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 that the chalazal
haustorium might act as a mediator of parental conflict, potentially by
controlling sucrose movement from the maternal parent into the endosperm.
 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 and thus proper seed development.