DNAs of the two mating-type alleles of Neurospora crassa are highly dissimilar

The mating-type alleles A and a of Neurospora crassa control mating in the sexual cycle and function in establishing heterokaryon incompatibility in the vegetative cycle. The A and a alleles were cloned, and they were shown to encode both the sexual functions and vegetative incompatibility. The mating-type clones contain nonhomologous DNA segments that are flanked by common DNA sequences. Neurospora crassa and all heterothallic and pseudohomothallic Neurospora species contain a single copy of one mating-type sequence or the other within each haploid genome. The six known self-fertile homothallic isolates contain an A homolog, but only one species also contains a homologous sequences. Homothallism in these species is not due to mating-type switching, as it is in Saccharomyces cerevisiae.