Reproduction in Ascomycetes
Second half of Chapter 7 in Alexopoulos et. al., 4th edition
Life cycle of a typical filamentous Ascomycete
ascogonium; trichogyne; antheridium; basal cells
paraphyses: text defines as sterile, basally attached cells; conflicts with Introductory Botany definition.
Formation of ascus: ascogenous hyphae, crozier; formation of clamp-like areas
Dikaryon formation
different ways to form (see handout); to this handout add gametangial fusion.
Sexual reproduction (in Ascomycetes, i. E., from formation of gamentagia, through ascogenous hyphae, ascus, and ascospore formation); different genes have to be present, thus if the strains are homogenic, they are incompatible; thus this system is called homogenic incompatibility
promotes outcrossing (see handout)
under control of Mating genes; these are located at loci with each locus called the MAT locus; in Neurospora the MAT locus also codes for color of ascospores with a causing lack of color and A causing melanin-containing spores
genes are either biallelic or multiallelic (bipolar, unifactorial; tetrapolar, befactorial)
alternate states within the same fungus of the same gene (i. E., the alleles) are often unrelated or markedly different in their DNA sequences and also may encode for different proteins (alternate forms of a locus which lack homology= idiomorph; see handout)
function:
in Saccharomyces encodes for proteins which bind to the DNA molecule causing the transcription (activated or repressed) of other genes (ultimately forms pheromonic proteins)
In Neurospora triggers events leading to: protoperithecial development; function of the trichogyne in containing the spematium (60 mutants formed); plasmogamy; gametes ability to function; perithecial development (200 mutants formed)
role of sex: promotion of genetic variation; repair of damage affecting both strands of a DNA molecule for which a template from a homologous chromosome is needed (in Saccharomyces)
Vegetative Incompatibility (somatic incompatibility; heterokaryon incompatibility; heterogenic incompatibility); in Ascomycetes homogenic and heterogenic incompatibility are considered different (since you have a distinct sexual phase with gametangia).
controlled by genes at loci (call v-c loci; or het loci); each locus has several genes
divided into fusion incompatibility (made of fus (fusion) loci and let (lethal) loci
post-fusion locus
post-fusion locus: often get barrage area—sparse mycelium due to destruction in fused cells and apotentially adjacent areas as well.
Either genes in same locus control somatic fusion; or these genes interacting along with genes at other non-v-c loci can control somatic fusion.
Mating genes can be identical, can have an effect, or can have no effect.
Function of vegetative incompatibility
prevent spread of viruses, protects individual agains harmful nuclei; prevents the exposure of the individual and its genome to risk (e. g. a gene exists which enables nuclei containing it to replace other nuclei in a heterokaryon); prevents introduction of deleterious mitochondria (a mitochrondrion exists which has a genome which renders the mitochrondrion defective in its respiratory funciton); prevents inclusion of plasmids which can cause senescence.
In nature this system prevents the fusion of genetically different individuals of the same species (these are called genets and often found in rotted wood differentiated by zone lines); sexual reproduction occurs when the ascogonia and antheridia begin to develop and these are not under the control of the somatic incompatibililty system.
Heterokaryosis and the Parasexual Cycle.
Heterokaryons formed by mutation; diploidization; hyphal fusion
Parasexual Cycle: 1) heterokaryon formation; 2) diploidization of nuclei; 3) haploidization
Occurrence: can be duplicated in laboratory (e. g. Botrytis cultures); however rare in nature and thus its effectiveness is not stressed in textbook.
However, Carlilr and Watkinson (The Fungi; 1994; Academic Press) "The initial colonization of a new area or a new species, however might sometimes be achieved by a single haploid strain. The resulting population would then at first lack vegetative incompatibility so, as mutants arose (which will occur since the hyphae resultant from such colonization would be haploid), heterokaryosis and parasexuality might bring about genetic exchange. There hence may be populations in which for a time parasexuality is important. In due course, however, genetic diversity would increase to a point where individuals would need to protect themselves from alien nucleic acid. Then mutations disrupting fusion with other strains might be favored and vegetative incompatibility might be re-established.