8.1 The process of sexual reproduction
Eventually, for the majority of fungi, karyogamy and meiosis take place and the nuclear products of meiosis are packaged into sexual spores. In many fungi sexual spores have thickened walls; that is, they are resistant spores that are often dormant, and formed in relatively small numbers. In some cases the whole gametangium (the zygospores of zygomycetes would be a typical example [CLICK HERE for a reminder of the images) develops into a resistant structure, in other cases the sexual spores (particularly ascospores) are resistant and have a period of obligate dormancy. However, in Basidiomycota, basidiospores are produced in large numbers and are dispersal spores, not dormant spores [CLICK HERE for a reminder of the appropriate section in Chapter 3].
As befits its use in traditional taxonomy there are numerous variations in sexual reproduction in fungi. The first of these variables is the presence or absence of incompatibility systems. For example, in the zygomycetes, Mucor mucedo is heterothallic (self-sterile), but its relative Rhizopus sexualis is homothallic (self-fertile).
There is then the matter of the morphology of the hyphal structures involved in the various stages and the manner in which the processes are carried out. For example, gametangia are morphologically alike in the true fungus (zygomycete) Mucor mucedo, but morphologically different in some of the Oomycota (kingdom Chromista) like Pythium, which is an important pathogen causing damping-off of seedlings.
Similarly, the duration of the various stages of sexual reproduction may vary and some may be prolonged, for example prolonged karyogamy in diploid yeasts and, as indicated above, prolonged plasmogamy in the dikaryotic heterokaryon of Basidiomycota.
Hormones are probably involved in regulating sexual reproduction in most organisms, and fungi are no exception. Unfortunately, only a few of the active chemicals have been isolated from fungi; however, all of the major chemical classes of hormones identified in animals and plants are also known in fungi [CLICK HERE to view our Resources Box on pheromones in fungi]:
- sterols in the Oomycete Achlya bisexualis, female mycelium produces antheridiol, male produces oogoniol [CLICK HERE to see earlier section];
- the sesquiterpene hormone sirenin produced by female zoogametes of Allomyces macrogynus to attract male zoogametes [CLICK HERE to see earlier section];
- chemotropism to volatile precursors in the trisporic acid pathway that attracts heterothallic (self sterile + and -) zygophores of Mucor mucedo to one another. On their own, neither strain can produce trisporic acid, but they ‘converse’, by exchanging a volatile precursor and collaborate in its biosynthesis [CLICK HERE to see our Resources Box on pheromones in fungi];
- peptide pheromones involved in yeast mating (see below; CLICK HERE to see it now);
- mating type pheromones of filamentous Ascomycota and Basidiomycota that are part of a G-protein signalling pathway (see below).
Mating systems (also called breeding systems) rely on nuclear genes that control progress towards meiosis in the heterokaryon established between vegetatively-compatible mycelia. Basic analysis of such systems depends on making experimental confrontations between mycelia and scoring whether or not the sexual stage is completed. Such experiments test for the phenotype of sexual reproduction, and the pattern of its occurrence and its inheritance allow deductions about the control of sexual reproduction. A mycelium that possesses genes that prevent mating between mycelia that are genetically identical will be self-sterile; since it ensures that different mycelia must come together for a successful mating to occur and this is why such a system is called heterothallism.
Many heterothallic fungi, indeed all known heterothallic Ascomycota, have only two mating types specified by a single locus with different ‘alleles’: Neurospora crassa, budding yeast Saccharomyces cerevisiae, and the (basidiomycete) grass rust Puccinia graminis are examples. In such cases the mating type of a culture depends on which ‘allele’ it has at the single mating type locus (involvement of one mating type locus gives rise to the alternative name of unifactorial incompatibility): successful mating only taking place between cells or mycelia that have different ‘alleles’ at the mating type locus. Of course, the diploid nucleus that results is heterozygous for the mating type factor, and meiosis produces equal numbers of progeny of each of the two mating types (hence yet another alternative name, bipolar heterothallism).
We put the word allele into quotes in the last few sentences because, although it is not evident from classical genetic analysis, one of the first things that molecular analysis revealed about the mating type factors is that the different forms of the mating type locus do not share the amount of DNA sequence homology you would expect of alleles. Their ‘alleles’ can be very different indeed, in some cases differing in length by thousands of base pairs. For this reason they have been called idiomorphs rather than alleles. Idiomorphic structure (not allelism) is common to all fungal mating type genes that are known.
In homothallic (self-fertile) fungi sexual reproduction can occur between genetically identical hyphae, but mating type factors may still be involved. Primary homothallism occurs in species completely lacking heterothallism, but secondary homothallism occurs in species that have an underlying heterothallism that is bypassed when spores are made.
Neurospora tetrasperma, Coprinopsis bisporus and Agaricus bisporus are good examples. In these cases, there are more post-meiotic nuclei than spores, so the spores become binucleate and heterozygous for mating type factors. Spore germination gives rise to heterokaryotic mycelia that are, consequently, able to complete the sexual cycle alone, that is they act like homothallic mycelia (but they are heterokaryons right from the start).
The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe exemplify a different process. Most strains are heterothallic with two mating types (see below), but in some strains mating occurs between progeny of a single haploid ancestor; that is, the culture appears to be ‘homothallic’. The apparent homothallism results from a switch, in a few cells in the population, from one mating type to the other (see section on Mating type switching in yeast below [CLICK HERE to see it now] ) so that the (still heterothallic) clone comes to contain cells of a different mating type.
Updated December 17, 2016