Recent evidence suggests that microbial populations in spatially and chemically fragmented habitats are not distributed ubiquitously, but rather exhibit geographic structure. Heterogeneous environments restrict gene flow among populations, which promotes genetic differentiation, local adaptation, and speciation. Hydrothermal vents are distributed in a patchy array due to topographic features, deep ocean currents, and variations in vent fluid chemistry. Though associations between chemosynthetic bacteria and their invertebrate hosts provide the basis for macrofaunal production at deep-sea hydrothermal vents, almost nothing is known about the distribution of genetic variation in the symbionts and how population structure of bacteria affects ecological interactions and the evolution of symbioses at vents. This research project will test models of gene flow for populations of thioautotrophic and methanotrophic endosymbionts of mussels inhabiting fast-, slow-, and discontinuous spreading centers by examining genetic structure along the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR) and within the Lau Back-arc Basin (Lau), respectively. Bathymodiolus mussels are found at nearly all hydrothermal vent sites and most likely acquire their symbionts from the environment in each generation, making them model systems for evaluating geographic structure of bacterial endosymbionts. Symbionts from 3 ridges, 5 vent fields per ridge, and mussels from 2-4 sites per vent field will be studied. For both types of symbiont, 4 DNA markers (16s rRNA, ITS, ftsZ, and either pmoA or soxY for the methanotroph and thioautotroph, respectively) will be sequenced from 20-30 mussel individuals per site. Population genetic, coalescent, and phylogeographic approaches will be employed in the data analyses.
Mechanisms that affect the population structure of bacterial endosymbionts have important implications for microbial biogeography, diversity, and the origin, evolution, and ecology of life at deep-sea hydrothermal vents. This research will yield the first high-resolution examination of gene flow among environmentally transmitted endosymbiont populations. Comparisons between hydrothermal vent fields will provide valuable information on the historical biogeography and barriers to dispersal for bacterial species. These findings will allow broader inferences regarding how deep ocean currents influence dispersal and the evolution of chemoautotrophic communities. Further, because hydrothermal vents serve as analogs to early Earth as well as extraterrestrial environments, results from this research will have implications for the origin of eukaryotic cell organelles and the ecology and evolution of microbes in extreme biomes. Annotated gene sequences will be deposited in GenBank and made publicly accessible through a website (www.endosymbiont.org) that the investigators use to present information on endosymbiosis to the scientific and public community. The investigators also give seminars designed for the general public on scientific topics, including microbial symbiosis and diversity, chemosynthesis and life on this planet and others. Undergraduates, graduate students, and postdoctoral fellows will have ample opportunity to participate in analyses of these gene sequences, both through their research and through inclusion in university courses.
