DNA-DNA Solution Hybridization Studies of the Bacterial Symbionts of Hydrothermal Vent Tube Worms (Riftia pachyptila and Tevnia jerichonana).

The giant tube worm, Riftia pachyptila (phylum Vestimentifera), is known only from four widely separated sulfide-rich deep-sea hydrothermal vent systems. This invertebrate is nourished by intracellular, chemoautotrophic bacterial symbionts which reside in a specialized trophosome tissue. The symbiont has not been cultured independently and is believed to be acquired de novo by host larvae of each generation. In the current study, R. pachyptila symbiont DNA was purified from the two most distant sites on the basis of its difference in density versus host DNA. These two standards were hybridized against trophosome DNAs of 13 individuals from the Guaymas Basin, Galapagos Rift, and 13 degrees N vents. This indicated that all R. pachyptila symbionts are conspecific and that the variability in DNA-DNA hybridization (relative binding ratio [RBR]) was comparable within or between widely separated vents. The symbiont of another tube worm, Tevnia jerichonana, was found to be the same as that of R. pachyptila, the first case in which distinct hosts possess the same sulfur bacterial symbiont. By contrast, Lamellibrachia sp. (same class as T. jerichonana) showed insignificant RBR with the R. pachyptila symbiont. DNA derived from solely eucaryotic tissue of R. pachyptila showed a surprisingly high RBR (20 to 50) with density-separated DNA standards. With DNAs obtained from physically separated symbionts, independent solution hybridization experiments confirmed the above-described conclusions. Possible explanations for this host-symbiont homology are discussed.