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 to "sexes" or differences in size between gametes.  Syngamy can only take place between gametes carrying different mating types.