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Section F

Sub division - Basidiomycotina

Basidiomycotina are mostly terrestrial and saprophytic or parasitic. It includes some 25,000 described species. This group includes most of the mushrooms , toadstools, stinkhorns, puffballs, and shelf fungi. And also contains important obligate parasites, two important plant pathogens the rusts and smuts.

This sub division shares many features in common with the Ascomycotina:-

  • Well-developed, branched, regularly septate hyphae.
  • Hyphae are initially uninucleate but soon become dikaryotic.
  • Presence of yeast stage and presence of macroscopic fruiting bodies, in some taxa.
  • Few species form blastic conidia.
  • Cell walls are usually chitinous.
  • As in the Ascomycota and Zygomycota, the most important feature is the production of basidiospores (sexual spore).
  • The basidiospores that are typically borne, exogenously, on horn-like sterigmata (singular-sterigma) of basidia (singular- basidium).

General characters of Basidiomycotina :-The mycelium of the Basidiomycotina in most species have three distinct phases during the life cycle of the fungus:-

  • Primary mycelium :- When it germinates, a basidiospore produces haploid primary mycelium . Initially the mycelium may be multinucleate, but septa soon form and the mycelium is divided into monokaryotic (uninucleate) cells. This septate mycelium grows by division of the terminal cell.
  • Dolipore septa with parenthesome are present in most of the genera.
  • Allows cytoplasmic movement but prevents nuclear migration from one compartment to the next.
  • Secondary mycelium :-Commonly a secondary mycelium forms upon conjugation of two sexually compatible hyphae ( heterokaryotic).
  • The dominant phase of the life cycle in most Basidiomycotina is a dikaryon, in which the two nuclei brought together in mating exist side-by-side in each cell.
  • But some times, septa are not formed after a mitosis in the terminal cell of primary hyphae as a results secondary mycelium is formed. Therefore all subsequent cells of the hyphae are binucleate (dikaryotic).
  • In both cases, the dikaryotic (binucleate) mycelium is formed, since karyogamy (the fusion of the gametic nuclei) dose not immediately follow plasmogamy (fusion of the protoplasts).
  • As the dikaryotic mycelium grows, the cells divide and more septa are formed between the new cells.
  • Each of the new cells in the secondary mycelium has one haploid nucleus from each parent. This is due to clamp connections, specialized structures unique to the Basidiomycotina.

  • These are loop-like hyphae which connect the cytoplasm of adjacent cells and through which nuclei move during cell division.
  • During cell division, one nucleus divides directly into the newly formed cell. The other nucleus divides inside the clamp connection and the two daughter nuclei migrate through the clamp connection in opposite directions to the two daughter cells, reestablishing the dikaryotic condition.
  • The tertiary mycelium which is also dikaryotic, arises directly from the secondary mycelium forms the fruiting bodies -basidiocarps.

Reproduction and life-cycle of Basidiomycotina :- Like all fungi, Basidiomycotina can undergo both asexual and sexual reproduction.

  • Asexual Reproduction :- Basidiomycotina reproduce asexually by either budding or asexual spore formation.
    • Budding occurs when an outgrowth of the parent cell is separated into a new cell. Any cell in the organism can bud.
    • The usual asexual spores formed by these fungi are arthrospores, sometimes called oidia.
    • Few species form blastic conidia. Conidia (asexual spore) formation, takes place at the ends of specialized structures called conidiophores. The septae of terminal cells become fully defined, dividing a random number of nuclei into individual cells. The cell walls then thicken into a protective coat. The protected spores break off and are disbursed and germinate.
    • Oidia are formed on specialised, erect hyphal branches known as oidiophores. Oidia can have two functions. They may germinate to form mycelium or they may function in the mating process. Asexual spores are usually haploid and the hyphae that form after germination are also haploid.
  • Sexual reproduction :-
    • No specialized sex organs form in the Basidiomycotina except in the rusts.
    • The sexuality in Basidiomycotina gets progressively subdued.
    • The specialized nature of sex organs is lost and the whole process is ultimately confined to copulation between vegetative mycelia (somatogamy).
    • The tertiary mycelium, which is also dikaryotic, arises directly from the secondary mycelium, and forms the basidiocarp. The spore forming basidia(basidia are not produced by asexual Basidiomycota) are produced by the terminal cell on millions of dikaryotic hyphae.
    • Basidiocarps vary greatly in appearance in different genera, but all bear basidia which are usually arranged in a layer of hymenium.
    • The basidia and basidiospores are produced on even sufaces or on hymenophores in form of teeth, tubes or leaf-like structures (gills) at the inferior side of caps or consoles (crustothecium, pileothecium).
    • In special parts of the fruit body (hymenia, hymenophores if existing) karyogamy occurs between the two haploid nuclei within a developing basidium. Then, the diploid nucleus undergoes meiosis to produce four haploid nuclei.
    •   Basidiospore formation on the basidium is exogenic (external) which means that the 4 basidiospores appear on small appendages (sterigma) on top of each basidium, in contrast to that, ascospore formation is endogenic.
    • Basidia of Hymenomycetidae are formed in layers called hymenia.
    • The hymenia are exposed on special surface enlarging structures (hymenophores) for spore discharge.
    • One of the most important characteristics of Basidiomycotina is the production of forcibly discharged ballistospores which are propelled into the air from the sterigma.
    • Ballistospores are sexual or asexual, and produced by basidia, hyphae, yeast cells, or even other ballistospores.
    • This type of spore discharge have been evolved very early in the evolutionary history of the Basidiomycotina as it is found in members of the earliest diverging lineages within the group.
    • Ballistospory is associated with forms that disperse their spores directly into the air. Most aquatic Basidiomycotina and forms that produce spores inside the fruiting body, such as puffballs, have lost ballistospory.
    • Basidiocarps is generally divided into 3 parts :-
      • Hymenium The typical hymenium consists of parallel arranged fertile elements: young basidia (basidioles), basidiospore forming basidia in various stages of developement and also sterile elements (cystidia).
      • Subhymenium – Below the hymenium a layer of isodiametric cells is present which is the subhymenium
      • Trama – The layer on which the subhymenium and the hymenium is found to be located is the trama. The trama is the main hyphal structure within the hymenophore (hymenophoral trama) and within the whole fruit body. 

