In this article we will discuss about:- 1. Characteristics of Basidiomycetes 2. Mating Systems of Basidiomycetes 6. Life Cycle Pattern.
Characteristics of Basidiomycetes:
The Basidiomycetes comprise the highest group of fungi which are often noticed conspicuously occurring in fields and forest areas. Most of them possess highly developed fructifications, known as basidiocarps which may be fleshy or woody.
The fungi commonly known as mushrooms, toadstools, puff-balls, and bracket fungi are the Basidiomycetes. To this group also belong the rusts and smuts, responsible for diseases of plants of economic importance. There are both parasitic and saprophytic fungi in this group.
Many used as food are cultivated in a large-scale for marketing. Again there are highly poisonous ones which should be carefully avoided. Some cause destruction of forest trees and ornamentals. Whereas others are responsible for the decay of fallen logs of wood. The Basidiomycetes resemble the Ascomycetes in possessing septate hyphae and production of non-motile spores and fructifications.
The fungi of this group are characterized by the production of spores known as basidiospores resulting from karyogamy and meiosis, and are borne externally on slender protuberances, the sterigmata (sing, sterigma). The sterigmata are developed from a more or less club-like structure, called a basidium (pl. basidia) for which the Basidiomycetes have been frequently named as club fungi.
The basidium was discovered by Levelle in 1837.
Mating Systems of Basidiomycetes:
About 10 per cent, of Basidiomycetes are homothallic.
Three types of homothallic behaviour may be distinguished:
(i) Primary homothallism:
In Coprinus sterquilinus, a single basidiospore germinates to form a mycelium which soon becomes organized into binucleate segments bearing clamp connections at the septa. There is no genetical distinction between the two nuclei in each cell, and mycelium is capable of forming fruit bodies.
(ii) Secondary homothallism:
In Coprinus ephemerus f. bisporus, the basidia bear two spores, but the spores are heterokaryotic. After meiosis two nuclei enter each spore and a mitotic division may follow. On germination, a single spore germinates to form a dikaryotic mycelium capable of producing fruit bodies.
(iii) Unclassified homothallism:
The four-spored wild mushroom Agaricus campestris and the two-spored cultivated mushroom Agaricus bisporus are homothallic in the sense that a mycelium derived from a single spore is capable of producing fruit bodies. In both species there is nuclear fusion in the basidium, followed by two nuclear divisions, presumably meiotic.
In a bisporus two nuclei enter each spore, but in A. campestris only one. However, paired nuclei, conjugate nuclear divisions and clamp connections have not been observed.
Amongst the remaining 90 per cent, of Basidiomycetes reported to be heterothallic, can be distinguished as follows:
About 25 per cent, of Basidiomycetes examined have been shown to be bipolar. Besides the Uredinales and most Ustilaginales, Coprinus comatus, Fomes roseus, and Polyporus betulinus have bipolar mating systems.
Coprinus lagopus, Polyporus abietinus, Schizophyllum commune, Crucibulum vulgare, and Cyathus striatus have tetrapolar mating systems.
Life Cycle Pattern of Basidiomycetes:
The general life-cycle pattern of the Basidiomycetes has resemblance with that of the Ascomycetes. The basidiospore on germination by germ tube gives rise to the haplophasic somatic body represented by primary mycelium. The primary mycelium often produces oidia.
The oidia may behave as spores and give rise to primary mycelia, or by spermatization with compatible primary mycelia take part in the formation of dikaryotic secondary mycelia. The secondary mycelium may also be formed by somatogamy between two compatible primary mycelia. The dikaryotic apical cell of the hypha of the secondary mycelium develops into basidium.
It may so happen that resting spores may be formed from the dikaryotic hyphal cells. Each resting spore on germination gives rise to a basidium. Karyogamy takes place in the basidium producing diplophasic condition which is followed immediately by meiosis and ultimately haploid basidiospores are produced. The basidiospores germinate to produce haplophasic somatic body.
Thus the life cycle is completed. The life cycle comprises
of the three phases the haplophase, dikaryophase and diplophase; of which the dikaryophase is very much prolonged and is nutritionally independent of the haplophase. The haplophase is also well represented. Just as in almost all the fungi the diplophase is very short and is followed by meiosis. The general life cycle pattern in the Basidiomycetes is presented in Figure 260.