Determinants of dispersal and phylogeographic history of a highly mobile cetacean species:The North Atlantic harbour porpoise.

Understanding the determinants of individual dispersal, population structure, and evolutionary history can provide insights about how species will evolve with climate change. However, such a task is complex for highly mobile marine species such as cetaceans for which it is intuitively difficult to infer what could limit their dispersal. The harbor porpoise (Phocoena phococena) is widely distributed in the North Atlantic. Its biogeographic history has been shaped by environmental variation during the last Glaciations with the divergence of three ecotypes (or sub-species) in the Eastern North Atlantic (ENA). However, we still do not know the degree to which populations in the ENA are connected to those in the Western North Atlantic (WNA), what environmental determinants drive the dispersal of porpoises, and whether distinct ecotypes exist in the WNA. Here we analyze the genetic diversity of 10 microsatellite loci and one-quarter of the mitogenome for an unprecedented sampling of 1,533 individuals. Using spatially explicit population genetics approaches, we investigated fine-scale population structure over the entire North Atlantic. Our study suggests that porpoises from the WNA and ENA are part of the same “continental shelf ecotype” that stretches from the northern Bay of Biscay to the WNA. We identified a clear signal of restricted dispersal in the mitogenome data supporting previous evidence of female philopatry. We also discovered a cryptic divergent mitochondrial lineage in one individual from Western Greenland suggesting a fourth distinct ecotype may exist. Finally, we reconstructed the phylogeographic history of these porpoises using coalescent simulations of population evolution, shedding light on the likely scenarios that shaped the current pattern of genetic diversity. These results provide key insights into the factors and processes shaping population structure in this species and will help model its evolution in the forecasted climate changes

No leading-edge effect in North Atlantic harbor porpoises: Evolutionary and conservation implications

Understanding species responses to past environmental changes can help forecast how they will cope with ongoing climate changes. Harbor porpoises are widely distributed in the North Atlantic and were deeply impacted by the Pleistocene changes with the split of three sub‐species. Despite major impacts of fisheries on natural populations, little is known about population connectivity and dispersal, how they reacted to the Pleistocene changes and how they will evolve in the future. Here, we used phylogenetics, population genetics, and predictive habitat modelling to investigate population structure and phylogeographic history of the North Atlantic porpoises. A total of 925 porpoises were characterized at 10 microsatellite loci and one‐quarter of the mitogenome (mtDNA). A highly divergent mtDNA lineage was uncovered in one porpoise off Western Greenland, suggesting that a cryptic group may occur and could belong to a recently discovered mesopelagic ecotype off Greenland. Aside from it and the southern sub‐species, spatial genetic variation showed that porpoises from both sides of the North Atlantic form a continuous system belonging to the same subspecies (Phocoena phocoena phocoena). Yet, we identified important departures from random mating and restricted dispersal forming a highly significant isolation‐by‐distance (IBD) at both mtDNA and nuclear markers. A ten times stronger IBD at mtDNA compared to nuclear loci supported previous evidence of female philopatry. Together with the lack of spatial trends in genetic diversity, this IBD suggests that migration‐drift equilibrium has been reached, erasing any genetic signal of a leading‐edge effect that accompanied the predicted recolonization of the northern habitats freed from Pleistocene ice. These results illuminate the processes shaping porpoise population structure and provide a framework for designing conservation strategies and forecasting future population evolution

No leading-edge effect in North Atlantic harbor porpoises: Evolutionary and conservation implications

International audienceAbstract Understanding species responses to past environmental changes can help forecast how they will cope with ongoing climate changes. Harbor porpoises are widely distributed in the North Atlantic and were deeply impacted by the Pleistocene changes with the split of three sub-species. Despite major impacts of fisheries on natural populations, little is known about population connectivity and dispersal, how they reacted to the Pleistocene changes and how they will evolve in the future. Here, we used phylogenetics, population genetics, and predictive habitat modelling to investigate population structure and phylogeographic history of the North Atlantic porpoises. A total of 925 porpoises were characterized at 10 microsatellite loci and one-quarter of the mitogenome (mtDNA). A highly divergent mtDNA lineage was uncovered in one porpoise off Western Greenland, suggesting that a cryptic group may occur and could belong to a recently discovered mesopelagic ecotype off Greenland. Aside from it and the southern sub-species, spatial genetic variation showed that porpoises from both sides of the North Atlantic form a continuous system belonging to the same subspecies (Phocoena phocoena phocoena). Yet, we identified important departures from random mating and restricted dispersal forming a highly significant isolation-by-distance (IBD) at both mtDNA and nuclear markers. A ten times stronger IBD at mtDNA compared to nuclear loci supported previous evidence of female philopatry. Together with the lack of spatial trends in genetic diversity, this IBD suggests that migration-drift equilibrium has been reached, erasing any genetic signal of a leading-edge effect that accompanied the predicted recolonization of the northern habitats freed from Pleistocene ice. These results illuminate the processes shaping porpoise population structure and provide a framework for designing conservation strategies and forecasting future population evolution