Photosynthetic Slugs

Animals that contain photosynthesizing algae within their bodies are by no means new to science. Perhaps the best known of these are the corals. The slug Elysia chlorotica is entirely different from those examples because rather than incorporating living algae into its tissues, it digests the algae and incorporates only the chloroplasts. Throughout its lifetime, it maintains the photosynthetic machinery within the chloroplasts in working order. Moreover, once the slug has eaten a healthy dose of chloroplasts, it never has to eat again. From then on, it can satisfy its energy needs by sunbathing.

One aspect of this that had puzzled scientists is that chloroplasts don’t last indefinitely. They must be re-supplied with chlorophyll and other compounds that are encoded in the algal nuclei. How were the slugs able to overcome this problem?

Mary Rumpho of the University of Maine sequenced DNA from the chloroplasts, confirming that the plastids did not contain the genes necessary to maintain themselves. She then looked at the slug’s genome and found that it contains at least one algal gene, and that this gene is present in the slug’s germline cells. In other words, although it is not clear how the slug got this gene (and suggestions have included viral transmission), it appears to be there to stay.

Not only is the gene present, but it is also active. Sidney K. Pierce of the University of South Florida in Tampa used a radioactive tracer to confirm that the slugs were in fact making their own chlorophyll.

Although gene transmission is common between types of bacteria, this could be the first case of horizontal gene transfer between plants and animals, a revolutionary finding.