Vertical transmission of a chemoautotrophic symbiont in the protobranch bivalve, Solemya reidi.

Invertebrates that contain endosymbiotic chemoautotrophic bacteria are widely distributed in a variety of reducing marine habitats (i.e., hydrothermal vents and cold seeps). The mechanisms of symbiont transmission and incorporation during early host development are poorly understood primarily because these critical early life stages have not been procured. Solemya reidi is a gutless protobranch bivalve found inhabiting pulpmill effluent and sewage outfall sites. Endosymbiotic sulfur oxidizing bacteria are contained within specialized host cells of the gill filaments. These bivalves have the advantage over their deep-sea counterparts in that they can be spawned and cultured axenically, providing the opportunity to examine the ontogenetic process of symbiont incorporation. The 16S rDNA of the symbiont from Solemya reidi was sequenced from a PCR product obtained using two bacterial-specific primers. Phylogenetic analysis of this sequence identified a hypervariable region of the molecule that was unique to this symbiont. An 18-base oligonucleotide probe was synthesized and tested for specificity and sensitivity to detect its symbiont-specific target. Immobilized cellular RNAs from a range of taxa were used to screen the specificity of the Solemya probe. At high stringency, the symbiont-specific probe hybridized only to RNA extracted from gill tissue of S. reidi. When matched with a bacterial-specific forward primer in PCR amplifications, the symbiont-specific probe successfully amplified S. reidi symbiont 16S rDNA genes in genomic DNA isolated from the host ovary, eggs, and 1- and 4-day-old larvae. Amplifications detected the presence of symbiont target in the germ tissue and larvae. Nonradioactive in situ hybridizations were performed on 3-day-old larvae with the symbiont-specific probe. Detection of hybrids localized the symbionts to the epithelial test cells of the larvae. These results suggest that the symbionts are vertically transmitted with the egg and are incorporated as a post-metamorphosis event.