Fruit bodies (Basidiocarps) of the different species show a great variety of different shapes :-

  • Holothecium:- No clear delimination of spore producing and sterile parts in the fruit body which is cushion-like, club-shaped (clavate), coralliform (coraloid) or irregularily lobed.
  • Crustothecium : -A fruit body which is more or less completely adnate to the substrate.
  • Pilothecium :-a fruit body which is differentiated in a stipe and a cap.

Subsection 2

Rusts:- Rusts (Pucciniales, previously known as Uredinales) shows greatest complexity in their life-cycle. Five different types of spores are formed on two different hosts in two unrelated host families. Such rusts are heteroecious (requiring 2 hosts) and macrocyclic. Autoecious rusts complete their life-cycles on one host intead of two. In order to complete its life history, this species produces the following five spore stages:-

  1. Spore Stage 0: Spermogonia (sing.=spermogonium) (consists of Receptive hyphae and Spermatia [sing.=spermatium]) :-Generally , basidiospores infect the alternate host the barberry , the mycelium forms pycnidia, called spermagonia. which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in host tissue (upper surface of a leaf). This stage, numbered "0", produces single-celled, minute 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 spermagonia to spermagonia, for crossing between the mating types.

  1. Spore Stage I: Aeciospores in aecia (sing.=aecium) :-After crossing , the dikaryons are formed and a second spore stage is formed, numbered "I" and called aecia. The aecia with the dikaryotic aeciospores are formed in dry chains in inverted cup-shaped bodies embedded in the lower surface of the barberry leaves. These aeciospores then infect the second host genus and cannot infect the host on which they are formed (in macrocyclic rusts).
  2. Spore Stage II: Urediospores in uredia (sing.=uredium) :- During the early spring ,the aeciospores infect the wheat plant. Infection of the wheat occurs in both the stem and leaves. Entry into the host occurs when the spores germinates and enters the plant through openings called stomata. After germination of spores, the mycelium in the host plant gives rise to clusters of the uredospores in dry pustules called uredinia. On the second host a repeating spore stage is formed, numbered "II", As the urediospores develop, they will burst the epidermis, exposing the characteristic, rusty-colored urediospores on the surface of the plant . The urediospore can infect other wheat plants throughout the spring and early summer.
  3. Spore Stage III: Teliospores in telia (sing.=telium) :- During late summer, the uredium gradually converts into the telium and begin to produce the two-celled, thick-walled teliospores a fourth spore type . The rusty-brown uredium becomes black as the teliospores are borne.The teliospore stage, with its thick wall is the over winter stage and will remain dormant for the winter.
  4. Spore Stage IV: Basidiospores on basidia (sing.=basidium) :- Under favorable condition, each cell is capable of germinating to produce basidia and basidiospores.In the Pucciniales, the basidia are cylindrical and become 3-septate (four celled) after meiosis, with each of the 4 cells bearing one basidiospore each. The basidospores disperse and start the infection process on host first again.

On the basis of life cycle patterns, the rusts are of three types :-

  • Macrocyclicrusts -Producing all 5 spores types.
  • Demicyclic rusts in which uredial stage absent
  • Microcyclic rusts Only two types of spores are formed -teleutospores and basidiospores)