In this article we will discuss about the life cycle of ascomycetes, explained with the help of a suitable diagram.
Despite their diversity in many features, the Ascomycetes possess certain common unifying characteristics, namely, the somatic body composed of a loose, indefinite mass of septate mycelium; the mode of asexrual reproduction; and sexual reproduction. In general, they have two reproductive phases; the asexual stage and the sexual stage.
Besides sexual process, some members of this class reproduce by budding and others by the development of spores, the conidia.
The conidia are produced profusely either directly from the somatic mycelium or from conidiophores after the organism has grown vigorously for quite some time. They are dispersed by external agencies, like, wind, insects, and similar others. Under favourable conditions each conidium germinates by germ tube which ultimately grows into somatic mycelium of the new individual.
This is how the fungus reproduces asexually. During sexual reproduction the changes involved in the process occur in regular sequence in cyclic order. The stage during which a fungus reproduces asexually is known as asexual stage or asexual cycle or conidial stage or imperfect stage.
Under favourable climatic conditions the asexual stage may be repeated resulting in the production of conidia in profuse quantities. This is a means how a large number of Ascomycetes propagate very quickly and disseminate themselves in distant localities. The conidia are very much susceptible to the alternation of climatic conditions.
In some cases, however, the conidia can stand unfavourable conditions and perennate. In course of development, the same mycelium from which, conidiophores and conidia were being developed now takes part in the formation of gametangia.
Gametangial union is followed by plasmogamy. Ultimately asci are formed, within which ascospores are produced by karyogamy and meiosis. The haplophase and diplophase are intervened by dikaryophase which may be short or long in duration. Depending on the organism, the asci may or may not be enclosed in an ascocarp.
Members of the Ascomycetes may be homothallic or heterothallic. In a homothallic species asci are formed in the same thallus, whereas in hererothallic species asci are developed by the interaction of two compatible thalli.
The basis for heterothallism is typically a single gene with two alleles ‘A’ and ‘a’ and because of seggregation during the meiotic division which precedes ascospore formation, the 8 ascospores normally present in an ascus will include 4 of one mating type and 4 of the other.
In certain 4-spored ascomycetous fungi, e.g., Neurospora tetrasperma and Podospora anserina, the ascospores are binucleate, and commonly contain nuclei of both mating types. Such ascospores on germination would give rise to fully fertile mycelia, and it would appear that the fungi are homothallic.
Occasionally uninucleate ascospores are also formed which on germination do not produce fertile mycelia and ascocarps are formed by interaction between mating types. Since basically these fungi are heterothallic in their mating behaviour, the term homothallism is used to describe the behaviour of their binucleate ascospores.
The ascospores are the organs through which most of the Ascomycetes perennate. The germination of ascospores takes place under favourable conditions by the production of one or more germ tubes which grow into somatic mycelium. The stage during which asci and ascospores are developed is known as sexual stage or sexual cycle or ascigerous stage or perfect stage.
A large number of Ascomycetes have both conidial and ascigerous stages by means of which they propagate and disseminate themselves.
But many Ascomycetes are known only in the ascigerous stage, their asexual stages either do not exist or have not been found. Usually both asexual and sexual stages are completed either parasitically or sapro- phytically depending on whether an organism is a parasite or a saprophyte.
But in many of the parasitic Ascomycetes the asexual stage is completed on the living tissue of the host and the sexual stage is formed only on the dead host tissue. Life cycle pattern in the Ascomycetes is presented in Figure 213.