Molecular systematics, population structure, and gene flow of deep-sea hydrothermal vent mussels (Mytilidae: Bathymodiolus) and associated sulfur-oxidizing endosymbionts

Bathymodiolus mussels are among the dominant constituents of deep-sea hydrothermal vent and cold-water sulfide/hydrocarbon seep communities which are based on chemoautotrophic production by microbes and are scattered worldwide as discrete habitat islands along the global mid-ocean ridge system, in back-arc spreading centers, and in subduction zones along continental margins. Herein I examine questions concerned with the dispersal, biogeography, and evolution of these mussels and their endosymbionts.; Chapter 1 examines factors that affect gene flow in vent mussels of the genus Bathymodiolus from eastern Pacific Ocean. Analyses of allozymes and mitochondrial DNA revealed significant barriers to gene flow along the ridge axis. All populations from 13°N to 11°S were homogeneous genetically and seemed to experience unimpeded extensive gene flow. In contrast, mussels from south of the Easter Microplate were highly divergent, possibly comprising sister-species that diverged after formation of the Microplate ∼4.5 MY ago.; Chapter 2 addresses phylogenetic relationships of Bathymodiolus species newly acquired from Blake Ridge with other Bathymodiolus species. Molecular phylogenetic analyses revealed that seep mussels from the Blake Ridge are conspecific to B. heckerae from the West Florida Escarpment and supported the hypothesis that vent mussels are not monophyletic to seep mussels and that evolutionary habitat transitions from seeps to vents are reversible.; Chapter 3 tests the hypothesis that a mussel hybrid zone in the Mid-Atlantic Ridge is a zone of secondary contact. Examination of nuclear and cytoplasmic gene markers demonstrated linkage disequilibrium that is consistent with the secondary contact hypothesis.; Chapter 4 tests the hypothesis that sulfur-oxidizing endosymbionts of vent mussels are transmitted vertically between generations. An absence of genetic coupling between host mitochondrial and symbiont DNA sequences rejected the ‘strictly cytoplasmic maternal co-transmission’ mode in these mussels. The alternative hypothesis of environmental acquisition of free-living form of the endosymbionts was supported by cytological examination of bacteriocytes of mussel gill that showed ongoing acquisition of symbionts.; Chapter 5 tests the hypothesis of cospeciation between vent-endemic deep-sea mussels and their endosymbiotic sulfur-oxidizing bacteria from World Oceans. Molecular phylogenetic analyses revealed incongruence between host and symbiont phylogenies, suggesting absence of cospeciation of the two participants in this symbiosis.