage is represented by the primary mycelium or homokaryon (B) which is formed by the germination of a basidiospore (A). The latter, finding conditions of temperature, food supply and moisture congenial for growth, germinates to form a hypha consisting of uninucleate cells.

It represents the primary mycelium and constitutes the hapiophase. It does not bear any sex organs-a feature in sharp contrast to the Phycomycetes and Ascomycetes, nor does it bear any basidia or basidiospores. The primary mycelium may multiply by conidia or sometimes by oidia.

The second stage is the secondary mycelium (Fig. 13.2). The cells of the secondary mycelium are binucleate. It represents the dikaryophase in the life cycle.

The Basidiomycetes, in fact, differ from the Ascomycetes in the increased prominence of the dikaryophase which is independent, long-lived and thus plays a prominent role in the life cycle.

The secondary mycelium originates from the primary mycelium as follows:

Most of the Basidiomycetes are heterothallic. It means the primary or homokaryotic mycelium in them is of two distinct strains which are called + and – strain.

The first step in the formation of the secondary mycelium involves the interaction between two compatible primary mycelia (Fig. 13.2). Two compatible hyphae (+ and – strain) from the neighbouring mycelia meet.

The intervening walls between the two adjacent cells at the point of contact dissolve. The protoplasts of the uninucleate cells intermingle in the fusion cell (plasmogamy). The two nuclei in the fusion cell do not fuse.

They lie side by side constituting a dikaryon. One of these is of + strain and the other of – strain. The binucleate cell thus established is known as the dikaryotised cell. Both the uninucleate and binucleate cells may be found in the same mycelium.

The first formed portions of a mycelium may be uninucleate and the later formed portions binucleate. The dikaryotic cell divides repeatedly by conjugate divisions to give rise to a secondary or dikaryotic mycelium (Fig. 13.2).

It consists of binucleate cells. During nuclear divisions of the dikaryotic cell special structures called the clamp connections are formed (Fig. 13.3). These clamp connections ensure that the sister nuclei of the dikaryon, at each division, separate into daughter cells.

The secondary mycelium differs from the primary mycelium in being long-lived, presence of clamp connections, vigorous growth, mode of branching, binucleate condition of cells, and development of fructifications in the Homobasidiomycetidae and teleutospores or brand spores (probasidia) in the Heterobasidiomycetidae.

Clamp Connections (Fig. 13.3):

When the dikaryotic cell (A) is ready to divide a pouch-like outgrowth arises from its wall (B), it arises midway between the two nuclei of the dikaryon. The two closely associated nuclei of the cell now divide simultaneously. This is called conjugate division.

One of the four daughter nuclei, generally the lower one of the upper pair, passes into the pouch (C). A septum appears at the base of the pouch (D). As a result the pouch is cut off from the main cell. It may now be called a clamp cell.

The clamp cell grows into hook like structure. Its tip bends over and finally fuses with the lateral wall (E) of the parent cell. The clamp cell now forms a bridge. It is called the clamp connection.

Another septum is laid down vertically under the bridge usually at about the level of the upper end of the clamp connection (E). It divides the parent cell into two daughter cells. The terminal daughter cell has two nuclei.

Each of these is a sister nucleus of the parent dikaryon. The lower or basal daughter cell possesses one nucleus. The fourth nucleus lies in the clamp connection. The nucleus of the clamp connection now migrates into the basal daughter cell. The latter also becomes binucleate (F).

The clamp connection thus simply functions as a bypass. It ensures that the sister nuclei formed by the conjugate division of the dikaryon separate into two newly formed daughter cells. The clamp connections are usually formed on the terminal cells of the hyphae of the secondary mycelium.

They generally persist on the old dikaryotic hyphae. The presence of hook like clamp connections is a safe criterion for distinguishing a secondary or dikaryotic mycelium from the primary or monokaryotic mycelium.

The clamp connections by some mycologists are considered homologous to the hooks of the ascogenous hyphae of the Ascomycetes.

This view is supported by the general occurrence of a clamp connection at the base of a basidium. Moore and Mclear (1962) studied the fine structure of septa of the dikaryotic hyphae of Basidiomycetes.

They found that the septum which is a cross wall flares sharply and broadly at the centre of the hypha to form a barrel- shaped structure with open ends.

This is the actual pore surrounded by a swollen rim which is a part of the annular septum. This type of septum is termed a dolipore septum. The septum and the swelling are covered by the cell membrane.

The opening on either side of the dolipore septum is guarded by a curved or crescent-shaped double membrane pore cap which in section looks like a parenthesis (round bracket).

The septal pore-cap is thus given the name parenthesome. The parenthesome is similar to the endoplasmic reticulum. The lower parenthesome is seen to be interrupted by gaps in the form of pores. The upper parenthesome may be continuous.

The dolipore parenthesome septum complex is unique to the Basidiomycetes. It maintains cytoplasmic continuity but prohibits nuclear migration.

The secondary mycelium in the fruit bodies of the higher Basidiomycetes becomes organised into specialised tissues. It is called the tertiary mycelium. The fructifications are thus formed of the tertiary mycelium.

The cells of the tertiary mycelium are also binucleate. The distribution of the dolipore parenthe-some septum complex in the Basidiomycetes is widespread.

It is characteristic of the primary, secondary, and generative tertiary mycelium of the Homo-basidiomycetes as well as of the basidiocarpic mycelium of the Heterobasidiomycetes. The major exceptions are the rusts and smuts.

The formation of basidia, the dikaryotic mycelium, the dolipore septum with the parenthesomes guarding the pore on both sides and clamp connections are the four diagnostic features of this class.

Diploidisation or Dikaryotisatton (Fig. 13.5):

The process by which the primary mycelium is converted into secondary or dikaryotic tnycelium is called diploidisation or dikaryotisation. The first step in diploidisation is the establishment of a dikaryon in the fusion cell (Fig. 13.2).

The dikaryotised ceil through repeated divisions by clamp connections gives rise to a secondary mycelium in which ‘ each cell possesses a dikaryon (two neclei).

Diploidisation takes place by the following methods:

1. By Hyphal Fusions. In this case fusion occurs between the vegetative cells of two neighbouring hyphae of the primary mycelia of opposite sexual strains (A).

2. By conjugation of basidiospores (B). Two basidiospores of opposite strains (Ustilago anthearum) meet and conjugate. The binucleate basidiospore formed in this way germinates to give rise to a secondary mycelium.

3. By the fusion of a germinating oidium of one strain with a cell of the primary mycelium of the opposite strain (C). The binucleate cell formed in this way by elongation and division by clamp connections develops into a secondary mycelium.

4. By fusion between a germinating basidiospore and a haploid cell of the basidium (D) as in U. violacea.

5. By fusion between the two haploid cells of opposite strains of the basidium (E) as in U. carbo.

6. By fusion between basidia formed by the germination of two smut spores (F).

Basidia are always produced from the binucleate cells of the secondary mycelium (Fig. 13.8).

Reproduction in Basidiomycetes:

Asexual Reproduction (Fig. 13.6):

It takes place by the following methods:

(i) By Conidia (Fig. 13.6 A-B):

The production of conidia is not of so common occurrence in the Basidiomycetes. They are produced in the rusts, smuts and some other Basidiomycetes. In the smuts, they are budded off from the basidiospores and the mycelium.

