Ecological genetics of the hydrothermal vent tubeworm, Ridgeia piscesae and the hydrocarbon seep mussel, Bathymodiolus childressi

by Carney, Susan Lynne.

Chemosynthetic communities far below the surface of the ocean present new opportunities for studies of the interaction of organisms with their environments. The hydrothermal vents of the northeast Pacific Ocean and the cold seeps of the Gulf of Mexico serve as the setting for the work in this dissertation. Populations of the vent tubeworm Ridgeia piscesae at the Juan de Fuca Ridge exhibit a number of diverse phenotypes. Like all deep-sea tubeworms, R. piscesae relies on a symbiosis with sulfuroxidizing bacteria within an organ in its trunk region and requires a constant supply of sulfide and oxygen from its surroundings. Different R. piscesae phenotypes each inhabit distinct microhabitats ranging from low temperature, low sulfide, diffuse flow, basalt substrate to vigorous flow, high temperature, sulfide chimneys. This study first uses a genome-wide DNA fingerprinting survey to test whether genetic variation undetected by previous studies underlies the two most extreme R. piscesae phenotypes from chimney and basalt environments. Our results confirm that this is a genetically undifferentiated population, supporting that this tubeworm species displays a high degree of phenotypic plasticity. Next, focusing on hemoglobin as a candidate gene, this study examines whether chimney and basalt R. piscesae differentially express the chains used to assemble their extracellular hemoglobin molecules, the direct link between the animal and its local environment. The gene expression analysis shows that R. piscesae on high-flow chimneys express higher levels of globin chains than animals from low-flow basalt environments, and within a phenotype, gene expression varies according to collection sites and different chemical environments. The genomic DNA sequences of the six globin chains examined in the expression analysis revealed no phenotype-specific iii nucleotide variation within or upstream of the genes, thus implicating trans-regulation of this tubeworm hemoglobin system. In the Gulf of Mexico, two mitochondrial and six nuclear markers were used to test whether gene flow between populations of the mussel Bathymodiolus childressi is limited over its range of depth, geographic distance, and environmental conditions. The results from this study yield no evidence for differentiation between B. childressi populations in the Gulf of Mexico, however they do suggest that the combination of geographic distance and depth may limit dispersal over the extremes of this species’ range. iv