Mettler R., Schaefer H. M., Chernetsov N., Fiedler W., Hobson K. A., Ilieva M., Imhof E., Johnsen A., Renner S. C., Rolshausen G., SerranoD., Wesołowski T., Segelbacher G. 2013. Contrasting patterns of genetic differentiation among Blackcaps (Sylvia atricapilla) with divergent migratory orientations in Europe. PLOS ONE 8,11: e81365.

Migratory divides are thought to facilitate behavioral, ecological, and genetic divergence among populations with different migratory routes. It is currently contentious how much genetic divergence is needed to maintain distinct migratory behavior across migratory divides. Here, we investigate patterns of neutral genetic differentiation among blackcap (Sylvia atricapilla) populations with different migratory strategies across Europe. We compare the level of genetic divergence of populations migrating to southwestern (SW) or southeastern (SE) wintering areas with birds wintering in the UK following a novel northwesterly (NW) migration route. Whereas this northwesterly direction has only been developed recently, the migratory divide between SW and SE wintering areas can be interpreted as a result of a re-colonization process after the last glaciation. Thus we predicted greater levels of genetic differentiation among the SW/SE populations. In disagreement with this prediction, a lack of genetic differentiation was found comparing SW/SE populations, suggesting that interbreeding likely occurs among blackcaps with different migratory orientations across a large area; therefore the SW/SE migratory divide can be seen as diffuse, broad band and is, at best, a weak isolating barrier. Conversely, weak, albeit significant genetic differentiation is evident between NW and SW migrants breeding sympatrically in southern Germany, suggesting a stronger isolating mechanism may be acting in this population. Significant isolation-by-distance was found among eastern blackcap populations, but not among western populations, revealing disparate patterns of genetic divergence among blackcap populations in Europe. We discuss possible explanations responsible for shaping the genetic structure among European blackcap populations and the influence of gene flow on the persistence of divergent migratory behaviors.