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Mating types are molecular mechanisms that regulate compatibility in sexually reproducing eukaryotes. They occur in isogamous and anisogamous species. Depending on the group, different mating types are often referred to by numbers, letters, or simply "+" and "-" instead of "male" and "female", that refer "sexes" or differences in size between gametes. Syngamy can only take place between gametes carrying different mating types.
Reproduction regulated by mating types is especially prevalent in fungi. Ascomycetes usually have two mating types referred to as "Mat 1-1" and "Mat 1-2" or in the case of yeast as "a" and "α" (alpha). 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 that regulate both the early and late stages of the sexual cycle. Heterothallic ascomycetes produce gametes that 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 fusion 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 on the other hand can have thousands of different mating types.
See also 
- C.J. Alexopolous, Charles W. Mims, M. Blackwell, Introductory Mycology, 4th ed. (John Wiley and Sons, Hoboken NJ, 2004) ISBN 0-471-52229-5