Mating types are the microorganism equivalent to sex in higher organisms[1] and occur in isogamous species.
Among fungi, mating type is determined by chromosomal regions called mating-type loci.
As an illustration, the model organism Coprinus cinereus has two mating-type loci called A and B.
At the A locus are 6 homeodomain proteins arranged in 3 groups of 2 (HD1 and HD2), which arose by gene duplication.
Functional heterodimers are necessary for a dikaryon-specific transcription factor, and its lack arrests the development process.
Filamentous ascomycetes usually have two mating types referred to as "MAT1-1" and "MAT1-2", following the yeast mating-type locus (MAT).
[8] Under standard nomenclature, MAT1-1 (which may informally be called MAT1) encodes for a regulatory protein with an alpha box motif, while MAT1-2 (informally called MAT2) encodes for a protein with a high motility-group (HMG) DNA-binding motif, as in the yeast mating type MATα1.
[9] The corresponding mating types in yeast, a non-filamentous ascomycete, are referred to as MATa and MATα.
[10] Mating type genes in ascomycetes are called idiomorphs rather than alleles due to the uncertainty of the origin by common descent.
The proteins they encode are transcription factors which regulate both the early and late stages of the sexual cycle.
Outcrossing, through complementation, could provide the benefit of masking recessive deleterious mutations in genes which function in the dikaryon and/or diploid stage of the life cycle.
[15] Studies on green algae have provided evidence for the evolutionary link between sexes and mating types.
[19]: 75 As of 2019, genomic conflict has been considered the leading explanation for the evolution of two mating types.
[20] Secondary mating types evolved alongside simultaneous hermaphrodites in several lineages.