heterothallism

Heterothallism is a mating system found in some fungi and algae where an individual thallus is unable to produce viable offspring through self-fertilization and requires mating with a compatible thallus of the opposite mating type. This contrasts with homothallism, where an individual thallus can undergo self-fertilization and produce viable offspring. The genetic basis for heterothallism typically involves one or more genes that control mating type. In many fungi, these are referred to as mating-type loci (MAT loci).

There are two primary forms of heterothallism: primary and secondary. Primary heterothallism, also known as unisexual heterothallism, is controlled by a single mating-type locus with multiple alleles, typically designated as 'a' and 'alpha' in ascomycetes. For successful reproduction, an 'a' thallus must mate with an 'alpha' thallus. Secondary heterothallism, or bipolar heterothallism, is controlled by two unlinked loci, each with multiple alleles. For example, a thallus might have mating type A1 and B1, and it can only mate with a thallus having A2 and B2, or A1 and B2, or A2 and B1, depending on the specific genetic system.

The evolutionary advantage of heterothallism is thought to lie in promoting outcrossing and increasing genetic diversity within a population. This genetic variation can be beneficial for adaptation to changing environments and for the development of resistance to pathogens. While heterothallism necessitates finding a compatible partner, it reduces the risks associated with inbreeding, such as the accumulation of deleterious mutations.