Genetic diversity and demographic instability in Riftia pachyptila tubeworms from eastern Pacific hydrothermal vents

Genetic differentiation (FST) among populations increased with geographical distances, as expected under a linear stepping-stone model of dispersal. Low levels of DNA sequence diversity occurred at all four loci, allowing us to exclude the hypothesis that an idiosyncratic selective sweep eliminated mitochondrial diversity alone. Total gene diversity declined with tectonic spreading rates. The southernmost populations, which are subjected to superfast spreading rates and high probabilities of extinction, are relatively homogenous genetically.Compared to other vent species, DNA sequence diversity is extremely low in R. pachyptila. Though its dispersal abilities appear to be effective, the low diversity, particularly in southern hemisphere populations, is consistent with frequent local extinction and (re)colonization events.Demographic instability influences the distribution of genetic diversity within and among discrete colonies of a widely distributed species [1,2]. Local extinction and (re)colonization events are expected to reduce the overall effective size of a metapopulation thereby limiting its capacity to retain genetic variation [3]. Furthermore, genetic drift contributes significantly less to geographical differentiation if the average persistence time of colonies is less than the time it takes to fix neutral alleles [4]. The number of colonists, their sources (i.e. migrant vs. propagule pools), and ongoing rates of gene flow all affect the degree to which demographic instability affects geographical differentiation [5-7]. Yet, these parameters are rarely known; instead they are commonly inferred from statistical analyses of genealogical variation that is assumed to be selectively neutral [8]. For a variety of practical reasons, recent studies have relied mostly on information from mitochondrial DNA variation, although the assumption of selective neutrality has been debated [9-11]. Independent genealogies involving nuclear genes are needed to distinguish the