The uredospores of rusts are also of conidial nature and function. The conidia in the Basidiomycetes are produced by the dikaryotic mycelium (A). They serve to propagate the dikaryophase in the life cycle.

(ii) By Oidia (Fig. 13.6 C):

These are small, hyaline thin-walled unicellular sections or fragments of the mycelium. They may be uninucleate or binucleate accordingly as they are produced by the breaking up of the primary or secondary mycelium.

They usually do not round up or secrete thick walls to become spore-like. They germinate by means of germ tubes. The latter grow into new mycelia. In some species the oidia are segmented from special, short lateral hyphal branches called the oidiophores (C).

They are segmented from the tip of the oidiophore in succession towards the base (basigenous succession). The oidia serve a double function. They may either germinate to form primary mycelia or bring about diploidisation.

In the latter case the germinating oidium acts as a spermatium and fuses with the hyphal cell of an opposite strain (Fig. 13.5 C).

(iii) Budding and fragmentation:

constitute the unimportant vegetative means of asexual reproduction in Basi-diomycetes.

The Basidiocarps (Fig. 13.7):

In the higher Basidiomycetes (class Homobasidiomycetidae) the secondary mycelium develops fruiting bodies called basidiocarps. The basidiocarps are usually massive aerial sporophores which bear basidia.

They are of various sizes, types, textures and forms. In texture they may be thin, crust-like, gelatinous, papery, thick and fleshy, leathery, corky, woody and spongy.

In size they range from small microscopic objects to macroscopic bodies 3 feet or more in diameter. In form they may be umbrella-shaped, fan-shaped, round and the like.

The portion of the secondary mycelium which forms the fructifications (basidiocarps) is, sometimes, called the tertiary or generative mycelium. It is dikaryotic.

The basidia which are characteristic reproductive structures of this class are of two types in general, the holobasidia and the phragmobasidia. The former are aseptate and thus unicellular and the latter are septate structures.

The holobasidia are characteristic of most of the Basidiomycetes particularly the gilled or fleshy fungi. They are developed in a palisade- like layer on the basidiocarp. This fertile layer is called the hymenium.

Interspersed among the basidia are the sterile hyphae known as the paraphysis. The hymenial layer may be exposed from the beginning or exposed towards maturity or remains closed throughout.

(a) Development of holobasidium (Fig. 13.8):

The simple, club-shaped or more or less cylindrical holo or homobasidium lacks septa, and has a rounded apex. It originates as a terminal cell of a binucleate hypha of the secondary or tertiary mycelium in the basidiocarp (Fig. 13.8).

The narrow elongated, binucleate young basidium is separated from the supporting hypha by a septum (a). A clamp connection is generally found at the basidium over the separating septum.

During further development, the young basidium increases in size and becomes broader. Its two nuclei fuse (karyogamy) to form a fusion nucleus or synkaryon (b). Soon after, the fusion nucleus (synkaryon) undergoes meiosis to form four haploid nuclei (d).

In the higher Basidiomycetes the basidium remains unseptate or single celled (e). It is thus called the holobasidium (homobasidium). The holobasidium is not morphologically differentiated into probasidium (hypobasidium) and metabasidium (epibasidium).

In this case the two terms denote the two different stages of development of the same structure (basidium). The early stage of development of the holobasidium when karyogamy takes place represents the probasidium and the later stage when meiosis takes place represents the metabasidium.

The holobasidium closely resembles the ascus in its development and cytology up to the initiation of basidiospores. It differs from the ascus in two respects, namely, the production of exogenous spores and their number which is four instead of eight.

(b) Formation of Basidiospores:

Into each basidiospore initial migrates a haploid nucleus from the basidium. During its passage through the narrow neck of the basidiospore initial the haploid nucleus in the mushrooms is said to assume vermiform shape (e).

It again becomes spherical in the spore. Subsequently each basidiospore initial is separated from its respective sterigma by a wall. Hawker (1967) reported that no cross wall is formed after the migration of the nucleus.

The protoplast of the spore secretes a new wall around it within and in initimate contact with the original wall of the basidiospore initial. The basidiospore wall thus appears two-layered.

The outer layer which represents the parent wall of the spore initial is known as perispore. The inner layer is called the epispore. It is the tine spore wall secreted by the spore protoplast. Usually the perispore and epispore are fused.

Typically the basidia are four spored structures (F). The basidiospores are borne externally. Each basidiospore has a small lateral outgrowth near the juncture with the sterigma. It is called the hilum. Of the four basidiospores two are of plus strain and two of minus strin.

Development of Phragmobasidium (Heterobasidium, Fig. 13.9):

The septate basidium or phragmobasidium is typical of the rusts and smuts which usually do not form any fructification or basidiocarps.

The phragmobasidia are formed by the germination of spores produced by the rounding up of the binucleate cells of the dikaryotic mycelium in smuts (A) to form spores.

These spores are usually thick-walled and are known as the smut or brand spores or teleutospores. Initially they are binucleate (B). These binucleate teliospores are sometimes called the probasidia (B).

Later the two nuclei fuse to form a zygote nucleus or synkaryon. The thick-walled smut spore with a synkaryon is called encysted probasidium or hypobasidium (C).

The latter germinates to produce a short germ tube, the promycelium or epibasidium. The zygotic nucleus undergoes meiosis and the resultant four haploid nuclei become uniformly distributed in the epibasidium (D).

Septa are laid between the nuclei dividing the epibasidium into four uninucleate cells. The basidium at this stage is divisible into two parts, the first formed basal hypobasidium (or probasidium) and the latter formed distal epibasidium (or metabasidium).

The phragmobasidium being septate is less like a typical ascus but the cytological events are again similar. Each epibasidial cell gives out a small, slender lateral projection, the sterigma.

The tip of the sterigma enlarges to form a sac-like swelling, the basidiospore initial. Meanwhile the haploid nucleus in each epibasidial cell divides mitotically into two.

One of these migrates into the developing basidiospore through its respective sterigma and the other remains in the basidial cell. The uninucleate basidiospore initials mature into basidiospores.

A few species lack sterigma. In them the basidiospores are sessile. Each basidium typically bears four basidiospores (E). The number, however, varies from one to many. The basidiospores generally are unicellular, uninucleate,’ haploid, tiny structures.

To begin with they are hyaline, single cells and may remain colourless or become pigmented. In form they may be oval, round or elongated. Dacromyces has septate basidiospores.

(c) Discharge of Basidiospores (Fig. 13.10 A-D):

The basidiospores which are exposed on hymenium are usually perched in an oblique manner (asymmetrically) on the tips of sterigmata. As a rule they are discharged forcibly and in quick succession by the “water drop mechanism”.

As the basidiospore matures, the turgid basidium forces out of the sterigma tip a liquid which begins to collect in the form of a droplet at the base of the basidiospore (A).

The droplet gradually grows bigger till it attains a certain size (B) and suddenly pushes off the basidiospore forcibly into the air to a short distance (C). The surface tension is said to provide the necessary force.

