Recombination and Variation at the A Mating-Type of Coprinus cinereus

The molecular basis of variability at the Coprinus cinereus A mating-type was investigated. The A mating-type controls sexual compatibility in this basidiomycete fungus and, like the S loci of many plants, displays quite high levels of variation in natural populations. The A mating-type is a complex locus made up of a number of genes arranged in gene pairs. We determined the role of recombination in generating diversity by using Southern blotting techniques to map 16 different A mating-types from locations around the world. Three variable regions in A segregate in natural populations and are in linkage equilibrium despite their close physical proximity. Recombination separating the two members of a gene pair was not observed, and these remain in linkage disequilibrium. Variation in A can be accounted for by recombination of highly variable blocks of DNA-containing gene pairs. The A matingtype provides an exceptional system for understanding the molecular evolution of variation in self-recognition genes.