Evolution of Deep-Sea Mussels (Bathymodiolinae) and Their Chemosynthetic Endosymbionts
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CitationFontanez, Kristina. 2012. Evolution of Deep-Sea Mussels (Bathymodiolinae) and Their Chemosynthetic Endosymbionts. Doctoral dissertation, Harvard University.
AbstractSymbiosis is one of the most widespread evolutionary strategies on Earth. In the deep-sea, symbioses between chemosynthetic bacteria and invertebrates are abundant at hydrothermal vents and cold seeps. These mutualisms, in which symbiont carbon fixation provides for host nutrition, are analogous to the ancient endosymbioses that resulted in the chloroplast and the eukaryotic mitochondrion. However, the evolutionary processes that led to the widespread dispersal of deep-sea organisms and the mechanisms by which symbioses are initiated and maintained are poorly characterized. This thesis examined the evolution of deep-sea mussels (Bathymodiolinae) and their chemosynthetic symbionts. Bathymodioline mussel taxonomy is in need of a comprehensive systematic revision because the majority of named genera do not constitute monophyletic groups. First, this thesis demonstrated that mussels found on the Northeast Pacific Ridges are members of Adipicola, a paraphyletic genus within Bathymodiolinae, refining the evolutionary history of this poorly characterized group. Second, an updated multi-locus phylogeny of bathymodiolines was presented and used to evaluate the statistical evidence for previously proposed hypotheses describing the directional evolution of bathymodioline traits. The results indicated that patterns of directional evolution in this group are not well supported and instead suggests that trait evolution has proceeded in a non-directional manner. Third, this thesis presented the first evidence of detection and abundance of bathymodioline symbionts in the deep-sea environment, providing direct evidence that these symbionts are environmentally acquired. Fourth, this thesis presented the first multi-locus phylogenies of bathymodioline symbionts and tested the hypothesis of environmental acquisition of symbionts in this group. The results demonstrated that symbiont and host lineages are decoupled, which is consistent with the environmental acquisition hypothesis. Finally, environmental acquisition implies that symbionts have opportunities to exchange genetic information with other bacterial strains and evidence for recombination in bathymodioline symbionts is also presented. This thesis advances our understanding of the evolutionary history of bathymodioline symbioses by clarifying host and symbiont evolutionary history and symbiont transmission strategy. In aggregate, these results suggest that bathymodiolines are more flexible with regard to the habitats they inhabit and the symbionts they harbor than previously understood.
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