Acquiring a Symbiont
Symbionts must be passed from one generation of their host to the next in order to be effective. In the case of Riftia pachyptila, for many years this process was misunderstood. Scientists knew that the symbiont had to re-infect each new generation; that it was not passed directly from the adults to their offspring. But they did not understand how this could be possible, since no one had been able to raise Riftia’s symbiont outside of Riftia.
RNA sequence analysis revealed that Riftia’s symbiont was closely related to other free living chemosynthetic bacteria (Stahl et al. 1984). Researchers continued investigate the bacteria themselves, attempting to understand its physiology, and eventually to raise them sans Riftia. Direct evidence that this symbiont could live free in the environment was provided by the Millikan et al. in 1999. They were able to find, and prove, the symbiont expressed, a gene for flagella. Flagella are necessary for single cell bacteria to move, and motion is essential to find and colonize juvenile Riftia. This finding supported the hypothesis that Riftia were re-colonized with each new generation. Finally, in 2008, scientists managed, though PCR amplifications and DNA sequence analysis, to find free living Riftia symbionts at the hydrothermal vents (Harmer et al. 2008).
Armed with this knowledge, scientists began studying the anatomy of the juvenile Riftia pachyptila to determine when exactly they gained the essential symbiont, and how. They first noted that juvenile Riftia, unlike their parents, possessed a digestive tract (Jones and Gardiner 1989). This led to a hypothesis that the baby Riftia would ingest their symbiont in the course of normal feeding. They thought the symbiont would somehow evade digestion, infect the walls of the digestive tract, and begin converting gut tissue into the highly specialized troposome (Gardiner et al. 1991). When they actually observed the development of Riftia larvae, scientists discovered that the symbionts first infect the skin and then work their way inward. The tissue of the larval Riftia responds to their invasion by creating the specialized troposome around the symbiont. When enough symbionts have become established, the juvenile Riftia undergoes massive apoptosis, sheds its temporary digestive system, and becomes dependent on the symbiont (Nussbaumer et al. 2006).
Credit: Andrea D. Nussbaumer, Charles R. Fisher and Monika Bright
Riftia pachyptila tubeworm larva, about 200 micrometers large.