Seria a população do Estreito de Magalhães, no extremo sul do Chile, o ponto de contato entre os leões-marinhos-sul-americanos dos oceanos Atlântico e Pacífico?

Abstract

South American sea lions (Otaria flavescens) are otariids with a wide distribution along the South American coast. Populations from the Atlantic and Pacific oceans show evidence of genetic structure with low gene flow in analyses using matrilineal markers, which led to the suggestion that they represent distinct Evolutionarily Significant Units. However, recent studies have indicated the existence of considerable gene flow at the southern tip of the continent, a region where the Strait of Magellan is located, and more specifically the population of Magdalena Island, which is the focus of the present study. To assess the relationship between South American sea lions from this island and populations from the Atlantic and Pacific oceans, five samples were obtained; however, only two sequences of the mitochondrial DNA (mtDNA) control region were recovered with good quality, each comprising 518 base pairs (bp). These sequences were aligned with 182 other sequences available for the species in the GenBank database, and 289 sites were retained after trimming the ends. Haplotypes were identified for the study location, and their relationships with populations from Atlantic and Pacific localities were inferred using a haplotype network. In addition, an analysis of molecular variance (AMOVA) was performed to evaluate the genetic structure of the studied populations through pairwise comparisons among countries: Brazil = 6, Uruguay = 13, Argentina = 104, Falkland Islands = 7, Magdalena Island = 2, Chile = 40, and Peru = 12. Finally, three scenarios with different groupings were tested to evaluate genetic structure: (1) Atlantic, Pacific, and Magdalena as three distinct groups; (2) Atlantic + Magdalena Island and Pacific; and (3) Pacific + Magdalena Island and Atlantic. The haplotype network revealed three main clusters: the first composed mainly of Pacific populations and some individuals from Argentina; the second consisting of haplotypes from Brazil, Uruguay, Argentina, and the Falkland/Malvinas Islands; and the third formed by haplotypes from Uruguay and Argentina. The two haplotypes from Magdalena Island were assigned to the first cluster, positioned alongside haplotypes from the southernmost region of the continent, including representatives from both Chile and Argentina. Pairwise comparisons indicated relatively high divergence between the Magdalena population and all Atlantic populations (FST: 0,41–0,67), and AMOVA results pointed to genetic structuring between the Atlantic and Pacific. Thus, the presence of shared haplotypes between the two oceans supports the conclusion that they do not constitute independent Evolutionarily Significant Units. Nevertheless, despite the divergence between populations from the two oceans, the Magdalena Island population is closely related to haplotypes from the southern extremes of both oceans, indicating that populations from the Strait of Magellan play a role in female-mediated gene flow in South American sea lions.