Fungal sexual reproduction and mating-type loci

Sexual reproduction is a hallmark of eukaryotes, generating diversity and variation through recombination and allele segregation, thereby facilitating natural selection. Unlike animals and plants, fungi do not have conventional male and female sexes but instead rely on mating types, which are determined by allelic differences at a specialized chromosomal region called the mating-type locus (MAT). The MAT locus typically exhibits great diversity in both organization and gene content, partially driven by reduced meiotic recombination in these regions during sexual reproduction. While ascomycetes predominantly have a bipolar mating system with a single MAT locus determining the mating type of the cell, there are also species that possess silent MAT cassettes that enable mating-type switching during vegetative growth. This process generates cells with compatible mating types, allowing mating between mother and daughter cells and resulting in inbreeding. In basidiomycetes, the ancestral mating system is tetrapolar, with two independent MAT loci (P/R and HD) collectively determining the mating type of the cell. Some species, however, have a bipolar mating system in which the P/R and HD loci have become completely linked genetically or even fused together, while others exhibit a so-called pseudobipolar mating system in which only partial genetic linkage has been established between the P/R and HD loci. Additionally, fungi employ a vast array of sexual reproductive strategies, including classical mating between cells of opposite mating types, as well as noncanonical modes such as unisexual, pseudosexual, and parasexual reproduction. In this Primer, we aim to introduce this fascinating diversity in mating-type determination and modes of sexual reproduction in fungi, with a focus on Cryptococcus species as a model. We then discuss how recent advances in genomics research have facilitated studies on fungal MAT loci and mating systems, highlighting key outstanding questions in the field and potential ways to address them.