Loss of Self‐Incompatibility by Mating‐Type Switching

Abstract

Sexual reproduction requires the fusion of compatible gametes. In many organisms without sexes, compatibility is described by the genes located at the mating‐types locus, and mating is restricted to haploid individuals carrying different alleles at this locus. To assure mating when no partners are around, a variety of mechanisms evolved that render two clonally derived cells compatible with each other, known as homothallism. This occurs mostly by incorporation of the two mating‐type alleles into the same haploid genome; however, this comes at a cost. Mating‐type switching reduces these costs by suppressing one of the mating‐type alleles. Clonal compatibility is maintained by structurally modifying the gene content or mating‐type conformation that defines the mating‐type identity of the cell. The presumed evolutionary steps from a self‐incompatibility to switching require incorporation of both mating‐type genes, silencing of one type, evolving a switching mechanism and optimising this mechanism. Key Concepts Mating types define compatibility between haploid individuals and are generally described by a single locus. In most species, only two mating types exist that strongly reduce the availability of compatible mates. Incorporation of the genes of both mating types (the mating‐type cassettes) into the same haploid genome can lead to self‐compatibility but comes at a large cost. Under low densities, the benefit of self‐compatibility can overcome the cost of carrying both mating‐type cassettes. Selection for suppression of one of the cassettes can restore a mostly outcrossing (heterothallic) phenotype, while probably retaining occasional selfing. After the evolution of silencing, switching during asexual growth is an efficient way of locally generating compatible genotypes. Suppression by dominance of mating type is likely to lead to switching by disruption or deletion. Suppression is determined by the location of a locus and likely selects for evolution of positional switching.