Seed sizes for mixed pollen and pure-species pollinations

Hybrid Seed Inviability (HSI) is a common barrier in angiosperms. Recent work suggests that the rapid evolution of HSI may, in part, be due to conflict between maternal and paternal optima for resource allocation to developing offspring (i.e. parental conflict). However, parental conflict requires that paternally-derived resource acquiring alleles impose a maternal cost. I test this requirement using three closely related species in the Mimulus guttatus species complex that exhibit significant HSI and differ in their inferred histories of parental conflict. I show that the presence of hybrid seeds significantly affects conspecific seed size for almost all crosses, such that conspecific seeds are smaller after developing with hybrids from fathers with a stronger history of conflict, and larger after developing with hybrids from fathers with a weaker history of conflict. This work demonstrates a cost of paternally-derived alleles and also has implications for species fitness in secondary contact.

Methods
Pollinations were completed for three species of Mimulus for a total of 9 different cross types: purely self-fertilized and mixed pollinations involving a roughly equal amount of self pollen and pollen from another species. The resultant fruits were then counted and quantified for: the number of seeds produced and their viability. Based on diagnostic seed characteristics, seeds were categorized as hybrid or conspecific. All conspecific seeds were measured for total area using ImageJ.