EVIDENCE FOR PHYLOGENETIC CONGRUENCE AMONG SULFUR-OXIDIZING CHEMOAUTOTROPHIC BACTERIAL ENDOSYMBIONTS AND THEIR BIVALVE HOSTS

Sulfur-oxidizing chemoautotrophic (thioautotrophic) bacteria are now k nown to occur as endosymbionts in phylogenetically diverse bivalve hos ts found in a wide variety of marine environments. The evolutionary or igins of these symbioses, however, have remained obscure. Comparative 16S rRNA sequence analysis was used to investigate whether thioautotro phic endosymbionts are monophyletic or polyphyletic in origin and to a ssess whether phylogenetic relationships inferred among these symbiont s reflect those inferred among their hosts. 16S rRNA gene sequences de termined for endosymbionts from nine newly examined bivalve species fr om three families (Vesicomyidae, Lucinidae, and Solemyidae) were compa red with previously published 16S rRNA sequences of thioautotrophic sy mbionts and free-living bacteria. Distance and parsimony methods were used to infer phylogenetic relationships among these bacteria. All new ly examined symbionts fall within the gamma subdivision of the Proteob acteria, in clusters containing previously examined symbiotic thioauto trophs. The closest free-living relatives of these symbionts are bacte ria of the genus Thiomicrospira. Symbionts of the bivalve superfamily Lucinacea and the family Vesicomyidae each form distinct monophyletic lineages which are strongly supported by bootstrap analysis, demonstra ting that host phylogenies inferred from morphological and fossil evid ence are congruent with phylogenies inferred for their respective symb ionts by molecular sequence analysis. The observed congruence between host and symbiont phylogenies indicates shared evolutionary history of hosts and symbiont lineages and suggests an ancient origin for these symbioses.