Evolution of multiple postzygotic reproductive barriers between species in the Mimulus tilingii complex

We highlight the potential for multiple strong postzygotic barriers to evolve among closely related species. Three putative species in the Mimulus tilingii complex (M. caespitosa, M. minor, and M. tilingii) are largely allopatric and restricted to high elevations, but until now, the evolutionary relationships among these species have not been not well understood. In this study, we show that the species in the M. tilingii complex are morphologically and genetically distinct, yet closely related (divergence time ~377ky). Through a comprehensive crossing experiment, we also find that species in the M. tilingii complex are strongly reproductively isolated from one another by several F1 postzygotic barriers, including hybrid seed inviability, hybrid necrosis, and F1 male and female sterility. We suggest that strict allopatry among these montane species within the M. tilingii complex might have facilitated the evolution of this strikingly high number of hybrid incompatibilities.