Bacterial Symbionts

Phylogenetic analyses based on 16S rRNA have revealed that two types of gamma Proteobacteria are housed within the bacteriocytes of bathymodioline mussels. One type of symbiont includes lineages that cluster with free-living, type I methanotrophs, while the others form a monophyletic group of chemoautotrophic endosymbionts associated with mytilid mussels and vesicomyid clams (Fig. 4). Overall, six of the sixteen species of Bathymodiolus host dual symbionts, while an additional three harbor only methanotrophs and another seven, along with Tamu fisheri, are associated strictly with chemoautotrophs (Table 1). The association between bathymodioline mussels and these two distinct bacterial clades presents a unique situation to address questions about the origin and evolution of symbioses.

Fig. 4. Bayesian phylogram of chemoautotrophic and methanotrophic endosymbionts hosted by bathymodioline mussels, inferred from 16S rRNA gene sequences (modified from DeChaine et al. 2005). Posterior probabilities (>90) are shown above branches. Taxa include symbiotic and free-living gamma proteobacteria. Clades of symbiotic chemoautotrophs and methanotrophs are boxed in gray. Bathymodiolus azoricus and B. puteoserpentis host the same dual symbionts, which are labeled M and C for methano-troph and chemoautotroph, respectively.

Fig. 4. Bayesian phylogram of chemoautotrophic and methanotrophic endosymbionts hosted by bathymodioline mussels, inferred from 16S rRNA gene sequences (modified from DeChaine et al. 2005). Posterior probabilities (>90) are shown above branches. Taxa include symbiotic and free-living gamma proteobacteria. Clades of symbiotic chemoautotrophs and methanotrophs are boxed in gray. Bathymodiolus azoricus and B. puteoserpentis host the same dual symbionts, which are labeled M and C for methano-troph and chemoautotroph, respectively.

To date, all methanotrophic endosymbionts of mussels form a monophyletic group nested within the type I free-living methanotroph clade (Fig. 4; DeChaine et al. 2005; Duperron et al. 2005a,b). Though the methanotrophs are generally host-specific, multiple host species can harbor the same methanotrophic symbiont phylotype and single mussel individuals can host multiple symbiont genotypes (DeChaine et al. 2005).

The bathymodioline chemoautotrophic symbionts cluster with symbionts of clams, to the exclusion of other chemoautotrophic vent symbionts (e.g., those of hydrothermal tubeworms and coastal mollusks; Fig. 4). As with the methanotrophs, diverse but related phylotypes have been uncovered for chemoautotrophic endosymbionts of mussels in the genus Bathymodiolus, including multiple phylotypes within an individual host as well as symbiont phylotypes shared among host species (Won et al. 2003 a; DeChaine et al. 2005; Duperron et al. 2005b). Though the chemoautotrophic symbionts of the Eastern Pacific Rise (EPR) mussel, B. thermophilus, were shown to fix CO2 and to utilize thiosulfate and hydrogen sulfide as energy sources and thus were labeled sulfur-oxidizing or thioautotrophic bacteria (Belkin et al. 1986; Nelson et al. 1995), energy substrates for chemoautotrophic endosymbionts of other mussels in the genus have not been determined. For a detailed discussion of chemoautotrophy, see the chapter on chemoautotrophic symbioses of vestimentiferan tubeworms (Stewart and Cavanaugh, this vol.) and the recent review by Cavanaugh et al. (2005).

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