On the causes of geographically heterogeneous parallel evolution in sticklebacks

The three-spined stickleback (Gasterosteus aculeatus) is an important model system for the study of parallel evolution in the wild, having repeatedly colonized and adapted to freshwater from the sea throughout the northern hemisphere. Previous studies identified numerous genomic regions showing consistent genetic differentiation between freshwater and marine ecotypes but these had typically limited geographic sampling and mostly focused on the Eastern Pacific region. We analysed population genomic data from global samples of the three-spined stickleback marine and freshwater ecotypes to detect loci involved in parallel evolution at different geographic scales. Most signatures of parallel evolution were unique to the Eastern Pacific and trans-oceanic marine-freshwater differentiation was restricted to a limited number of shared genomic regions, including three chromosomal inversions. On the basis of simulations and empirical data, we demonstrate that this could result from the stochastic loss of freshwater-adapted alleles during the invasion of the Atlantic basin and selection against freshwater-adapted variants in the sea, both of which can reduce standing genetic variation available for freshwater adaptation outside the Eastern Pacific region. Moreover, the elevated linkage disequilibrium associated with marine-freshwater differentiation in the Eastern Pacific is consistent with secondary contact between marine and freshwater populations that evolved in isolation from each other during past glacial periods. Thus, contrary to what earlier studies from the Eastern Pacific region have led us to believe, parallel marine-freshwater differentiation in sticklebacks is far less prevalent and pronounced in all other parts of the species global distribution range.