Developmental evidence for parental conflict in driving Mimulus species barriers

The endosperm, a tissue that nourishes the embryo in the seeds of flowering plants, is often disrupted in inviable hybrid seeds between closely related species. A key question is whether parental conflict is a major driver of this common form of reproductive isolation.  Here, we performed reciprocal crosses between pairs of three monkeyflower species (Mimulus caespitosa, M. tilingii, and M. guttatus). The severity of hybrid seed inviability varies among these crosses, which we inferred to be due to species divergence in effective ploidy. By performing a detailed time series of seed development, we discovered parent-of-origin phenotypes that provide strong evidence for parental conflict in shaping endosperm evolution. We found that the chalazal haustorium, a tissue within the endosperm that occurs at the maternal-filial boundary, shows pronounced differences between reciprocal hybrid seeds formed from Mimulus species that differ in effective ploidy. These parent-of-origin effects suggest...