Sharp phylogeographical differentiation near the southern range edge of the silverside Odontesthes nigricans: Distinct peripheral populations and incipient speciation?

Abstract

The silverside Odontesthes nigricans is the Atheriniform species that reach the southernmost latitude in Patagonia. Its wide distribution ranges from temperate waters in the Atlantic coast of South America (39°S, Buenos Aires Province, Argentina) to the Sub-Antarctic environment of Cape Horn (56°S). A previous study has shown significant intraspecific differences on growth parameters and otolith morphology between the southern peripheral population of O. nigricans in the Beagle Channel and the populations in the Atlantic coast of Tierra del Fuego. In this work, we report the first data on population genetics of O. nigricans along the southern part of its distributional range, including the peripheral populations of Beagle Channel. To examine the genetic variability, we analyzed 224 sequences of 815 bp of mitochondrial control region generated from individuals captured at two sites in the Beagle Channel and six sites at the Atlantic coast. The genetic structure assessment revealed a sharp phylogeographic division between Beagle Channel and the Atlantic coast of Patagonia with a 39.83% of variance among groups. The observed genetic diversity was lower in Beagle Channel than in the Atlantic coast sites and the demographic analyses are compatible with a scenario of past population expansion, especially in Beagle Channel. We hypothesize two possible scenarios: one is a recent colonization of the Beagle Channel from the Atlantic Coast followed by divergence promoted by adaptation to local conditions. On the other hand, population differentiation may have been attained or initiated in allopatric conditions during Quaternary glacial phases and the populations may now be experiencing a secondary contact near the eastern tip of Tierra del Fuego. These hypotheses should be addressed in the future with genomics tools. By now, these populations should be considered as different units for conservation and management purposes.