Abstracts (first author)
Selection on flower colour in a hybrid zone reveals interwoven patterns of genomic divergence at a linked cluster of loci
To understand how selection establishes reproductive barriers between populations, we need to identify the loci regulating the selected traits. Knowing that, how are sets of co-adapted alleles maintained when gene-flow occurs between distinct populations? One view is that genetic linkage maintains fit haplotypes, resulting in a genomic signature of selection as an “island of divergence”. To address these issues we study a hybrid zone between two Antirrhinum species with different flower colours. Various phenotypes result from this hybridization, but are restricted to a narrow region (~1km) and do not spread to the parental populations. A gene regulating magenta flower pigmentation, ROS, was found to have a steep cline correlated with a flower colour cline, suggesting this trait is under selection. A second tightly-linked locus also regulating this trait, EL, has now been mapped. A genomic comparison of diversity between the Antirrhinum species shows a signature of selection in the region containing both loci (ca. 180kb~0.9cM). Surprisingly, this “island of divergence” includes non-shared and shared polymorphisms, questioning how the parental ROS-EL haplotypes are maintained in face of gene-flow. By genotyping plants across the hybrid zone we show that even though LD between them is high (r~0.9), recombinants do occur (~12% in the zone). This suggests that gene-flow between the two populations was enough to break fixed differences within the “island”, resulting in a rugged divergence profile that includes fixed polymorphisms (linked to the functional loci) interspersed with shared ones. To our knowledge this is one of a few cases where a cluster of loci under selection has been finely mapped and its behaviour studied at a population-level. This analysis empirically demonstrates the significant impact gene-flow has in breaking associations of clusters of putatively co-adapted alleles, providing key insights to our interpretation of genomic signatures of selection.