The long distance dispersal is, however, dependent on air currents. The basidiospore carries the water drop with it.

(d) Germination of Basidiospores (Fig. 13.1):

On falling on a suitable substratum the basidiospore germinates. It puts out a germ tube (Fig. 13.1 A). The latter develops into a primary mycelium (Fig. 13.1 B). In a few species the basidiospores bud off into secondary basidiospores or conidia.

Kinds of Basidia (Fig. 13.11):

The basidia vary in form in different groups of Basidiomycetes. In general they are of two types, namely, unseptate or holobasidia (D) and septate or phragmobasidia (A).

1. Holobasidium (D):

It is a single-celled, unseptate, club-shaped structure with a rounded apex. The holobasidia are formed from the terminal binucleate cells of the secondary mycelium (Fig. 13.8) which enlarge to form club-shaped basidia borne in a palisade-like layer on the basidiocarp.

Probasidium is the name given to the young basidium in which nuclear fusion occurs. At its top the basidium usually bears four, sometimes only two, basidiospores, each at the end of a short, slender process called the sterigma. Two of these are of one strain and two of the other. Holobasidium is characteristic of the order Agaricales (mushrooms and toad stools).

2. Phragmobasidium (Fig. 13.11 A):

It is septate and is further divided into three kinds accordingly as the division is by transverse or vertical septa or have a deeply incised apex.

(a) Stichobasidial type:

The phragmobasidium in this type is cylindric and transversely septate. It is differentiated into two parts, the first formed portion and the latter formed portion.

The former is called probasidium or hypobasidium and the latter metabasidium or epibasidium. The septa in some genera are formed in the hypobasidium (A).

Each cell of the hypobasidium produces an unseptate epibasidium laterally. It bears terminally a basidiospore on a short sterigma. This kind of basidium is typical of the Auriculariales.

The epibasidia are formed only on one side of the hypobasidium (A). In Ustilago (Fig. 13.9 E) diploid brand spore represents the hypobasidium. It germinates to form the epibasidium which is transversely septate.

The terminal cell of the epibasidium produces a sterigma at its apex. The three other cells of the epibasidium push out lateral sterigmata one each more than half way up the cell. In U. maydis, the sterigmata are absent (Fig. 13.9 E).

The basidiospores are borne directly on the epibasidial cells. In Tilletia the brand spore (hypobasidium) germinates to form a transversely septate four-celled epibasidium which bears an apical cluster of sickle-shaped, septate sporidia.

In rusts also the transverse septa are fonned in the epibasidium. Each cell of the epibasidium bears a basidiospore on a lateral sterigma.

(b) Chiastobasidial type (Fig. 13.11 B):

In this case, the phragmobasidium is vertically septate. The septa are formed in the hypobasidium which is more or less rounded. Each cell of the hypobasidium is prolonged into a long slender branch at its apex. It is the epibasidium.

The latter bears a basidiospore at its tip on a slender sterigma. This type of basidium is found in Exidia nucleata (Tremallales).

(c) Tuning Fork Type (Fig. 13.11 C):

The probasidium or hypobasidium is borne at the end of a binucleate hypha. It is narrow and elongated with a wall thicker than the parent hypha. Its tip is prolonged into two long arms known as the epibasidia.

The whole structure thus resembles a tuning fork type. Each epibasidium at its tip bears a small pointed sterigma supporting an obliquely perched basidiospore.

This type of basidium is of universal occurrence in the family Dacrymycetaceae (Dacrymyces deliquescens). The phragmobasidium in this type lacks septation. The epibasidia grow from the tip of the hypobasidium.

Sexual Reproduction (Fig. 13.12):

The development of sex organs, antheridia and ascogonia, is universally absent throughout the class. Majority of the species are heterothallic. Morphologically the mycelia are alike but they are different in their sexual behaviour.

This rudimentary difference in sex, shown at the time of sexual fusion, is designated by plus and minus signs. These signs are called the sexual strains. Either of these mycelia, if cultured artificially, remains sterile.

They form no fructifications. Fructifications are formed only if two mycelia of opposite strains come into contact. The sexual process is thus extremely simplified.

It consists of three fundamental processes characteristic of sexual reproduction, namely, sexual fusion or plasmogamy, karyogamy and meiosis.

1. Plasmogamy (Fig. 13.12 A-B):

It means the union of two protoplasts whereby the sexual nuclei of opposite strains come close together in a pair within the same cell. In Basidiomycetes plasmogamy is accomplished either by somatogamy (A) or by spermatisation (B).

(a) Somatogamy (Fig. 13.12 A):

Two somatic hyphae of the primary mycelia of opposite strains come in contact (b). The walls between the adjacent cells at the point of contact dissolve (c).

The two nuclei come to lie side by side in the fusion cell which thus becomes binucleate (c). This sexual union or plasmogamy by fusion of somatic cells is called somatogamy.

The binucleate or dikaryotic cell thus formed, by successive divisions and clamp formation (d) at each division, develops into a secondary mycelium. Plasmogamy thus is basically the means to initiate the dikaryophase in the life cycle.

In the homothallic species plasmogamy takes place by the formation of tubular connections between the somatic cells of the same mycelium.

(b) Spermatisation (Fig. 13.12 B):

It is another method whereby plasmogamy occurs. Plasmogamy by spermatisation exclusively takes place in the rusts. The rusts produce numerous, tiny, uninucleate, non-motile, spore-like bodies called the spermatia.

They are formed in flask- shaped organs, the spermagonia (Fig. 14.15) developed on the upper surface of the leaf of the second host. The spermatia are carried by the various agencies.

Generally, they are carried by the insects to the special receptive hyphae of opposite strain produced in another spermagonium. The spermatia adhere to these hyphae at the tips (a) or laterally (b).

At the point of contact the wall dissolve and a pore is formed (c). The contents of the spermatium, which function as a male gamete, migrate through the pore into the receptive hypha and make it binucleate (c).

The receptive hypha thus functions as a female organ. Plasmogamy by the union of a spermatium with a receptive hypha is known as spermatisation.

Some mycologists use this term in a wider sense to refer to the contact of detached non-motile cells such as spermatia, microconidia, conidia and oidia with trichogyne or a receptive hypha.

The terminal binucleate or dikaryotic cells of the hyphae of the secondary mycelium develop into basidia (Fig. 13.8). The two nuclei in the dikaryotic cell fuse. This fusion of the two nuclei is called karyogamy (Fig. 13.8 b).

The resultant diploid fusion nucleus is called a synkaryon. The young basidium containing the synkaryon is called the probasidium. It represents the transitory diplophase.

The synkaryon in the probasidium soon undergoes two nuclear divisions (Fig. 13.8 c-d). These divisions constitute meiosis. Meiosis restores the haploid condition in the life cycle.

Karyogamy and meiosis take place in the basidium at different stages of development. The basidium is thus homologous to the ascus of Ascomycetes.


