Rapid genetic divergence in postglacial populations of threespine stickleback (Gasterosteus aculeatus): the role of habitat type, drainage and geographical proximity

The goal of this study was to evaluate the role of common ancestry, and of geographical or reproductive isolation, in genetic divergence in population s of threespine sticklebacks (Gasterosteus aculeatus). Using seven DNA micr osatellite loci we compared the effects of habitat type, drainage system an d geographical proximity on genetic distance among 16 populations situated in an area in Schleswig-Holstein (Germany) that became deglaciated approxim ate to 12 000 years ago. Stickleback population structure correlated only w eakly with drainage system, whereas the primary divergence was among habita t types. Phylogenetic analysis revealed that lake (n = 7) and river (n = 5) populations formed two distinct clades (Cavalli-Sforza's and Edwards' chor d distance, 82-100% bootstrap support) at approximately equal genetic dista nces to a third clade, comprising putative estuarine (n = 4) ancestors. All ele frequencies in lake and river populations represented different subsets of the genetically more diverse estuarine populations. In nested AMOVAS ap proximately twice the genetic variance was distributed among lake vs. river vs. estuarine populations as compared with the combined effects of drainag e system and geographical distance. Limited gene flow between habitat types must have been established after postglacial colonization, suggesting ecol ogical hybrid inferiority or behavioural mating barriers between ecotypes. Within estuarine and lake populations, population differentiation followed an isolation-by-distance model. Given the high observed heterozygosities wi thin the 16 study populations (H-O = 0.65-0.87), the mean divergence betwee n lake and river population pairs (F-ST = 0.18 +/- 0.007) would be reached after 300-6000 generations in a stepwise mutation model, depending on the s ize of N-e. This demonstrates both the utility of hypervariable microsatell ites for detecting recent population divergences and the danger of operatin g at temporal or spatial scales which are beyond their resolution.