Phylogeography of deep sea vestimentiferans and a population genetics study of two species Lamellibrachia luymesi Seepiophila jones

by McMullin, Erin R.

First discovered in 1977 on the Galapagos Rift, vestimentiferans are a group of deep-sea annelids found in a variety of environments worldwide. Vestimentiferan communities are isolated pockets of high biomass in the otherwise nutrient-poor deep-sea. Chemosynthesis, not photosynthesis, is the underlying energy source for vestimentiferans, which entirely lack a digestive tract and rely on internal sulfide-oxidizing symbionts for fixed carbon. Symbionts appear to be acquired by the motile vestimentiferan larvae before they settle and become sessile adults. The ability of both these organisms and their symbionts to disperse across the sometimes considerable distances between sulfidic environments has been a topic of study for over a decade. This study addresses the question of dispersal by two different methods, with a biogeographical approach of all vestimentiferan species, and through the population genetic analysis of two species within the Gulf of Mexico. Species distributions can reveal barriers to gene flow, and the range over which an organism maintains a single species reflects the ability of individuals in that species to interbreed between populations. In the case of the vestimentiferan symbionts, which do not interbreed, symbiont strain distribution reflects the presence of a particular bacterial strain near the vestimentiferan larvae before symbiont acquisition. Vestimentiferan biogeography reveals that many species have very large species ranges, indicating the exchange of at least one migrant per generation between sites as distant as 6000km. However, different vestimentiferan species are found between sites that are geographically close but that are at very different depths, suggesting depth may be a barrier to either dispersal or survival of larvae between sites. The same vestimentiferan symbiont strains are found worldwide, with different host species in sites as distant as the Florida Escarpment and the coast of Oregon containing the same symbiont. However, as with the vestimentiferan hosts, evidence suggests that depth may affect the presence of symbiont strains. Symbiont strains may in fact be ubiquitously distributed throughout the world’s oceans but only within a certain depth range, and the particular strain a vestimentiferan host contains may depend predominantly on the depth at which that host lives. Biogeography serves to define species limits, but does not directly measure gene flow within a species. Microsatellite markers were developed for use in a population genetics study of Lamellibrachia luymesi and Seepiophila jonesi of the Louisiana Slope of the Gulf of Mexico. iv Microsatellite markers are regions of repetitive DNA that tend to be highly polymorphic within a species. Five polymorphic microsatellite loci were isolated from L. luymesi and seven from S. jonesi; these loci had between 7 and 50 alleles. Samples were collected from aggregations of tubeworms from nine hydrocarbon seep sites from the Louisiana Slope, and screened with the microsatellite loci to reveal departures from Hardy Weinberg Equilibrium. No obvious population structure was found between aggregations within a sample site in either species. L. luymesi showed some evidence for isolation by distance across the 480km from which it was sampled, but S. jonesi showed no evidence for isolation by distance over 580km. Both L. luymesi and S. jonesi, however, showed a generalized excess of homozygous individuals which did not reflect underlying population substructure. These data suggest that both species have high geneflow between aggregations and sample sites, but that L. luymesi dispersal is somewhat more limited than S. jonesi. The generalized homozygote excess observed reflects a departure from HWE the cause of which is not clearly identified in this study. Cohorts may exist within an aggregation that were not revealed with this sampling plan, leading to mating between related individuals. A second hypothesis to explain the observed homozygote excess may be strong selection against heterozygous larvae, as is suggested in some marine bivalves. Unlike most of the deep sea, vestimentiferan communities are characterized by very high biomass. These communities, however, depend on resources that are patchily distributed across the globe. Biogeographic data suggest that both vent and seep vestimentiferans have strong dispersal capabilities, and the population genetic analyses presented here support the same conclusion. However, vestimentiferan dispersal is affected by at least one physical barrier, depth, and may also be affected by water currents. The polymorphic markers isolated in this study are a tool that can be used to address the presence of physical barriers between vestimentiferan communities, and can also be used to reveal non random mating within a species, as in the Gulf of Mexico study. v