Contents

A recent classification [4] adopted by a coalition of 67 mycologists recognizes three subphyla (Pucciniomycotina, Ustilaginomycotina, Agaricomycotina) and two other class level taxa (Wallemiomycetes, Entorrhizomycetes) outside of these, among the Basidiomycota. As now classified, the subphyla join and also cut across various obsolete taxonomic groups (see below) previously commonly used to describe Basidiomycota. According to a 2008 estimate, Basidiomycota comprise three subphyla (including six unassigned classes) 16 classes, 52 orders, 177 families, 1,589 genera, and 31,515 species. [5]

Traditionally, the Basidiomycota were divided into two classes, now obsolete:

Previously the entire Basidiomycota were called Basidiomycetes, an invalid class level name coined in 1959 as a counterpart to the Ascomycetes, when neither of these taxa were recognized as divisions. The terms basidiomycetes and ascomycetes are frequently used loosely to refer to Basidiomycota and Ascomycota. They are often abbreviated to "basidios" and "ascos" as mycological slang. [ citation needed ]

Agaricomycotina Edit

The Agaricomycotina include what had previously been called the Hymenomycetes (an obsolete morphological based class of Basidiomycota that formed hymenial layers on their fruitbodies), the Gasteromycetes (another obsolete class that included species mostly lacking hymenia and mostly forming spores in enclosed fruitbodies), as well as most of the jelly fungi. This sub-phyla also includes the "classic" mushrooms, polypores, corals, chanterelles, crusts, puffballs and stinkhorns. [6] The three classes in the Agaricomycotina are the Agaricomycetes, the Dacrymycetes, and the Tremellomycetes. [7]

The class Wallemiomycetes is not yet placed in a subdivision, but recent genomic evidence suggests that it is a sister group of Agaricomycotina. [8] [9]

Pucciniomycotina Edit

The Pucciniomycotina include the rust fungi, the insect parasitic/symbiotic genus Septobasidium, a former group of smut fungi (in the Microbotryomycetes, which includes mirror yeasts), and a mixture of odd, infrequently seen, or seldom recognized fungi, often parasitic on plants. The eight classes in the Pucciniomycotina are Agaricostilbomycetes, Atractiellomycetes, Classiculomycetes, Cryptomycocolacomycetes, Cystobasidiomycetes, Microbotryomycetes, Mixiomycetes, and Pucciniomycetes. [10]

Ustilaginomycotina Edit

The Ustilaginomycotina are most (but not all) of the former smut fungi and the Exobasidiales. The classes of the Ustilaginomycotina are the Exobasidiomycetes, the Entorrhizomycetes, and the Ustilaginomycetes. [11]

Genera included Edit

There are several genera classified in the Basidiomycota that are 1) poorly known, 2) have not been subjected to DNA analysis, or 3) if analysed phylogenetically do not group with as yet named or identified families, and have not been assigned to a specific family (i.e., they are incertae sedis with respect to familial placement). These include:

  • AnastomycesW.P.Wu, B.Sutton & Gange (1997)
  • AnguillomycesMarvanová & Bärl. (2000)
  • AnthoseptobasidiumRick (1943)
  • ArcisporaMarvanová & Bärl. (1998)
  • ArrasiaBernicchia, Gorjón & Nakasone (2011)
  • BrevicellopsisHjortstam & Ryvarden (2008)
  • CelatogloeaP.Roberts (2005)
  • CleistocybeAmmirati, A.D.Parker & Matheny (2007)
  • CystogloeaP. Roberts (2006)
  • DacryomycetopsisRick (1958)
  • EriocybeVellinga (2011)
  • HallenbergiaDhingra & Priyanka (2011)
  • HymenoporusTkalčec, Mešić & Chun Y.Deng (2015)
  • KryptastrinaOberw. (1990)
  • MicrostellaK.Ando & Tubaki (1984)
  • NeotyphulaWakef. (1934)
  • NodulosporaMarvanová & Bärl. (2000)
  • ParaphelariaCorner (1966)
  • PunctulariopsisGhob.-Nejh. (2010)
  • RadulodontiaHjortstam & Ryvarden (2008)
  • RestilagoVánky (2008)
  • SinofavusW.Y.Zhuang (2008)
  • ZanchiaRick (1958)
  • ZygodesmusCorda (1837)
  • ZygogloeaP.Roberts (1994)

Unlike animals and plants which have readily recognizable male and female counterparts, Basidiomycota (except for the Rust (Pucciniales)) tend to have mutually indistinguishable, compatible haploids which are usually mycelia being composed of filamentous hyphae. Typically haploid Basidiomycota mycelia fuse via plasmogamy and then the compatible nuclei migrate into each other's mycelia and pair up with the resident nuclei. Karyogamy is delayed, so that the compatible nuclei remain in pairs, called a dikaryon. The hyphae are then said to be dikaryotic. Conversely, the haploid mycelia are called monokaryons. Often, the dikaryotic mycelium is more vigorous than the individual monokaryotic mycelia, and proceeds to take over the substrate in which they are growing. The dikaryons can be long-lived, lasting years, decades, or centuries. The monokaryons are neither male nor female. They have either a bipolar (unifactorial) or a tetrapolar (bifactorial) mating system. This results in the fact that following meiosis, the resulting haploid basidiospores and resultant monokaryons, have nuclei that are compatible with 50% (if bipolar) or 25% (if tetrapolar) of their sister basidiospores (and their resultant monokaryons) because the mating genes must differ for them to be compatible. However, there are sometimes more than two possible alleles for a given locus, and in such species, depending on the specifics, over 90% of monokaryons could be compatible with each other.

The maintenance of the dikaryotic status in dikaryons in many Basidiomycota is facilitated by the formation of clamp connections that physically appear to help coordinate and re-establish pairs of compatible nuclei following synchronous mitotic nuclear divisions. Variations are frequent and multiple. In a typical Basidiomycota lifecycle the long lasting dikaryons periodically (seasonally or occasionally) produce basidia, the specialized usually club-shaped end cells, in which a pair of compatible nuclei fuse (karyogamy) to form a diploid cell. Meiosis follows shortly with the production of 4 haploid nuclei that migrate into 4 external, usually apical basidiospores. Variations occur, however. Typically the basidiospores are ballistic, hence they are sometimes also called ballistospores. In most species, the basidiospores disperse and each can start a new haploid mycelium, continuing the lifecycle. Basidia are microscopic but they are often produced on or in multicelled large fructifications called basidiocarps or basidiomes, or fruitbodies), variously called mushrooms, puffballs, etc. Ballistic basidiospores are formed on sterigmata which are tapered spine-like projections on basidia, and are typically curved, like the horns of a bull. In some Basidiomycota the spores are not ballistic, and the sterigmata may be straight, reduced to stubbs, or absent. The basidiospores of these non-ballistosporic basidia may either bud off, or be released via dissolution or disintegration of the basidia.

In summary, meiosis takes place in a diploid basidium. Each one of the four haploid nuclei migrates into its own basidiospore. The basidiospores are ballistically discharged and start new haploid mycelia called monokaryons. There are no males or females, rather there are compatible thalli with multiple compatibility factors. Plasmogamy between compatible individuals leads to delayed karyogamy leading to establishment of a dikaryon. The dikaryon is long lasting but ultimately gives rise to either fruitbodies with basidia or directly to basidia without fruitbodies. The paired dikaryon in the basidium fuse (i.e. karyogamy takes place). The diploid basidium begins the cycle again.

