Crossing data and genomic data for: 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 of sympatric
species 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 possible 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 producing foundational
information about reproductive isolation and genomic divergence in this
understudied group, we add new diversity and phylogenetic resolution to
our understanding of the mechanisms of plant speciation.