The signature of balancing selection: Fungal mating compatibility gene evolution

A key problem in evolutionary biology has been distinguishing the contribut ions of current and historical professes to the maintenance of genetic vari ation. Because alleles at self-recognition genes are under balancing select ion, they exhibit extended residence times in populations and thus may prov ide unique insight into population demographic history. However, evidence f or balancing selection and extended residence times has almost exclusively depended on identification of transspecific polymorphisms; polymorphisms re tained in populations through speciation events. We present a broadly appli cable approach for detecting balancing selection and apply it to the bl mat ing type gene in the mushroom fungus Coprinus cinereus, The comparison of n eutral molecular variation within and between allelic classes was used to d irectly estimate the strength of balancing selection. Different allelic cla sses are defined as encoding different mating compatibility types and are t hus potentially subject to balancing selection. Variation within an allelic class, where all alleles have the same mating compatibility type, provided an internal standard of neutral evolution, Mating compatibility in this or ganism is determined by the complex A mating type locus, and bl is one of s everal redundantly functioning genes. Consequently, we conducted numerical simulations of a model with two subloci and varying levels of recombination to show that balancing selection should operate at each sublocus, Empirica l data show that strong balancing selection has indeed occurred at the bl l ocus, The widespread geographic distribution of identical b1 alleles sugges ts that their association with differing A mating types is the result of re cent recombination events.