Meiosis Edit

Coprinopsis cinerea is a basidiomycete mushroom. It is particularly suited to the study of meiosis because meiosis progresses synchronously in about 10 million cells within the mushroom cap, and the meiotic prophase stage is prolonged. Burns et al. [12] studied the expression of genes involved in the 15-hour meiotic process, and found that the pattern of gene expression of C. cinerea was similar to two other fungal species, the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. These similarities in the patterns of expression led to the conclusion that the core expression program of meiosis has been conserved in these fungi for over half a billion years of evolution since these species diverged. [12]

Cryptococcus neoformans and Ustilago maydis are examples of pathogenic basidiomycota. Such pathogens must be able to overcome the oxidative defenses of their respective hosts in order to produce a successful infection. The ability to undergo meiosis may provide a survival benefit for these fungi by promoting successful infection. A characteristic central feature of meiosis is recombination between homologous chromosomes. This process is associated with repair of DNA damage, particularly double-strand breaks. The ability of C. neoformans and U. maydis to undergo meiosis may contribute to their virulence by repairing the oxidative DNA damage caused by their host's release of reactive oxygen species. [13] [14]

Many variations occur. Some are self-compatible and spontaneously form dikaryons without a separate compatible thallus being involved. These fungi are said to be homothallic, versus the normal heterothallic species with mating types. Others are secondarily homothallic, in that two compatible nuclei following meiosis migrate into each basidiospore, which is then dispersed as a pre-existing dikaryon. Often such species form only two spores per basidium, but that too varies. Following meiosis, mitotic divisions can occur in the basidium. Multiple numbers of basidiospores can result, including odd numbers via degeneration of nuclei, or pairing up of nuclei, or lack of migration of nuclei. For example, the chanterelle genus Craterellus often has six-spored basidia, while some corticioid Sistotrema species can have two-, four-, six-, or eight-spored basidia, and the cultivated button mushroom, Agaricus bisporus. can have one-, two-, three- or four-spored basidia under some circumstances. Occasionally, monokaryons of some taxa can form morphologically fully formed basidiomes and anatomically correct basidia and ballistic basidiospores in the absence of dikaryon formation, diploid nuclei, and meiosis. A rare few number of taxa have extended diploid lifecycles, but can be common species. Examples exist in the mushroom genera Armillaria and Xerula, both in the Physalacriaceae. Occasionally, basidiospores are not formed and parts of the "basidia" act as the dispersal agents, e.g. the peculiar mycoparasitic jelly fungus, Tetragoniomyces or the entire "basidium" acts as a "spore", e.g. in some false puffballs (Scleroderma). In the human pathogenic genus Cryptococcus, four nuclei following meiosis remain in the basidium, but continually divide mitotically, each nucleus migrating into synchronously forming nonballistic basidiospores that are then pushed upwards by another set forming below them, resulting in four parallel chains of dry "basidiospores".

Other variations occur, some as standard lifecycles (that themselves have variations within variations) within specific orders.

Rusts Edit

Rusts (Pucciniales, previously known as Uredinales) at their greatest complexity, produce five different types of spores on two different host plants in two unrelated host families. Such rusts are heteroecious (requiring two hosts) and macrocyclic (producing all five spores types). Wheat stem rust is an example. By convention, the stages and spore states are numbered by Roman numerals. Typically, basidiospores infect host one, also known as the alternate or sexual host, and the mycelium forms pycnidia, which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in the host tissue (such as a leaf). This stage, numbered "0", produces single-celled spores that ooze out in a sweet liquid and that act as nonmotile spermatia, and also protruding receptive hyphae. Insects and probably other vectors such as rain carry the spermatia from spermagonium to spermagonium, cross inoculating the mating types. Neither thallus is male or female. Once crossed, the dikaryons are established and a second spore stage is formed, numbered "I" and called aecia, which form dikaryotic aeciospores in dry chains in inverted cup-shaped bodies embedded in host tissue. These aeciospores then infect the second host, known as the primary or asexual host (in macrocyclic rusts). On the primary host a repeating spore stage is formed, numbered "II", the urediospores in dry pustules called uredinia. Urediospores are dikaryotic and can infect the same host that produced them. They repeatedly infect this host over the growing season. At the end of the season, a fourth spore type, the teliospore, is formed. It is thicker-walled and serves to overwinter or to survive other harsh conditions. It does not continue the infection process, rather it remains dormant for a period and then germinates to form basidia (stage "IV"), sometimes called a promycelium. In the Pucciniales, the basidia are cylindrical and become 3-septate after meiosis, with each of the 4 cells bearing one basidiospore each. The basidiospores disperse and start the infection process on host 1 again. Autoecious rusts complete their life-cycles on one host instead of two, and microcyclic rusts cut out one or more stages.

Smuts Edit

The characteristic part of the life-cycle of smuts is the thick-walled, often darkly pigmented, ornate, teliospore that serves to survive harsh conditions such as overwintering and also serves to help disperse the fungus as dry diaspores. The teliospores are initially dikaryotic but become diploid via karyogamy. Meiosis takes place at the time of germination. A promycelium is formed that consists of a short hypha (equated to a basidium). In some smuts such as Ustilago maydis the nuclei migrate into the promycelium that becomes septate (i.e., divided into cellular compartments separated by cell walls called septa), and haploid yeast-like conidia/basidiospores sometimes called sporidia, bud off laterally from each cell. In various smuts, the yeast phase may proliferate, or they may fuse, or they may infect plant tissue and become hyphal. In other smuts, such as Tilletia caries, the elongated haploid basidiospores form apically, often in compatible pairs that fuse centrally resulting in "H"-shaped diaspores which are by then dikaryotic. Dikaryotic conidia may then form. Eventually the host is infected by infectious hyphae. Teliospores form in host tissue. Many variations on these general themes occur.

Smuts with both a yeast phase and an infectious hyphal state are examples of dimorphic Basidiomycota. In plant parasitic taxa, the saprotrophic phase is normally the yeast while the infectious stage is hyphal. However, there are examples of animal and human parasites where the species are dimorphic but it is the yeast-like state that is infectious. The genus Filobasidiella forms basidia on hyphae but the main infectious stage is more commonly known by the anamorphic yeast name Cryptococcus, e.g. Cryptococcus neoformans and Cryptococcus gattii.

The dimorphic Basidiomycota with yeast stages and the pleiomorphic rusts are examples of fungi with anamorphs, which are the asexual stages. Some Basidiomycota are only known as anamorphs. Many are yeasts, collectively called basidiomycetous yeasts to differentiate them from ascomycetous yeasts in the Ascomycota. Aside from yeast anamorphs, and uredinia, aecia and pycnidia, some Basidiomycota form other distinctive anamorphs as parts of their life-cycles. Examples are Collybia tuberosa [15] with its apple-seed-shaped and coloured sclerotium, Dendrocollybia racemosa [16] with its sclerotium and its Tilachlidiopsis racemosa conidia, Armillaria with their rhizomorphs, [17] Hohenbuehelia [18] with their Nematoctonus nematode infectious, state [19] and the coffee leaf parasite, Mycena citricolor [17] and its Decapitatus flavidus propagules called gemmae.


