Host-symbiont attributes in the bivalve family Thyasiridae: insights from a primitive and plastic association

Factors leading to the establishment and maintenance of long-term, stable associations between animals and bacteria are not well understood; examining relatively simple symbiotic relationships could help us understand how symbioses might evolve. In the bivalve family Thyasiridae, some species form symbioses with sulphur-oxidizing gammaproteobacteria that are maintained extracellularly among gill cell microvilli. Symbionts receive reduced sulphur, carbon dioxide and oxygen for autotrophic production from the animal's ventilation activities, and are periodically engulfed and digested by the host's epithelial cells. Recent work on chemosymbiotic thyasirids from Bonne Bay, NL has revealed an unexpected degree of diversity among thyasirids as well as among their symbionts. Also, studies of thyasirid populations, combined with the discovery of key gene sequences in the first sequenced thyasirid symbiont genome, suggest how symbionts are transmitted between generations. Finally, a seasonal study of the relative abundance of symbionts and of remnants of digested bacteria in host cells (evidence of symbiont digestion) reveals that the association is highly dynamic. These new findings allow us to refine our model of chemosymbiosis evolution in thyasirids.