Molecular coevolution of a sex pheromone and its receptor triggers reproductive isolation in Schizosaccharomyces pombe.

The diversification of sex pheromones is regarded as one of the causes of prezygotic isolation that results in speciation. In the fission yeast Schizosaccharomyces pombe, the molecular recognition of a peptide pheromone by its receptor plays an essential role in sexual reproduction. We considered that molecular coevolution of a peptide-mating pheromone, M factor, and its receptor, Map3, might be realized by experimentally diversifying these proteins. Here, we report the successful creation of novel mating-type pairs by searching for map3 suppressor mutations that rescued the sterility of M-factor mutants that were previously isolated. Several strong suppressors were found to also recognize WT M factor. The substituted residues of these Map3 suppressors were mapped to F204, F214, and E249, which are likely to be critical residues for M-factor recognition. These critical residues were systematically substituted with each of the other amino acids by in vitro mutagenesis. Ultimately, we successfully obtained three novel mating-type pairs constituting reproductive groups. These novel mating-type pairs could not conjugate with WT maters. Furthermore, no flow of chromosomally integrated drug-resistance genes occurred between the novel and the WT mating pairs, showing that each experimentally created reproductive group [e.g., M factor(V5H) and Map3(F214H)] was isolated from the WT group. In conclusion, we have succeeded in creating an artificial reproductive group that is isolated from the WT group. In keeping with the biological concept of species, the artificial reproductive group is a new species.