Genes and Genomics

J. Stephen Horton , . Scott E. Gold , in Applied Mycology and Biotechnology , 2005

2.2 Mating Types

Basidiomycetes generally have four functional mating specificities that combine to generate full sexual compatibility. Their cousins of the Ascomycota classically have 2 mating types (eg. a and α in Saccharomyces and A and a in Neurospora). Many excellent reviews are available on the topic of fungal mating types ( Brown and Casselton 2001 Casselton and Olesnicky 1998 Casselton 2002 Fraser and Heitman 2004 ) and so the issue will not be belabored here, rather we will give a brief outline for the basidiomycetes as the mating type genes are crucial developmental factors for these fungi. The basidiomycetes have taken sexual promiscuity to new heights. For example it is estimated that Coprinopsis cinerea (Coprinus cinereus) and Schizophyllum commune have 12,000 and 20,000 different mating specificities all of which are inter-compatible but self-incompatible, respectively ( Brown and Casselton 2001 ). This makes the probability of coming upon a compatible partner nearly 100%. Most mushrooms species have no inhibition to anastomosis in the homokaryotic stage and this makes biological sense since nearly all fusions will be sexually productive.

There are two functional classes of mating loci in the basidiomycetes. These are classically described as the a and b or A and B loci. Unfortunately in the smut model Ustilago maydis and related species the mating loci are named in reverse for function when compared to the Hymenomycetes including the mushrooms. In the mushrooms, the A loci encode homeodomain containing transcription factors while the B loci encode lipopeptide pheromones and transmembrane pheromone receptors. In the smuts the a locus encodes the pheromones and receptors and the b locus encodes the homeodomain containing proteins. In the smuts the a and b loci may be genetically linked or unlinked. This situation leads to two major groups the bipolar smuts in which there are only two mating specificities that segregate at meiosis and the tetrapolar smuts in which meiosis produces four mating specicities in the progeny. It has been shown that at least for some bipolar smuts the a and b loci are both present but are not separable by recombination ( Bakkeren et al. 1992 Bakkeren and Kronstad 1994 Lee et al. 1999 ). The tetrapolar mating system of the smuts is described later in this chapter. The arrangement of the A and B genes in the mushrooms is quite complex often with linked sub-loci. In some species the A and B genes may be distributed between distinct subloci known as Aα, Aβ, Bα and Bβ ( Fowler et al. 2004 Pardo et al. 1996 ). The paradigm in C. cinerea is that each A locus encodes three pairs on homeodomain containing proteins while each B locus encodes three cassettes each with a pheromone receptor gene and usually two pheromones ( Brown and Casselton, 2001 ). The pairs are made up of canonical members called HD1 and HD2, which must form at least one nonself heterodimer with those of another monokaryon for completetion of successful A mating function. None of the HD1 proteins are able to form a productive heterodimer with the HD2 proteins encoded by the same monokaryon. Similarly at the B locus the pheromones produced by a monokaryon will not interact with any of the receptors produced by that strain. Thus the basidiomycetes are exquisitely designed to find compatible partners in nature and yet recognize self as incompatible.


Basidiomycetes: Meaning, Features and Significance| Fungi

Basidiomycetes are fairly a large group of fungi represented by about 1100 genera consisting of 16,000 species and classified under the sub-division Basidiomycotina. Many of the Basidiomycetes are the familiar larger fleshy fungi such as mushrooms, toadstools, puffballs, geasters, stinkhorns, earth stars, bird’s nest fungi, jelly fungi bracket fungi, rusts, and smuts.

Majority of the Basidiomycetes are saprophytes causing decay of litter, wood or dung. A few forms are found as symbionts forming mycorrhizae in trees, whereas some ones are destructive parasites destroying a wide range of woody and herbaceous plants. Rusts and smuts are pathogenic to plants causing important diseases and are restricted to live tissues of their respective host plants.

Salient Features of Basidiomycetes:

(i) Thallus is usually mycelial (some are yeasts called basidiomycetous yeasts). Hyphae are septate. In some cases, a number of hyphae lying parallel to one another are joined together to form thick strands enveloped in a sheath or cortex and behave as a unit or tissue. This modified hyphal form is called rhizomorph.

(ii) The mycelium of most of the Basidiomycetes passes through three distinct stages of development, which are called primary mycelium (monokaryotic and develops from the germination of sexual spore called basidiospore), secondary mycelium (dikaryotic and develops from primary mycelium after the process of plasmogamy during sexual reproduction), and tertiary mycelium (represented by well-matured secondary mycelia that compose the fruiting bodies called basidiomata of complex Basidiomycetes).

(iii) Septa in hyphae are simple or characteristic dolipore. The dolipore septum flares sharply and broadly in the middle portion forming a barrel-shaped structure with open ends, which is covered by membranous structure called paranthesome or septal pore cap.

(iv) Cell wall is made up of chitin and glucans.

(v) In all the Basidiomycetes, except the rusts, a specialized structure called clamp-connection is formed on the secondary mycelium. Clamp-connection is a device for the perpetuation of dikaryophase in the secondary mycelia in Basidiomycetes.

(vi) The asexual reproduction takes place by oidia, conidia, or chlamydospore formation. Out of the several types of spores developed in the life cycle of rusts and smuts, some (e.g., uredospore) function as asexual spores. The higher taxa of this class lack asexual reproduction.

(vii) No specialized sex organs develop in Basidiomycetes and sexual reproduction takes place by conjugation of nuclei of two different strains.

(viii) During sexual reproduction, the dikaryotic cell is formed by spermatization, somatogamy, clamp- connection, or Buller phenomenon.

(ix) Karyogamy does not occur just after plasmogamy because there exists a prolonged dikaryophase between plasmogamy and karyogamy. The fusion of nuclei of two different strains occurs within the basidial mother cell.

(x) Meiosis takes place just after karyogamy resulting in haploid daughter nuclei that are used in the formation of basidiospores.

(xi) Basidiospores are the characteristic haploid sexual spores of basidiomycetous fungi, which are produced exogenously on the surface of basidia after karyogamy and meiosis within nuclei lying inside basidia. If the sexual spore of a fungus is a basidiospore, the fungus is a Basidiomycete regardless of any other character. This one character distinguishes Basidiomycetes from all other fungi.

(xii) Except rusts and smuts, the Basidiomycetes usually produce fruiting bodies called basidiomata (sing. basidioma), which were earlier called basidiocarps.

Significance of Basidiomycetes:

(i) Rusts and smuts cause many diseases of cereals and other economically important plants resulting in great loss in production that brings famine in certain parts of the world.

(ii) Mushrooms are edible and are enjoyed by mycophagists for food and flavour. Agaricus, Pleurotus, and Volvoriella are cultivated on large scale throughout the world. Mushroom cultivation industry is growing fastly.

(iii) Some mushrooms (called toadstools) such as Amanita spp. are deadly poisonous, while others such as Psilocybe spp. produce hellucinogenic chemical substances.

(iv) The polypores cause enormous damage to forest trees as parasites and to timber as saprophytes.

