Cospeciation of chemoautotrophic bacteria and deep sea clams

Abstract

Vesicomyid clams depend entirely on sulfur-oxidizing endosymbiotic bacteria for their nutriment. Endosymbionts that are transmitted cytoplasmically through eggs, such as these, should exhibit a phylogenetic pattern 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 with the host genealogy, based on clam mitochondrial cytochrome oxidase subunit I and large subunit (16S) rDNA sequences. This phylogenetic evidence supports the hypothesis of cospeciation and a long term association between the participants in this symbiosis.

Figure 1

ML phylogenies for nine species of vesicomyid clams and their associated endosymbionts. The clam species were characterized in previous allozyme and mitochondrial COI studies (17, 26). Host and symbiont trees were not drawn to the same rate scale (scale below each tree). Numbers above nodes are the percentage bootstrap support from 1,000 resamplings. Small letters and symbols below nodes mark topological comparisons between the two trees. All LnLi values presented are based on unrooted 9 taxa topologies. (A) The vesicomyid tree was based on combined data from portions of the mitochondrial COI gene (516 bp) and large subunit 16S rDNA (513 bp) gene, and an empirically derived ts/tv ratio of 3.0. (B) The bacterial tree (LnLi −2808.31) was based on a portion of small subunit 16S rDNA (1,433 bp), and an empirically derived ts/tv ratio of 3.75.