The evolution of multiple, independent and multispecific mating-type loci is a feature unique to homobasidiomycete fungi. To propose a model of evolution, data assembled for the wood-rotting fungus Schizophyllum commune were analyzed. In one mating-type locus, pheromone receptors and several pheromones are encoded which have been investigated in some detail and can be used to understand the ligand-receptor interactions and activation of signal transduction which are essential to sexual propagation. Previous models for the evolution of new alleles were complicated and involved three subsequent steps (without selectable phenotype) prior to the establishment of a new stable pheromone-receptor pair. This paper presents a model for the evolution of new specificities by recombination and selection that incorporates the multi-state receptor activation recently established for S. commune, explaining differential responses to different pheromones in one receptor molecule. The model takes into account the occurrence of multiple pheromone genes in each locus and unilateral nuclear donor/acceptor strains that may in nature act as steps in the evolution of new specificities. A second homobasidiomycete fungus, Coprinus cinereus, was similarly characterized at the molecular level. Data acquired in this system support the conclusion that the presented model can be generalized.