Sexual life cycles:
Sexual life cycles in the strict sense, i.e. involving nuclear fusion and meiosis, occur only in those ascomycetes which possess asci, because it is within the young ascus that these events occur. Ascospores of most ascomycetes contain one or more haploid nuclei, and therefore most (but by no means all) ascomycetes have a haploid vegetative mycelium. The mycelium is often capable of asexual reproduction, e.g. by fragmentation, budding or by the formation of conidia, chlamydospores, sclerotia, etc. The structure and formation of conidia is described below. Some yeasts, e.g. Saccharomyces cerevisiae, show an alternation of diploid and haploid yeast-like states and here the diploid state is the commonly encountered form in contrast to Schizosaccharomyces in which the vegetative cells are haploid.
The mating behaviour of ascomycetes may be homothallic or heterothallic. In homothallic ascomycetes the mycelium derived from a single ascospore is capable of reproducing sexually, i.e. by developing asci. Examples are Emericella nidulans, Pyronema domesticum and Sordaria fimicola.
However, the homothallic condition does not preclude outcrossing as is shown by the formation of hybrid asci containing black (wild type) and white (mutant) ascospores in crosses between different strains of Sordaria
fimicola, In heterothallic ascomycetes the ascus usually contains four ascospores of one mating type and four of the other. The two mating types differ at a single allele and the mating types may be designated A and a, a and a, or ( þ).
Sexual reproduction
Sexual reproduction occurs following plasmogamy between cells of the two mating types. Plasmogamy is of three main types:
1. Gametangio-gametangiogamy.
Fusion occurs between differentiated gametangia. An example is Pyronema domesticum where fusion is between the trichogyne, a filamentous extension of the large, swollen ‘female’ gametangium (the ascogonium) and a less swollen ‘male’ gametangium, the antheridium, which donates nuclei to the trichogyne and thereby to the ascogonium .
2. Gameto-gametangiogamy.
Fusion takes place between a small unicellular male gamete (spermatium) and a differentiated female gametangium (ascogonium). The spermatium is rarely capable of independent germination and growth and may only germinate to produce a short conjugation tube which fuses with the wall of the ascogonium. An example is Neurospora crassa in which the spermatium fuses with a trichogyne.
3. Somatogamy.
Fusion takes place between undifferentiated hyphae, i.e. there are no recognizable sexual organs. This type of sexual behaviour is shown by Coprobia granulata, whose orange ascocarps are common on cattle dung.
Asexual life cycles:
Most fungi which were formerly classified in the artificial group Deuteromycotina or Fungi Imperfecti are conidial forms (anamorphs) of Ascomycota, although a few have affinities with Basidiomycota. Evidence for a relationship to Ascomycota comes from morphological similarity and from DNA sequence comparisons. Morphological similarities include the structure of the mycelium, the layering of the hyphal wall as seen by electron microscopy, the finestructure of nuclear division, and also close resemblances of conidial structure and development. Some genera contain species which reproduce by asexual means only, whilst closely similar forms have sexual as well as asexual reproduction. Examples include Aspergillus and Penicillium, which are anamorphs of several genera of Ascomycota and Fusarium which is the anamorph of Gibberella and Nectria . It is presumed that fungi which reproduce only by conidia have lost the capacity to form ascocarps in the course of evolution.
Parasexual reproduction:
This is a process in which genetic recombination can occur through nuclear fusion and crossing over of chromosomes during mitosis. Meiosis does not occur, and instead haploidization takes place by the successive loss of chromosomes during mitotic divisions.
The essential steps include
(i) nuclear fusion between genetically distinct haploid nuclei in a heterokaryon to form diploid nuclei;
(ii) multiplication of the diploid nuclei along with the original haploid nuclei;
(iii) the development of a diploid homokaryon;
(iv) genetic recombination by crossing-over during mitosis in some of the diploid nuclei; and
(v) haploidization of some of the diploid nuclei by progressive loss of chromosomes (aneuploidy) during mitosis.
This process was discovered in Emericella (Aspergillus) nidulans, which can reproduce sexually by forming asci and asexually by forming conidia.
By changing the nutrient content of the medium on which the fungus is grown, the development of asci and therefore of normal sexual reproduction can be prevented.
Genetic mapping based on gene recombination following conventional sexual reproduction has been compared with mapping based on parasexual recombination and has yielded identical results. Parasexual recombination is known to occur not only in Ascomycota but also in Oomycota and Basidiomycota. It makes possible genetic recombination in organisms not known to reproduce by sexual means and helps us to understand why purely asexual fungi such as many species of Aspergillus and Penicillium have achieved success and have continued to flourish in the course of evolution. However, because parasexual reproduction is comparatively rare in nature, it is probably only a partial substitute for sexual reproduction, so that purely asexual species are more prone to accumulating deleterious mutations