Introgression and Parental Conflict Underlie Repeated Occurrences of Postzygotic Isolation

Postzygotic reproductive isolation is often thought of as a byproduct of neutral divergence, but it can also evolve rapidly and dynamically. One major contributor to the evolution of intrinsic postzygotic reproductive isolation is intragenomic conflict, such as conflict between maternal and paternal interests in resource allocation to developing offspring (i.e., parental conflict). One potential reproductive barrier caused by parental conflict is early onset hybrid inviability in developing seeds. Here we show that hybrid seed inviability is segregating within the Mimulus guttatus species complex. Using crosses between 25 populations of Mimulus guttatus , we show that patterns of hybrid seed inviability follow a geographic rather than phylogenetic pattern. We use these crosses, as well as germination assays and embryo rescue experiments, to show that these results are consistent with the parental conflict theory. Using genome sequences from across the complex, we find that high-conflict lineages are not monophyletic, and the repeated occurrence of hybrid seed inviability is, in at least one case, driven by introgression. Our study supports the idea that parental conflict and introgression of selfish elements can contribute to the rapid, repeated, and strong evolution of intrinsic postzygotic reproductive isolation.