Prezygotic reproductive isolation between Mimulus guttatus and Mimulus glaucescens

Reproductive isolation is critical to speciation and therefore biodiversity. The immediate goal of such studies is to identify which barriers contribute most to overall reproductive isolation between species, in order to gain a broader understanding of the process of speciation. Mimulus guttatus and M. glaucescens have postzygotic barriers to hybridization, but their prezygotic barriers remain unstudied. I examined potential premating prezygotic barriers such as phenology, pollinator behavior and floral morphology, as well as possible post-pollination prezygotic barriers such as pollen adhesion, germination and pollen tube growth rates between species. We found no significant barriers to hybridization between species for any traits, although we did identify subtle differences in herkogamy and corolla tube width. We conclude that prezygotic barriers to reproduction between M. guttatus and M. glaucescens are weak, and contrast with evidence for postzygotic barriers such as reduced seed set and delayed flowering in hybrids. Additional research should focus on microhabitat preferences and a population genetic analysis to determine whether introgression can occur between natural populations.