(v) Mycelia of basidiomycetous fungi play significant role in decomposition of organic matter and recycling of nutrients. It is because they bear ability to produce various extracellular enzymes which break down complex chemicals like cellulose and lignin.


Phylum Basidiomycota: Life Cycle

Although some basidiomycetes produce asexual spores, asexual reproduction is far more common in the phylum Ascomycota. Therefore, we will discuss a generalized life cycle covering sexual reproduction. Turn your attention to the basidiomycete life cycle in Figure 5.


Figure 5. A basidiomycete life cycle. (Click image to enlarge)

In steps 1 and 2, two haploid hyphae of opposite mating types fuse to produce a dikaryotic hypha. In step 3, under favorable environmental conditions (optimum temperature and moisture levels), the dikaryotic hypha is capable of producing a fruitbody in this case, a gilled mushroom. In step 4, basidia begin to form on the surface of the gills, and in step 5, karyogamy occurs to form a diploid nucleus in each basidium. In step 6, meiosis immediately follows karyogamy and each of four genetically distinct, haploid nuclei migrates into appendages and develops into basidiospores. Basidiospores are released and wind dispersed. They germinate into haploid hyphae and the cycle begins again in step 7.


9.1: Basidiomycota- The Club Fungi - Biology

Unit Four. The Evolution and Diversity of Life

18.7. Basidiomycetes

The phylum Basidiomycota contains the most familiar of the fungi among their 22,000 named species—the mushrooms, toadstools, puffballs, and shelf fungi. Many mushrooms are used as food, but others are deadly poisonous. Some species are cultivated as crops—the button mushroom Agaricus bisporus is grown in more than 70 countries, producing a crop in 1998 with a value of over $15 billion. Also among the basidiomycetes are bread yeasts and plant pathogens including rusts and smuts. Rust infections resemble rusting metal, whereas smut infections appear black and powdery due to their spores.

The life cycle of a basidiomycete (figure 18.8a) starts with the production of a hypha from a germinating spore. These hyphae lack septa at first, just as in zygomycetes. Eventually, however, septa are formed between each of the nuclei—but as in ascomycetes, there are holes in these cell separations, allowing cytoplasm to flow freely between cells. These hyphae grow, forming complex mycelia, and when hyphae of two different mating types (+ and -) fuse, they form cells in which the nuclei remain separate—they do not fuse into one nucleus. Recall that if two distinct nuclei occur within each cell of the hypha, it is called dikaryotic, indicated by the n + n tan area of the cycle. The dikaryotic hypha that results goes on to form a dikaryotic mycelium. The mycelium forms a complex structure made of dikaryotic hyphae called the basidiocarp, or mushroom (figure 18.8b).

Figure 18.8. Life cycle of a basidiomycete.

(a) Basidiomycetes usually reproduce sexually, with the fusion of nuclei in the basidia to produce a zygote. Meiosis follows syngamy and produces basidiospores that eventually form a basidiocarp (b).

The two nuclei in each cell of a dikaryotic hypha can coexist together for a very long time without fusing. Unlike the other two fungal phyla, asexual reproduction is infrequent among the basidiomycetes, which typically reproduce sexually.

In sexual reproduction, zygotes (the only diploid cells of the life cycle) form when the two nuclei of dikaryotic cells fuse (on the right-hand side of the cycle). This occurs within a club-shaped reproductive structure called the basidium (plural, basidia). Meiosis occurs in each basidium, forming haploid spores called basidiospores. The basidia occur in a dense layer on the underside of the cap of the mushroom, where the surface is folded like an accordion. It has been estimated that a mushroom with an 8-centimeter cap can produce 40 million spores per hour!

Key Learning Outcome 18.7. Mushrooms are basidiomycetes, which form club-shaped reproductive structures called basidia.


Club Fungi

The familiar mushrooms found on pizzas and in the lawn are part of a group of fungi called club fungi, or basidiomycetes. The phylum Basidiomycota consists of 16,000 different species of fungi, including the shelf or bracket fungi found on dead trees and the less well known puffballs, bird’s nest fungi, and stinkhorns. All of these are fruiting bodies, a reproductive structure where spores are produced and released, and are called basidiocarps. They contain the basidia (from the Greek basis, meaning pedestal) which is the familiar club-shaped structure that produce basidiospores.

Reproduction

Club fungi do reproduce asexually occasionally however, they usually reproduce sexually. The monokaryotic hyphae of two different mating types meet to fuse into a dikaryotic mycelium, which can continue its existence for hundreds of years. In mushrooms, the dikaryotic mycelium radiates out and produces basidiocarps in an ever larger “fairy ring.” In puffballs, spores are produced in the parchment-like membranes and released through pores or when the membrane breaks down. In bird’s nest fungi, falling raindrops provide the force necessary to send the nest’s basidiospore-containing “eggs” flying through the air to land on some other vegetation so it can be eaten by an animal. They pass through the digestive tracks of animals unharmed and are distributed over a large area. Stinkhorns resemble a mushroom with a spongy stalk and a slimy, compact cap. They give off an incredibly horrible odor that attracts flies. When the flies land to feed on a sweet jelly produced by the stinkhorn, they pick up the spores that they later distribute.

Types of Club Fungi

Club fungi come in several types, including mushrooms, puffballs, bird’s nest fungi, and stinkhorns. Two other types of club fungi that are of great economic importance are smuts and rusts. Smuts and rusts are club fungi that parasitize cereal crops and cause huge crop loss every year. They don’t form basidiocarps, and their small numerous spores resemble soot.

Some smuts enter the seeds and live in the plant, becoming visible only near maturity. Others affect the plant externally. The corn smut mycelium grows between the kernels of corn and secretes substances that cause the development of tumors on the ears of the corn. Rusts usually have a more complicated life cycle because they usually require two different plant host species to complete the cycle. For instance, black stem rust of wheat uses barberry bushes as an alternate host, and blister rust of white pine uses currant and gooseberry bushes. Eradicating these alternate hosts in areas where these rusts are a problem usually keeps the rusts in check. Wheat rust is kept in check by producing new resistant strains of wheat continuously to keep up with the adaptable, mutating rust.


Different Kinds of Fungi

Fungi are found to exhibit both sexual and asexual mode of reproduction. They are mainly classified into seven phyla or divisions on the basis of the types of spores, and the nature of the reproductive structures they form.

Chytridiomycota

Chytridiomycota can be found all over the world, and they are commonly known as chytrids. The name is derived from the Greek word chytridion, meaning ‘little pot’, which refers to the pot-like structure that contains the unreleased spores. They produce mobile zoospores for propagation. The movement of these spores is facilitated by the single flagellum present on their body. Chytrids are quite distinct from other divisions of fungi, and they are composed of four main clades.

Blastocladiomycota

The fungi belonging to the phylum Blastocladiomycota were initially included in a clade that constituted the phylum Chytridiomycota. But recently, on the basis of the results of molecular data and the characteristic of their ultrastructures, they are placed as a sister clade to Zygomycota and Glomeromycota. They can be saprotrophs, and they can exhibit sporic meiosis.

