Abstract
The complementarity of historical and contemporary processes contributes to understanding the genetic structure of continuously distributed marine species with high dispersal capabilities. Cephalorhynchus eutropia, has a continuous coastal distribution with strong genetic differentiation identified by nuclear DNA markers. We explored the historical dimension of this genetic differentiation between northern and southern populations to evaluate phylogeographic structure. Additionally, we conducted mtDNA and microsatellite analyses to detect past and recent demographic changes. The southern population was characterized by lower genetic diversity with a signal of population expansion, likely associated with ice retreat and habitat extension after the Last Glacial Maximum (LGM). In contrast, structure within the northern population was more consistent with stable historical population size. Approximate Bayesian Computation analyses suggested that during the LGM, C. eutropia persisted in the northern area; while the south was colonized by dispersal ~11,000 years ago followed by population expansion. This study shows that Chilean dolphin population structure is consistent with predictions from the Expansion-Contraction biogeographic model, with a poleward post-glacial shift revealed in current genetic structure. The results also confirm the validity of the population units previously identified, demonstrating their historical origin and highlighting the utility of integrating genetic markers with different temporal scale resolutions.
Highlights
In this context, historical processes such as differential effects of glaciation and sea-level changes may greatly influence biogeographic patterns, shaping the distribution and population structure of many species[12,13]
We analyzed the historical biogeography of C. eutropia to test whether population structure as influenced by the Last Glacial Maximum (LGM) is more consistent with the predictions of the expansion-contraction model under the “In situ refugia” or “Northern shift” scenarios
The interpretation of the structure and distribution patterns of genetic diversity is one of the research objectives of fields including molecular ecology, population genetics and landscape genetics[37]. These patterns become more predictable in cases when the genetic structure pattern is concordant with geographic structure[38] as has been described for example in the mammal Leopardus guigna in southern South America, where geographical barriers such as the presence of the Andes Mountains and the Chacao Channel have shaped its current genetic population structure[39], and in the fish, Orestias ascotanis, endemic to the Altiplano, where population structure reflects a long-term consequence of habitat fragmentation[40]
Summary
Historical processes such as differential effects of glaciation and sea-level changes may greatly influence biogeographic patterns, shaping the distribution and population structure of many species[12,13]. The southern area is a protected area of fjords and channels dominated by Sub-Antarctic water and the melting of resident glaciers[32], where Chilean dolphins are mainly located in protected fjords and channels[30] This biogeographic boundary is associated with the oceanic divergence between the Humboldt and the Cape Horn currents, where the West Wind Drift reaches the coast and divides into the northward flowing Humboldt Current and the poleward flowing Cape Horn Current. The geographic break detected in C. eutropia is coincident with the northern limit of the LGM ice sheet[34] During this period, western Patagonia was covered by a vast ice sheet that extended from the southernmost latitudes to Chiloé Island, south of 43°S, descending from the Andes and reaching the edge of the continental shelf[35]. We analyzed the historical biogeography of C. eutropia to test whether population structure as influenced by the LGM is more consistent with the predictions of the expansion-contraction model under the “In situ refugia” or “Northern shift” scenarios
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