Secondary contact provides the ultimate test of whether new species have, in fact, diverged enough as to maintain their distinctness. Depending on the extent of reproductive isolation that populations have evolved in allopatry, taxa that come into secondary contact can hybridize and result in potential introgression (or genetic exchange between taxa). While genomic studies have shown that introgression is common, we are trying to understand the ecological and genomic factors that contribute to the extent of introgression between two taxa, and how introgression may vary across space, between taxa, or across the genome. For example, spatial variation in environmental factors that affect reproductive barriers, such as flowering time in plants, may lead to variation in introgression across a zone of secondary contact. Likewise, differences in the reproductive biology of interacting taxa may affect asymmetry in the direction of introgression. Here, we investigate sources of variation in introgression across a zone of secondary contact between two subspecies that have diverged in their mating system. We pair whole-genome-resequencing with range-wide sampling to characterize the magnitude of introgression. We find that there is substantial variation in introgression across a zone of secondary contact – from sites with no introgression, to sites with recent introgression, and sites with more ancient introgression. We find that variance in spring precipitation strongly predicts the extent of introgression across the contact zone. We also find strong asymmetry in introgression between the subspecies, with there being more introgression from the selfer to the outcrosser than the reverse. We describe how biological and genomic consequences of mating system shifts might mediated this asymmetry. Our results suggest that both environmental and reproductive factors influence the outcome of secondary contact.
