Mating types regulate fusion (syngamy) between gametes or haploid structures that function as gametes. Only gametes with different mating types can fuse, analogous to self-incompatibility systems in angiosperm plants and corals (Charlesworth, 1994; Idnurm et al., 2008). In some species, individuals produce gametes that are all compatible (homothallism), whereas in other species individuals produce gametes that are only compatible with gametes of a different mating type and not with gametes of the same mating type (heterothallism). Homothallism is most common in the Zygomycota and Ascomycota (Lin & Heitman, 2007). Almost all heterothallic ascomycetes have a system with two different mating types, which usually implies that each individual is compatible with half the population (if both mating types occur in equal frequencies, which is the case whenever sexual reproduction occurs frequently). Many Basidiomycota (especially the mushroom-forming ones; Kües et al., 2011), some Physarum species (Collins, 1975) and at least one ascomycete Gibberella cingulata (Cisar & TeBeest, 1999) have a system with more than two, sometimes up to hundreds, different mating types (see also ‘Sexual Selection in Mushroom Fungi’ in the main text). In a population with many mating types, gametes are compatible with almost all unrelated gametes in a population.
Fungal mating types are genetically defined. At the mating-type locus, one or often multiple tightly linked genes are encoded. In ascomycetes, the genes of the two mating types are not homologous and are therefore referred to as idiomorphs (Metzenberg & Glass, 1990). Depending on the species, the genes at the mating-type loci can both regulate functions that operate during mating such as extracellular signalling (Kothe, 2008; Raudaskoski & Kothe, 2010), cell fusion (Glass et al., 2000; Fraser et al., 2007), inheritance of cytoplasmic genes (Yan & Xu, 2003) and establishment of a diploid or heterokaryotic individual (Crowe, 1963; Fraser et al., 2007) and after zygote formation, for instance in regulating cell division (Raper, 1985), sexual reproduction (Van Heeckeren et al., 1998) and virulence (Kwon-Chung et al., 1992) (for an extensive overview of the known molecular mechanisms see Heitman et al., 2007). Basidiomycetes have evolved a unique bifactorial mating-type system with two unlinked mating-type loci, which both have to be different for successful mating to occur (Raper, 1966; James et al., 2006). Different groups within the basidiomycetes have reversed to a unifactorial system, either by losing one of the mating-type loci or by recombination causing linkage between the two loci (Kües et al., 2011).
Inter- and intra-mating-type sexual selection
In sexually reproducing populations, the mating types are expected to be at equal frequencies. Using the same reasoning that predicts equal investment in the sexes (Fisher, 1958), we can predict that negative frequency–dependent selection will select for equal investment in the different mating types (May et al., 1999). However, in contrast to the different sexes, there are no inherent differences between different mating types in investment in gametes, so that an innate asymmetry as present between the sexes is unlikely. Therefore, equal investment in mating types also means equal frequencies of mating types. This is the case for species with two mating types and also for species with multiple mating types. Only when events of sexual reproduction are separated by long stages of vegetative growth or many rounds of asexual reproduction, and one mating type has a higher asexual growth rate, increased virulence or reduced mortality, systematic skews will be possible in the mating-type ratio.
In many species of basidiomycetes, more than two mating-type alleles are present (Kües et al., 2011). Due to negative frequency–dependent selection, all alleles in a well-mixed population are expected to have equal ratios (1/n for n alleles), because more common alleles have a reduced population level compatibility. If mating occurs randomly, each mating-type allele will then obtain 1/n of the matings. A mating-type allele that is more successful in conquering gametes than other mating types can increase in the population to a higher frequency than 1/n. Competition in this situation will be between the different mating types. Even though this is a clear example of sexual selection, because fitness is increased solely by increasing the number of mates obtained, this case does not comply to the definition by Jennions & Kokko (2010) as given in the main text. According to the general definition, sexual selection occurs among the members within one class (i.e. within one sex or one mating type), but in this special form of sexual selection, it occurs between classes at the level of the whole population. An example of this specific case occurs in basidiomycetes as discussed in ‘Sexual Selection in Mushroom Fungi’ in the main text.