Study of genetic gradients among populations of Atlantic anchovy (Engraulis encrasicolus L.) located along marine ecotones

摘要

The Quaternary climate oscillations had a major role in shaping the geneticarchitecture of living species. In the marine realm, the apparent lack of physicalbarriers to dispersal allows organisms to track optimum physiological conditions bydisplacing their distribution ranges. The European anchovy Engraulis encrasicolus isa small pelagic fish that has a broad distribution range in the Atlantic Ocean. Despitethe high ability for dispersal, this species exhibit an unusual population structure andtwo mitochondrial clades clinally distributed along the eastern Atlantic and theMediterranean. In the present thesis, we investigated North Atlantic anchovy’sresponse to climate cycles at the leading edges of the distribution range. These smallpelagic fishes massively followed suitable thermal conditions cyclically over thePleistocene and therefore were able to preserve high levels of genetic diversity. Wefurther explored the variation of the mitochondrial clades of the European anchovyand found that the anti-tropically distributed clade is under positive selection,suggesting that temperature is shaping the contemporary distribution of mtDNA cladefrequencies. The Old World Anchovies (OWA) complex, of which the Europeananchovy is part, has taxa distributed in the Pacific and Atlantic oceans. This complexoriginated at 3.16 Ma in the Indo-Pacific during the late Pliocene and split in twogroups, one that remained in the Pacific Ocean and one that colonized the AtlanticOcean during the Pleistocene (0.62 Ma). The genetic patterns among the OWAindicate no genetic diferentiation between putative species from the Atlantic Ocean,and low levels of ongoing geneflow between Atlantic and Pacific anchovies. Withinthe Pacific Ocean, two well supported mitochondrial clades reveal ancient trans-Equatorial migrations, while nuclear loci support contemporary admixture.