Developmental evidence for parental conflict in driving Mimulus species barriers

Abstract

The endosperm, a tissue that nourishes the embryo in the seeds of flowering plants, is often disrupted in inviable hybrid seeds between species presumed to have divergent histories of parental conflict. Despite the potential importance of parental conflict in plant speciation, we lack direct evidence of its action in driving species barriers. 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 determined was due to species divergence in effective ploidy. By performing a time series of seed development, we assessed whether regions within the endosperm were potential targets of parental conflict. We found that the chalazal haustorium, a tissue within the endosperm that occurs at the maternal-filial boundary, develops abnormally in hybrid seeds when the paternal parent has the greater effective ploidy. Within these Mimulus species, parental conflict might target the chalazal haustorium to control sucrose movement from the maternal parent into the endosperm. Consequently, conflict may be exposed in crosses between species. Our study suggests that parental conflict in the endosperm may function as a driver of speciation by targeting regions and developmental stages critical for resource allocation.