Evolution of the mating type gene pair and multiple sexes in .

The multiple mating type system of the Ciliate is a self/non-self recognition system, whose specificity resides in a head-to-head, functionally distinct pair of genes, . We have now sequenced and analyzed these mating type genes in nine additional species. We conclude that and are derived from a common ancestral gene and have co-evolved for at least ∼150 Myr. We show that , a perpetual selfer (unisexual) species, has a single mating type gene pair, whose and genes likely have different mating type specificity. We document the recent replacement of a complete different set of mating type specificities for another, illustrating how quickly this can happen. We discuss how varying conditions of reproductive stress could result in evolutionary co-adaptations of and genes and changes in mating type determination mechanisms.