Multiple versions of the A mating type locus of Coprinus cinereus are generated by three paralogous pairs of multiallelic homeobox genes.

The A mating type locus of Coprinus cinereus determines mating compatibility by regulating essential steps in sexual development. Each A locus contains several genes separated into two functionally independent complexes termed A alpha and A beta, and the multiple alleles of these genes generate an estimated 160 A mating specificities. The genes encode two classes of homeodomain-containing proteins designated HD1 and HD2. In this report we describe two newly cloned loci, A2 and A5, and compare them with A42, A43 and A6 that we have described previously. An A beta-null locus, retaining just a single active HD1 gene from the alpha-complex, was generated by mutation. Using this as a transformation host, gene combinations that promote A-regulated development were identified. We demonstrate that each A locus contains members of three paralogous pairs of HD1 and HD2 genes. Different allelic versions of gene pairs are compatible but paralogous genes are incompatible. The genes present in four uncloned A loci were deduced using Southern analyses and transformations with available cloned genes. The combined analysis of nine A factors identifies sufficient A gene alleles to generate at least 72 A mating specificities.