Genetic population structure of European Eel: Panmixia and Its Consequences

European (Anguilla anguilla) and American (A. rostrata) eel spawn in a widely overlapping region of the southern Sargasso Sea. Their offspring are advected by ocean currents towards continental coasts and freshwater bodies, and in the case of European eel both juvenile and adult spawning migrations encompass more than 5,000 kilometers. The genetic population structure of European eel and the evolution of both species represent a classical biological mystery.  Despite being distributed from northern Europe to northern Africa, some studies have suggested that European eel is panmictic, whereas others have suggested weak but statistically significant genetic differentiation representing either isolation-by-distance or isolation-by-time. However, all studies have been based on continental samples rather than samples from the Sargasso Sea. Recently, we studied microsatellite DNA markers in eel larvae collected at four transects in the spawning region. The results provided no evidence for genetic differentiation and absence of both isolation-by-distance and isolation-by-time. Similar results were obtained for continental samples of juvenile eels, thus providing strong evidence that the species is indeed panmictic. This raises the question how eels can cope with environments ranging from subarctic to subtropical climates. Does the species show extreme phenotypic plasticity or does local selection take place in different environments that is subsequently obliterated in the genetic ”melting-pot” of the Sargasso Sea? We are currently addressing this issue using a Restriction site Associated DNA (RAD) based genome scan of eel larvae, juvenile and adult eels from different environments and will present preliminary results. Finally, the time and mechanisms of speciation of European and American eel have been debated, with hypotheses ranging from sympatric speciation mediated by continental drift to allopatric speciation mediated by population declines and range contractions during recent glaciations. By sequencing mitogenomes from individuals of both species we aim to date speciation and unravel the associated mechanisms, and we will present preliminary results.