Abstract
BackgroundComparative phylogeography recently performed on the mitochondrial cytochrome oxidase I (mtCOI) gene from seven deep-sea vent species suggested that the East Pacific Rise fauna has undergone a vicariant event with the emergence of a north/south physical barrier at the Equator 1-2 Mya. Within this specialised fauna, the tube-dwelling polychaete Alvinella pompejana showed reciprocal monophyly at mtCOI on each side of the Equator (9°50'N/7°25'S), suggesting potential, ongoing allopatric speciation. However, the development of a barrier to gene flow is a long and complex process. Secondary contact between previously isolated populations can occur when physical isolation has not persisted long enough to result in reproductive isolation between genetically divergent lineages, potentially leading to hybridisation and subsequent allelic introgression. The present study evaluates the strength of the equatorial barrier to gene flow and tests for potential secondary contact zones between A. pompejana populations by comparing the mtCOI gene with nuclear genes.ResultsAllozyme frequencies and the analysis of nucleotide polymorphisms at three nuclear loci confirmed the north/south genetic differentiation of Alvinella pompejana populations along the East Pacific Rise. Migration was oriented north-to-south with a moderate allelic introgression between the two geographic groups over a narrow geographic range just south of the barrier. Multilocus analysis also indicated that southern populations have undergone demographic expansion as previously suggested by a multispecies approach. A strong shift in allozyme frequencies together with a high level of divergence between alleles and a low number of 'hybrid' individuals were observed between the northern and southern groups using the phosphoglucomutase gene. In contrast, the S-adenosylhomocysteine hydrolase gene exhibited reduced diversity and a lack of population differentiation possibly due to a selective sweep or hitch-hiking.ConclusionsThe equatorial barrier leading to the separation of East Pacific Rise vent fauna into two distinct geographic groups is still permeable to migration, with a probable north-to-south migration route for A. pompejana. This separation also coincides with demographic expansion in the southern East Pacific Rise. Our results suggest that allopatry resulting from ridge offsetting is a common mechanism of speciation for deep-sea hydrothermal vent organisms.
Highlights
Comparative phylogeography recently performed on the mitochondrial cytochrome oxidase I gene from seven deep-sea vent species suggested that the East Pacific Rise fauna has undergone a vicariant event with the emergence of a north/south physical barrier at the Equator 1-2 Mya
A physical barrier to dispersal can disrupt the relationship between genetic differentiation and migration rate: all models show that the establishment of a long-term physical barrier to dispersal abruptly reduces or stops gene flow between populations and leads to a rapid change in allele frequencies on either side of the barrier that may be dampened or accelerated depending on the type of selection at specific loci
Deep-sea hydrothermal vents represent a patchily distributed habitat suggestive of a one-dimensional steppingstone model over 60 000 kilometres of globe-encircling ridge crests [10]. Because of their dependence on ‘hot’ sulphidic fluids from vent chimneys, deep-sea hydrothermal vent species display an island-like distribution along ridges
Summary
Comparative phylogeography recently performed on the mitochondrial cytochrome oxidase I (mtCOI) gene from seven deep-sea vent species suggested that the East Pacific Rise fauna has undergone a vicariant event with the emergence of a north/south physical barrier at the Equator 1-2 Mya. A physical barrier to dispersal can disrupt the relationship between genetic differentiation and migration rate: all models show that the establishment of a long-term physical barrier to dispersal abruptly reduces or stops gene flow between populations and leads to a rapid change in allele frequencies on either side of the barrier that may be dampened or accelerated depending on the type of selection at specific loci. This often leads to the fixation of mutations (divergence) and reciprocal monophyly for many loci, provided that no lineage extinction or population admixture occurs [8]. In most cases, allozyme studies have failed to detect isolation-by-distance among populations of hydrothermal vent species (e.g. [14,15]), suggesting that an n-island model [1] is a better description of gene flow in vent systems
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