In this article we will discuss about the life cycle of basidiomycetes with the help of suitable diagrams.
Mycelium of Basidiomycetes:
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.
Basidia:
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:
The club-shaped holobasidium pushes out four slender projections at its top {d). These are the sterigmata. The tip of each sterigma swells to form a sac-like swelling, the basidiospore initial.
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.
2. Karyogamy:
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.
3. Meiosis:
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.