Title: Ecological factors affect pollen-pistil compatibility and hybridization dynamics
Description: Individuals are classified as different species if they are reproductively isolated from one another; however, the degree of reproductive isolation is not a stable quantity between species but can exhibit considerable plasticity in response to diverse environmental factors. In plants, a key stage of reproductive isolation occurs after pollination but before ovule fertilization, and it is regulated by interactions between pollen and pistils. Both the growth environment and the pollen competitive environment have documented effects on pollen-pistil compatibility within species, but it is not known how they influence reproductive compatibility between species. I tested whether ecological factors that influence mating patterns within species also influence hybridization dynamics between species. The degree of reproductive isolation between Phlox cuspidata and P. drummondii was contingent on the abiotic environment and the pollen competitive environment. Heterospecific pollen was more successful when mothers and fathers were grown in different soil Ca environments. These results suggest that when species predictably inhabit different environments, environmental heterogeneity can decrease reproductive isolation by enhancing the performance of pollen dispersed across species and environments. Selfing as opposed to outcrossing increased hybridization in both self-compatible P. cuspidata and self-incompatible P. drummondii . The timing of conspecific versus heterospecific pollen deposition also affected hybridization frequency, such that early self-pollination decreased hybridization in P. cuspidata and a delay in the deposition of compatible conspecific pollen increased hybridization in P. drummondii.

Other mechanisms that affect nonrandom mating within species may also affect reproductive isolation between species. A literature review identified style length as a predictor of nonrandom mating within species. Species with longer styles exhibited more equal paternity across pollen donors. Given its effect on mating patterns within species, style length may also influence the dynamics of reproductive isolation between species. In summary, reproductive isolation is contingent on ecological factors that vary within the ranges of species. Moreover, mechanisms that determine the degree of reproductive isolation between species appear to be present within species prior to secondary contact.