COSPECIATION OF CHEMOAUTOTROPHIC BACTERIA AND DEEP-SEA CLAMS

Vesicomyid clams depend entirely on sulfur-oxidizing endosymbiotic bac teria for their nutriment. Endosymbionts that are transmitted cytoplas mically through eggs, such as these, should exhibit a phylogenetic pat tern that closely parallels the phylogeny of host mitochondrial genes. Such parallel patterns are rarely observed, however, because they are obscured easily by small amounts of horizontal symbiont transmission or occasional host switching. The present symbiont genealogy, based on bacterial small subunit (16S) rDNA sequences, was closely congruent w ith the host genealogy, based on clam mitochondrial cytochrome oxidase subunit I and large subunit (16S) rDNA sequences. This phylogenetic e vidence supports the hypothesis of cospeciation and a long term associ ation between the participants in this symbiosis.