Patterns of evolutionary divergence and the genetics of hybrid sterility between two species of Mimulus

In this study, I investigated the genetics and dynamics of species divergence between two closely related species of monkeyflower, Mimulus nasutus and M. guttatus. In the first part, I examined DNA sequence variation at two nuclear loci to characterize the divergence history of the two species. High sequence similarity among populations of the selfing M. nasutus points to a single evolutionary origin. Consistent with their distinct mating systems, all genetic variation in M. nasutus is distributed among populations, whereas M. guttatus exhibits appreciable levels of nucleotide diversity within populations. The finding that several M. guttatus sequences share complete identity with sequences from M. nasutus suggests that recent introgression may have occurred. I argue that exceptionally high nucleotide diversity in M. guttatus is consistent with a long-term history of directional introgression from M. nasutus to M. guttatus throughout the divergence of these two species.;In the second and third parts of this study, I investigated the genetics of hybrid sterility between M. guttatus and M. nasutus. In an interspecific cross between two inbred lines, I showed that nearly complete hybrid male sterility is caused by a simple Dobzhansky-Muller incompatibility. I genetically mapped this sterility effect: the M. guttatus allele at the hybrid male sterility 1 (hms1) locus acts dominantly in combination with recessive M. nasutus alleles at the hybrid male sterility 2 (hms2) locus to cause hybrid male sterility. In addition, hms1 and hms2 cause a significant reduction in hybrid female fertility, suggesting that sex-specific hybrid defects might share a common genetic basis. I also characterized intraspecific variation for the hms1-hms2 incompatibility in natural populations of M. nasutus and M. guttatus. My results suggest that the hms2 incompatibility allele is geographically widespread within M. nasutus. In contrast, M. guttatus is polymorphic for the hms1 locus and the incompatibility allele appears to be extremely geographically restricted. I found evidence for the presence of the hms1 incompatibility allele in only two M. guttatus populations. Sampling extensively within one of these M. guttatus populations demonstrated that the hms1 locus is polymorphic and that the incompatibility allele appears to segregate at an intermediate frequency.