Mating type

Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in multicellular organisms such as fungi.

Definition
Mating types are the microorganism equivalent to sex in higher organisms and occur in isogamous species. Depending on the group, different mating types are often referred to by numbers, letters, or simply "+" and "−" instead of "male" and "female", which refer to "sexes" or differences in size between gametes. Syngamy can only take place between gametes carrying different mating types.

Occurrence
Reproduction by mating types is especially prevalent in fungi. Filamentous ascomycetes usually have two mating types referred to as "MAT1-1" and "MAT1-2", following the yeast mating-type locus (MAT). 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. The corresponding mating types in yeast, a non-filamentous ascomycete, are referred to as MATa and MATα.

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. Heterothallic ascomycetes produce gametes, which present a single Mat idiomorph, and syngamy will only be possible between gametes carrying complementary mating types. On the other hand, homothallic ascomycetes produce gametes that can fuse with every other gamete in the population (including its own mitotic descendants) most often because each haploid contains the two alternate forms of the Mat locus in its genome.

Basidiomycetes can have thousands of different mating types.

In the ascomycete Neurospora crassa matings are restricted to interaction of strains of opposite mating type. This promotes some degree of outcrossing. 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.

Evolution
Mating types likely predate anisogamy, and sexes evolved directly from mating types or independently in some lineages.

Studies on green algae have provided evidence for the evolutionary link between sexes and mating types. Such as when in 2006 Japanese researchers found a gene in males of Pleodorina starrii that’s an [https://en.m.wiktionary.org/wiki/orthologue#:~:text=orthologue%20(plural%20orthologues),sequences%20found%20in%20different%20species. orthologue] to a gene for a mating type in the Chlamydomonas reinhardtii. In Volvocales, the plus mating type is the ancestor to female.

In ciliates multiple mating types evolved from binary mating types in several lineages. As of 2019, genomic conflict has been considered the leading explanation for the evolution of two mating types.

Secondary mating types evolved alongside simultaneous hermaphrodites in several lineages.