Cline coupling and uncoupling in a stickleback hybrid zone

Abstract

Strong ecological selection on a genetic locus can maintain allele frequency differences between populations in different environments, even in the face of hybridization. When alleles at divergent loci come into tight linkage disequilibria, selection acts on them as a unit and can significantly reduce gene flow. For populations interbreeding across a hybrid zone, linkage disequilibria between loci can force clines to share the same slopes and centers. However, strong ecological selection can push clines away from the others, reducing linkage disequilibria and weakening the barrier to gene flow. We looked for this cline uncoupling effect in a hybrid zone between stream resident and anadromous sticklebacks at two genes known to be under divergent natural selection (Eda and ATP1a1) and five morphological traits that repeatedly evolve in freshwater stickleback. We used 10 anonymous SNPs to characterize the shape of the zone. We found that the clines at Eda, ATP1a1, and four morphological traits were concordant and coincident, suggesting that direct selection on each is outweighed by the indirect selection generated by linkage disequilibria. Interestingly, the cline for pectoral fin length was much steeper and displaced 200m downstream, and two anonymous SNPs also had steep clines.