Experimental evolution of mating discrimination in budding yeast.

Assortative mating, when individuals of similar phenotypes mate, likely plays a key role in preventing gene flow during speciation. Reinforcement occurs when two previously geographically separated (allopatric) groups meet after having evolved partial postzygotic isolation; they are selected to evolve or enhance assortative mating to prevent costly intergroup matings that produce only maladaptive or sterile hybrids. Studies in Drosophila have shown that the genetic architectures of mating discrimination could differ significantly with or without reinforcement, suggesting that the evolution of assortative mating may be more complicated than expected. To study the evolution of assortative mating, we evolved mating discrimination in populations of the budding yeast, Saccharomyces cerevisiae. After 36 cycles of selection, these cells are five times more likely to mate with each other than to their ancestors, despite detectable one-way gene flow between the selected and reference populations. Several individual cultures evolved mating discrimination by changing their mating kinetics, with some mating more rapidly and others more slowly than the ancestral population. Genetic analysis indicates that multiple mutations have accumulated to produce the altered mating preference. Our results show that subtle details of mating behavior can play an important role in the evolution of reproductive isolation.