Basidiomycetes: The Club Fungi | Structure & Characteristics

 

1. They are the most advanced and most commonly seen fungi. Their fructifications are often large and conspicuous, e.g., mushrooms, toadstools, puff balls, bracket fungi etc.
2. Basidiomycetes are among the best decomposers of wood. They are able to decompose both cellulose and lignin. Lignin is not metabolised by most other fungi and even bacteria. Ganoderma species causes decay of wood even on standing trees.
3. Motile structures or cells are absent.
4. Mycelia are of two types, primary and secondary. Primary mycelium contains monokaryotic cells and is short lived.
5. Monokaryotic phase or primary mycelium may multiply by oidia, conidia-like spores and pycniospores.
6. Secondary mycelium is long lived and dominant phase of life cycle. It is represented as dikaryophase. It consists of profusely branched septate hyphae.
7. Septa possess dolipores or central pores with barrel-shaped outgrowths (except rusts and smuts)
8. Handle like outgrowths are found on the sides of septa. They are called clamp connections. Clamp connections are meant for proper distribution of dikaryons at the time of cell division.
9. Secondary mycelium can perennate in the soil or wood by means of sclerotia or rhizomorphs.
10. Dikaryophase or secondary mycelium may multiply by different types of spores — chlamydospores, aecidiospores, uredospores, teleutospores etc.
11. There is often differentiation of two mating types (+) and (–) in thallus.
12. Sexual reproduction does not involve sex organs. Instead fusion occurs between basidiospores and other monokaryotic spores, between a spore or spermatium and a hypha or between two hyphal cells of primary mycelia.
13. Karyogamy and meiosis occur in club-shaped structures known as basidia (singular — basidium). The name of the class is based after them.
14. A basidium commonly produces four meiospores or basidiospores exogenously at the tips of fine outgrowths called sterigmata or directly on the basidium.
15. The fungi may or may not produce fructifications called basidiocarps. The basidiocarps vary Trom microscopic forms to large macroscopic structures. Some puff balls and brackets can be over 50 cm in diameter.

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(i) Smuts: They produce thick-walled, black-coloured resting spores called smut spores. Smuts are of two types, covered and loose. In covered smut, the spore mass remains within the membranous covering of sorus, eg., Ustilago hordei (covered smut of barley), Ustilago maydis (smut of corn). In loose smut the spores are exposed while attached to the host, e.g., Ustilago nuda tritici (loose smut of wheat), U. avenae (loose smut of oat). (ii) Mushrooms: They are edible and non edible Agaricales which possess umbrella like basidiocarp. Common examples of edible mushrooms are Agaricus campestris, A. bisporous, Volvariella volvacea (Paddy straw mushroom), Pleurotus ostreatus etc. (iii) Toadstools: Toadstools are poisonous mushrooms which generally have white spores. Amanita caesarea (Caeser’s mushroom) was used in poisoning Roman emperor Caesar. The other toadstools are Amanita phalloides (Death cup), A. muscaria (Fly agaric) and Gynomitra esculenta (heat labile carcinogenic toxin). (iv) Rusts: They are characterised by the formation of rusty pustules containing the spores. Puccinia graminis tritici — Black rust of wheat. Puccinia glumarum or P. striiformis — Yellow rust of wheat. P. recondita — Brown rust of wheat (v)   (a) Puccinia is heteroecious obligate parasite which completes its life cycle on two hosts; Primary host — wheat Secondary or alternate host — Barberry (b)   lt has macrocyclic life cycle with five types of spores: uredospores (n + n), teleutospores (n+ n) on wheat plant, basidiospore (n) in soil, pycniospores (n) and aeciospores (n + n) on barberry leaf. (vi) Hallucinogens : Psilocybe mexicana (Sacred mushroom) has hallucinating properties similar to LSD. It is used by Mexican Indians during certain religious ceremonies. (vii) Armillaria (largest fungi): A. mellea (Honey mushroom) is a serious root parasite of both hardwoods and conifers. The fungus develop rhizomorphs into the phloem of the host and hence, blocks the food supply. (viii) Puffballs: The basidiocarp is a stalked rounded structure which, upon ripening. releases out pufis of spores. The fructification may grow above or below the substratum, e.g., Lycoperdon obiongisporum, L. giganteum. (ix) Bracket fungi (Shelf fungi) : They are basidiomycetes whose basidiocarps or fructifications appear on tree trunks, logs, lumber etc. just as brackets or shelves, e.g., Fomes applanatus, Polyporus sulphureus, Ganoderma. (x) Predator fungi : e.g., Dactylaria, Arthrobotrys. (xi) Stink horn. Phallus impudicus (Dead man’s finger). Spore mass produces a stinking odour to attract flies.

Life History of a Mushroom

Agaricus campestris is the common field mushroom which has edible basidiocarp. The fungus is saprotrophic. The vegetative or assimilative part of mycelium is subterranean. It is found in moist humus rich soil of open and fields, grassland, piles of straw or within rotting logs.

The mycelium multiplies by fragmentation; occasionally by oidia and chlamydospores.

Life cycle of mushroom contains two types of mycelia, primary and secondary. Sex organs do not differentiate. Primary mycelium is short lived. It consists of septate hyphae having monokaryotic cells.

The mycelia are heterothallic, [having two mating types, (+) and (–)]. The hyphae of two mating types come in contact and show somatogamy. However, only plasmogamy occurs at this time. It gives rise to a dikaryotic cell that grows, divides and produces a long–lived and extensive dikaryotic or secondary mycelium. The hyphae of secondary mycelium show clamp connections and dolipore septa. Its cells possess two haploid nuclei instead of single diploid nucleus.

Under favourable conditions, hyphae of secondary mycelium collect at places and give rise to rounded or pyriform compact masses of hyphae called buttons. The buttons enlarge and produce aerial fructifications or basidiocarps. The latter are popularly termed as mushrooms. In contrast the secondary mycelium, from which mushrooms develop, is known as spawn. The basidiocarps or mushrooms often lie in rings. The latter are called as fairy rings, its diameter increases centrifugally every year.

Each basidiocarp or mushroom is cream to pinkish brown in colour and consists of two parts, stipe and pileus. The stipe or stalk is fleshy. It is slightly swollen at the base. Pileus is umbrella-like cap of the mushroom. In the button stage, the pileus is connected to stipe by membrane called veil or velum. It ruptures during growth of pileus. However, its remains can be seen on the upper part of stipe as annulus. The pileus is circular in outline. Its upper surface is more or less convex. The under surface is flat or concave. It bears 300-600 radiating rows of vertical plates named gills (lamellae).

The two sides of vertically placed gills are lined by thousands of club-shaped basidia alongwith sterile paraphyses. The two, together constitute the fertile layer or hymenium of the gill. Hymenium is subtended by compact subhymenium. The centre consists of interwoven hyphae called trama. Each basidium functions as the site for both karyogamy and meiosis. The two nuclei fuse to form a short-lived diploid synkaryon.

 

The latter, then divides meiotically giving rise to four haploid nuclei, two of (+) strain and two of (–) strain. The free end of the basidium now develops four peg-like outgrowths called sterigmata. Each sterigmata bears an ovoid pinkish-purple meiospore termed as basidiospore. A droplet appears at the tip of sterigmata which creates tension and hanging basidiospores are carried away by air currents. The basidiospores are liberated successively for several days. After falling on a suitable substratum, each basidiospore gernminates to produce monokaryotic primary mycelium.