Despite scalding-hot water and poisonous hydrogen sulfide, deep-sea clams prosper along fissures in the sea bottom. But they can't do it alone: The clams survive because symbiotic bacteria inside their gill cells convert the poisons to nutrients. Now biologists report in today's Proceedings of the National Academy of Sciences that the relationship is so close that the species may be evolving together. Other experts suggest that the bacteria could be losing their identity as a separate species altogether, becoming an essential part of the clam cells.
Symbiosis is common in nature: for example, flowers provide nectar for bees, and the bees pollinate the flowers. Sometimes, two species will become so interdependent that neither can survive without the other. When this happens, one might expect that if the host organism evolves into two species, its symbiont would follow suit--a process called "cospeciation" or "congruent evolution." But if the symbiont can live in more than one host species or can survive on its own, then it may not evolve in step with one host.
According to Andrew Peek, a molecular biologist at the University of California, Irvine, deep-sea clams seemed like a good candidate for cospeciation because neither the clam nor the bacteria that live inside its cells have ever been found apart from one another. Moreover, there is evidence that the clams pass the bacteria from one generation to the next in their eggs. To test the cospeciation hypothesis, Peek analyzed the DNA of nine different clam species and their symbiotic bacteria, then determined statistically the most likely family trees of each organism. Out of 135,135 possible family trees, nine were identified as the most likely ones for the clams and for the bacteria--and they were exactly the same. Thus Peek concludes it is overwhelmingly likely that the bacteria and clams evolved together.
The finding "addresses indirectly the evolution of organelles" within cells, says physiological ecologist Charles Fisher of Pennsylvania State University, University Park. Mitochondria--organelles that power all animal cells--are thought to have once been symbiotes that lost their ability to live outside the cell. The bacteria in the deep-sea clams could be following the same route, he says.