29 Oral - Platform Session #3 Speciation, Hybridization, and Introgression
Thursday June 09, 9:30 AM - 9:45 AM

Authors:
Shelley Sianta; David Moeller; Yaniv Brandvain

Affiliation: University of Minnesota

Keywords:
Speciation & hybridization

Secondary contact of taxa that have diverged in allopatry has historically been touted as the final stage of speciation, deciding the fate of incipient species. Ongoing work in the field of speciation genomics demonstrates that introgression upon secondary contact is common. However, it remains unclear whether differences in ecological, reproductive and genomic features of hybridizing taxa have repeatable effects on introgression. For example, shifts in mating system, from predominantly outcrossing to predominantly self-fertilizing, are predicted to affect both the magnitude and asymmetry of introgression. The few studies that have documented introgression between a selfer and an outcrosser have found asymmetric patterns, with more introgression occurring from the selfer to the outcrosser. Many of these studies, however, involve either sampling few sympatric populations and/or few individuals, making it unclear whether this pattern is generalizable. Moreover, it is unclear what mechanisms may govern this pattern. Here, we study two subspecies of Clarkia xantiana that have diverged in mating system in allopatry and that have recently come into secondary contact. Along their narrow zone of contact, we employ a high-density sampling scheme across four contact zone sites (n = 442 individuals). We test for the presence of introgression, asymmetry in the magnitude of introgression, and the extent of independence of introgression across the contact zone sites. Despite finding no evidence for early generation hybrids, we find introgression in all contact zone sites and a signal for independence of introgression among contact zone sites. While we detect introgression in both subspecies within each contact zone site, there is strong asymmetry in introgression, with more introgression from the selfer to the outcrosser. Differences in track lengths of introgressed DNA suggest that introgression from the outcrosser to the selfer is either more recent and/or selection against introgressed DNA is more efficient because of reduced effective recombination in the selfer. Lastly, we test the hypothesis that differences in the architecture of genetic load between the outcrosser and selfer contribute to patterns of introgression within each subspecies. This study highlights the predictability of mating system shifts on patterns of introgression upon secondary contact.