Neocallimastigomycota

Initially, Neocallimastigomycota belonged to the phylum Chytridiomycota. The fungi that belong to this phylum are generally found in the digestive tract of herbivorous animals. The fungi of this phylum are anaerobic, i.e., they can thrive in the absence of oxygen. They can exist both on land and in water. Like Chytridiomycotas, they form zoospores that contain single or multiple flagella.

Zygomycota

Most of the fungi belonging to the phylum Zygomycota are saprobes. They are commonly known as sugar or pin molds. They can reproduce both sexually and asexually. For sexual reproduction, they produce zygospores, while asexual reproduction is carried out by means of sporangiospores. Some common type of fungi that belong to this phylum are, black bread mold, mucor, rhizomucor, rhizopus, and pilobolus species.

Glomeromycota

This phylum contains approximately 200 species of fungi, which mainly reproduce asexually. They draw nutrition from plants, and form arbuscular mycorrhizas with higher plants. The fungus basically penetrates the cortical cells of the roots of a plant, and form arbuscular mycorrhizas. These specialized structures help the plant derive nutrients from the soil. According to scientific estimates, the symbiotic relationship between plants and glomeromycotas dates back to 400 million years.

Ascomycota

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Ascomycotas are also known as sac fungi. Ascomycota is the largest phylum of fungi that contains more than 30,000 species. The fungi of this phylum reproduce sexually, and produce ascospores in a sac-like structure, called ascus. However, some species are found to reproduce asexually. Some common Ascomycotas are, mushrooms, morels, yeasts, and truffles. They can be saprotrophs and parasites. They can also establish symbiotic relationships with plants. Aspergillus, Penicillium, and Claviceps are some important genera that belong to the phylum Ascomycota.

Basidiomycota

The members of this phylum are also called club fungi or basidiomycetes. They produce basidiospores for reproduction. The spores are produced in a specialized club-like structure, called basidium. The phylum includes several species like mushrooms, rust, and smut fungi.

Fungi play an important role in the ecosystem as decomposers. Though some of them are pathogens, many of them are widely used for the preparation and preservation of food like wine, beer, bread, cheese, and soy products. Fungi like mushrooms and truffles are an important food source. Several species of fungi are also used to produce antibiotics like penicillin and cephalosporin, and some important vitamins.

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9.1: Basidiomycota- The Club Fungi - Biology

I. Fungal Form and Function

A. Fungal Body Plans Figure 22-1

1. Most fungi are multicellular.

2. The food&endashabsorbing part of the fungus is a mesh of branching filaments called the mycelium.

a. Each tubular filament is a hypha with chitinous walls.

b. Interconnections and perforations called septa allow cytoplasmic flow necessary for transport to nonabsorptive parts of the body.

1. Fungi are heterotrophs that use organic matter.

a. Some (saprobes) get their nutrients from nonliving matter.

b. Others (parasites) thrive on tissues in living hosts.

2. All fungi rely on extracellular enzymatic digestion and absorption.

3. Fungi are valuable decomposers in the environment.

1. Asexual reproduction is mostly by spores produced in sporangia.

2. Sexual reproduction proceeds through the formation of gametes in gametangia as well as by spores.

II. Beneficial Associations Between Fungi and Plants

A. Lichens

1. Lichens are mutualistic associations (symbiotic relationship) between fungi and cyanobacteria, green algae, or both.

a. The fungus parasitizes the photosynthetic alga upon which it depends entirely for its food.

b. The algae derive very little benefit other than a protected place to survive.

2. Lichens live in inhospitable places such as bare rock and tree trunks.

a. By their metabolic activities, lichens can change the composition of their substrate.

b. They are unusually sensitive to air pollution

1. A mycorrhiza is a symbiotic relationship in which fungi hyphae surround roots of shrubs and trees.

a. The hyphae of exomycorrhizae do not penetrate the cells of the root.

b. The hyphae of endomycorrhizae do penetrate the cells of the root.

2. Because of its extensive surface area, the fungus can absorb mineral ions and facilitate their entry into the plant.

III. Major Groups of Fungi TABLE 22-1

A. Over 80,000 species of fungi have been identified, and there are many more.

B. The major groups -- zygote fungi (Zygomycetes), sac fungi (Ascomycetes), club fungi (Basidiomycetes), egg fungi (Oomycota) and one "catch&endashall" category "imperfect" fungi (Deuteromycota).

C. About 430 million years ago fungi started invading the land.

IV. Zygomycetes (Zygomycota)

A. The name is taken from the structure that forms in sexual reproduction.

1. Sexual reproduction begins when two hyphae (different mating strains) grow toward each other and fuse.

a. Gametangia form and make haploid nuclei, which later fuse to form a zygote.

b. A zygosporangium forms around the zygote later it releases haploid spores.

2. Each spore germinates to produce a stalked structure that releases spores that will grow into new mycelia.

B. Many are saprobes in the soil on decaying plant matter others, such as black bread mold, live on stored food.

A. This group includes both single&endashcelled and multicelled forms.

1. Single&endashcelled yeasts are useful in baking (carbon dioxide production makes the bread "rise") and for alcoholic&endashbeverage production.

2. Multicelled forms include edible morels and truffles.

3. Claviceps purpurea is an ascomycete that attacks rye plants. It produces several toxins, one of which is the actual ingredient in LSD. Infected rye made into flour and consumed can produce convulsions, hallucinations, and even death. Another toxin, ergot, is currently used in drugs which induce labor and control hemorrhaging after childbirth.

B. Reproduction is both asexual and sexual.

1. Yeasts produce buds multicelled species form spores called conidia.

2. In sexual reproduction, they form ascocarps, which bear spore&endashproducing sacs called asci.

VI. Club Fungi (Basidiomycota)

A. A Sampling of Spectacular Diversity

1. This group includes commonly seen puffballs and shelf fungi, rusts and smuts, and edible mushrooms.

2. Some are famous for their hallucinogenic and/or deadly toxins.

1. Club fungi produce spores on the outer surface of basidia borne on a short&endashlived structure called the basidiocarp (usually visible above ground).

2. When spores land on a suitable site, they germinate to produce extensive underground mycelia, which then reproduce sexually, resulting in a dikaryotic stage.

VII. "Imperfect" Fungi (Deuteromycota)

A. This group includes all fungi lacking a known sexual phase.

B. Examples include the yeast Candida albicans, which causes vaginal infections.

C. Penicillium, a conidium (asexual spore) producing species famous for production of an antibiotic of similar name. Also used in the production of camembert and roquefort cheeses.

D. Aspergillus produces citric acid used in candies and soft drinks. It's also used for fermenting soybeans for soy sauce.

E. Other imperfect fungi are human parasites causing diseases such as ringworm and athlete's foot.

VIII. Egg Fungi (Oomycota) also listed under Funguslike protists

A. Water molds (Class Oomycota) attack aquatic animals (such as goldfish) or land plants (such as potatoes and grapes).

1. They produce extensive mycelia, some of which become modified to form gamete&endashproducing structures.

2. The diploid zygote develops into a resting spore that will germinate into a mycelium&endashproducing motile, asexual spores.


Watch the video: Club fungi intro (May 2022).