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Abstract
Comparative phylogeography of deep-sea hydrothermal vent species has uncovered several genetic breaks between populations inhabiting northern and southern latitudes of the East Pacific Rise. However, the geographic width and position of genetic clines are variable among species. In this report, we further characterize the position and strength of barriers to gene flow between populations of the deep-sea vent mussel Bathymodiolus thermophilus. Eight allozyme loci and DNA sequences of four nuclear genes were added to previously published sequences of the cytochrome c oxidase subunit I gene. Our data confirm the presence of two barriers to gene flow, one located at the Easter Microplate (between 21°33′S and 31°S) recently described as a hybrid zone, and the second positioned between 7°25′S and 14°S with each affecting different loci. Coalescence analysis indicates a single vicariant event at the origin of divergence between clades for all nuclear loci, although the clines are now spatially discordant. We thus hypothesize that the Easter Microplate barrier has recently been relaxed after a long period of isolation and that some genetic clines have escaped the barrier and moved northward where they have subsequently been trapped by a reinforcing barrier to gene flow between 7°25′S and 14°S.
Highlights
Genetic structure is easier to detect and understand in a onedimensional system than in a two-dimension space [1]
Polymorphism and divergence from DNA sequences Population structure across the Easter Microplate was found for two genes (Fig. 2, 3), mtCOI (4.4% divergence) and EF1a (1.0% divergence), with two divergent clades corresponding to East Pacific Rise (EPR) and Pacific-Antarctic Ridge (PAR, i.e., 31uS-38uS) populations, respectively
Previous and present works strongly support the hypothesis that the population structure of Bathymodiolus thermophilus is impacted by two barriers to gene flow along the EPR: the so-called Easter Microplate barrier ([15], [24], present manuscript) and the 7u259S–14uS barrier
Summary
Genetic structure is easier to detect and understand in a onedimensional system than in a two-dimension space [1]. Even in a one-dimension space, detecting a genetic cline with a correlation between genetic differentiation and geographical distance does not always mean that populations are following an isolation-by-distance (IBD) model. Such a correlation can be due to the presence of barriers to dispersal (e.g., [2]) or secondary contacts between previously isolated populations under expansion (e.g., [3]). When detecting a genetic cline, one should consider the possibility that the location of this cline may be due to the presence of a natural barrier to dispersal because clines are expected to be trapped by such a barrier [4], [5]. Clines often typify adaptive gradients [9], [10] or hybrid zones (i.e., regions containing recombinant individuals between genetically differentiated populations)
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