Strong postmating reproductive isolation in Mimulus section Eunanus

Postmating reproductive isolation can help maintain species boundaries when premating barriers to reproduction are incomplete. The strength and identity of postmating reproductive barriers are highly variable among diverging species, leading to questions about their genetic basis and evolutionary drivers. These questions have been tackled in model systems but are less often addressed with broader phylogenetic resolution. In this study we analyze patterns of genetic divergence alongside direct measures of postmating reproductive barriers in an overlooked group within the model monkeyflower genus, Mimulus. Within this Mimulus brevipes species group, we find substantial divergence among species, including a cryptic genetic lineage. However, rampant gene discordance and ancient signals of introgression suggest a complex history of divergence. In addition, we find multiple strong postmating barriers, including postmating prezygotic isolation, hybrid seed inviability, and hybrid male sterility, leading to complete or substantial postmating isolation in all species pairs. Hybrid seed inviability appears linked to differences in seed size, providing a window into the developmental mechanisms underlying this reproductive barrier. While geographic proximity and incomplete mating isolation may have allowed gene flow within this group in the distant past, strong postmating reproductive barriers today are likely to prevent any ongoing hybridization. By investigating reproductive isolation in a phylogenomic context, we highlight the benefits of understudied species groups to our understanding of speciation.