Changes in mate recognition through alterations of pheromones and receptors in the multisexual mushroom fungus Schizophyllum commune

Schizophyllum commun, has thousands of mating types defined in part by nume rous lipopeptide pheromones and their G-protein-coupled receptors. These mo lecules are encoded within multiple versions of two redundantly functioning B mating-type loci, B alpha and B beta. Compatible combinations of pheromo nes and receptors, produced by individuals of different B mating types, tri gger a pathway of fertilization required for sexual development. Analysis o f the B beta2 mating type locus revealed a large cluster of genes encoding a single pheromone receptor and eight different pheromones. Phenotypic effe cts of mutations within these genes indicated that small changes in both ty pes of molecules could significantly alter their specificity of interaction . For example, a conservative amino acid substitution in a pheromone result ed in a gain of function toward one receptor and a loss of function with an other. A two-amino-acid deletion from a receptor precluded the Mutant phero mone from activating the mutant receptor, yet this receptor was activated b y other pheromones. Sequence comparisons provided clues toward understandin g how so man), variants of these multigenic loci could have evolved through duplication and mutational divergence. A tree-step model for the origin of new variants comparable to those found in nature is presented.