The sixth transmembrane region of a pheromone G-protein coupled receptor, Map3, is implicated in discrimination of closely related pheromones in Schizosaccharomyces pombe

Pheromone Response and Mating Behavior in Fission Yeast

Most ascomycete fungi, including the fission yeast Schizosaccharomyces pombe, secrete two peptidyl mating pheromones: C-terminally modified and unmodified peptides. S. pombe has two mating types, plus and minus, which secrete two different pheromones, P-factor (unmodified) and M-factor (modified), respectively. These pheromones are specifically recognized by receptors on the cell surface of cells of opposite mating types, which trigger a pheromone response. Recognition between pheromones and their corresponding receptors is important for mate discrimination; therefore, genetic changes in pheromone or receptor genes affect mate recognition and cause reproductive isolation that limits gene flow between populations. Such genetic variation in recognition via the pheromone/receptor system may drive speciation. Our recent studies reported that two pheromone receptors in S. pombe might have different stringencies in pheromone recognition. In this review, we focus on the molecular mechanism of pheromone response and mating behavior, emphasizing pheromone diversification and its impact on reproductive isolation in S. pombe and closely related fission yeast species. We speculate that the "asymmetric" system might allow flexible adaptation to pheromone mutational changes while maintaining stringent recognition of mating partners. The loss of pheromone activity results in the extinction of an organism's lineage. Therefore, genetic changes in pheromones and their receptors may occur gradually and/or coincidently before speciation. Our findings suggest that the M-factor plays an important role in partner discrimination, whereas P-factor communication allows flexible adaptation to create variations in S. pombe. Our inferences provide new insights into the evolutionary mechanisms underlying pheromone diversification.

Evolution and molecular bases of reproductive isolation

The most challenging problem in speciation research is disentangling the relative strength and order in which different reproductive barriers evolve. Here, we review recent developments in the study of reproductive isolation in yeasts. With over a thousand genome-sequenced isolates readily available for testing the viability, sterility, and fitness of both intraspecies and interspecies hybrid crosses, Saccharomyces yeasts are an ideal model to study such fundamental questions. Our survey demonstrates that, while chromosomal-level mutations are widespread at the intraspecific level, anti-recombination-driven chromosome missegregation is the primary reproductive barrier between species. Finally, despite their strength, all of these postzygotic barriers can be resolved through the asexual life history of hybrids.