Neutral and nonneutral mitochondrial genetic variation in deep-sea clams from the family vesicomyidae.

Nucleotide sequences at two mitochondrial genes from 57 individuals representing eight species of deep-sea clams (Vesicomyidae) were examined for variation consistent with the neutral model of molecular evolution. One gene, cytochrome oxidase subunit I (COI), deviated from the expectations of neutrality by containing an excess of intraspecific nonsynonymous polymorphism. Additionally, one species, Calyptogena kilmeri, showed a significant excess of rare polymorphism specifically at the COI locus. In contrast, a second mitochondrial gene, the large-subunit 16S ribosomal RNA gene (16S), showed little deviation from neutrality either between or within species. Together, COI and 16S show no deviation from neutral expectations by the HKA test, produce congruent phylogenetic relationships between species, and show correlated numbers of fixed differences between species and polymorphism within species. These patterns of both neutral and nonneutral evolution within the mitochondrial genome are most consistent with a model where intraspecific nonsynonymous polymorphism at COI is near neutrality. In addition to examining the forces of molecular evolution, we extend hypotheses about interspecific relationships within this family for geographical locations previously unexamined by molecular methods including habitats near the Middle Atlantic, the Aleutian Trench, and Costa Rica.

Sulfide-Driven Autotrophic Balance in the Bacterial Symbiont-Containing Hydrothermal Vent Tubeworm, Riftia pachyptila Jones.

Hydrothermal vent tubeworms, Riftia pachyptila Jones, were maintained alive and studied on board ship using flow-through pressure aquaria. Simultaneous measurements of O2, ΣCO2, ΣH2S fluxes showed that the intact symbioses reach maximum rates of uptake of ΣCO2 (>2 µmole g-1 h-1) at about 90 µM ΣH2S. Measurements were made of hemolymph and coelomic fluid ΣCO2, ΣH2S, thiosulfate, pH, and hemoglobin concentrations in worms kept under various conditions of O2 and ΣH2S. Normal hemolymph pH appears to be about 7.5 and is not affected by ΣH2S and ΣCO2 concentrations within the ranges observed. We conclude that Riftia is specialized to provide sulfide to its symbionts with minimal interaction of sulfide with the animal metabolism. The uptake of sulfide is apparently by diffusion into the hemolymph, facilitated by the sulfide-binding properties of the hemoglobins. Both ΣCO2 and PCO2 are elevated in the hemolymph above their levels in the medium, although they are reduced under autotrophic conditions. Thus inorganic carbon is apparently concentrated from the medium into the hemolymph by an unknown mechanism.