Files
Abstract
Despite their importance in speciation, major questions involving postzygotic barriers still remain, such as why they evolve and how much they contribute to species divergence. Here, we investigate the genetic mechanisms and evolutionary drivers of postzygotic isolation among a group of closely related taxa in the genus Mimulus. In the M. tilingii complex, the three species are largely allopatric and grow only at high elevations. In Chapter II, we determined that these three species are morphologically and genetically distinct, and diverged ~400kya. Additionally, each species pair in the M. tilingii complex is nearly completely reproductively isolated by many postzygotic barriers, including F1 hybrid seed inviability, F1 hybrid necrosis, and F1 male and female hybrid sterility. In Chapter III, we investigated the developmental, genetic, and evolutionary processes that contribute to the first-acting postzygotic barrier: F1 hybrid seed lethality. We performed a detailed developmental assessment of hybrid seed inviability between species in the M. tilingii complex, and a more distant relative, M. guttatus. We determined that between any species pair, hybrid seed development was disrupted in regions of the seed that potentially control nutrient acquisition. This finding provides empirical evidence for the classic theory that parental conflict over resource allocation targets nutrient acquiring tissues within the seed. Finally, in Chapter IV, we investigated the genetic basis of severe F1 hybrid sterility between species in the M. tilingii complex and discovered that this postzygotic barrier is largely explained by underdominant chromosomal rearrangements. This finding might suggest a role for genetic drift in fixing such underdominant rearrangements. Alternatively, if maladaptive gene flow has occurred between these species, these costly rearrangements might have evolved via strong selection for suppressed recombination among locally adapted loci. Together, our findings provide context for how strong postzygotic barriers may evolve and contribute to speciation.