Thursday, November 20, 2008
Evolutionary novelty: Photosynthetic slug
All of biology, from genes to species, is united by common descent. Therefore new biological entities – novelties – must come from the modification of existing entities. Lightening does not strike and impart new features into organisms; new features evolve from existing ones. New research in PNAS provides fascinating new insights into the evolutionary origin of a 'photosynthetic slug'.
Given new features evolve from existing ones, one way novelties originate is through duplication and divergence. Another way is through new combinations of existing biological entities. In fact, biological entities can be recombined at many levels. Protein domains fuse to form new genes, genes become expressed together in new combinations in developmental time or space, even species can merge together to form new species, as occurred at the origin of eukaryotic cells when one species merged with a bacteria that became our cells’ energy factories, the mitochondria.
Imagine if evolution happened to produce a photosynthetic animal, and ask, what are some of the ways it might happen? One likely way is to utilize existing organisms (or their genes) that already have the ability to convert light energy into chemical energy. This is exactly what has happened during the evolution of the gastropod mollusk Elysia chlorotica, a green “sea slug”. Like other types of animal including reef-building corals, E. chlorotica harbors the photosynthetic machinery of other organisms. In the case of reef corals, a symbiotic relationship with dinoflagellates provides photosynthetic ability. But in the green sea slug, only the photosynthetic machinery itself is sequestered, by ingesting an algae, and using the algae’s plastids, the photosynthetic sub-cellular structure of the algae (interestingly, the plastid joined the algal cell in an ancient novel merger of species).
This presents a puzzle. The algae’s plastid, which is being used by the green sea slug for photosynthesis, does not itself contain all of the machinery required for photosynthesis. Instead, many of the photosynthesis genes reside in the algae, only some reside in the plastid. Yet the green sea slug can photosynthesize for months using only the plastid, even in the absence of algae, and therefore in the absence of the algae’s photosynthesis machinery. How is this possible?
The authors found that at least one gene (psbO) is integrated into the genome of the green sea slug. This gene is identical in sequence to an algal gene, yet the sequence adjacent to the gene in the slug analyses make clear that the gene is in the slug’s genome and not an experimental artifact, like contamination.
As the authors indicate, this work raises many interesting questions. How does the slug’s gene target the plastids? What about all the other genes absent from the plastid that are required for photosynthesis – are those transferred to the slug, too? Or could some of the slug’s genes replace the function of the missing genes? What is the specific mechanism for horizontal transfer of genes from one species to another? Clearly, the authors are thinking about these interesting questions, and likely future research will provide us with answers.
I noticed that Carl Zimmer already posted on this article in his fine blog, The Loom
As I wrote in an article with Michael Rose called “The New Biology: Beyond the Modern Synthesis”, acceptance of biological mergers was slow, perhaps because the modern synthesis viewed genomes as sleekly functional, finely tuned to current utility. As such, moving genes from one genome to another seems like it should be suboptimal, and therefore rare. No one doubts the importance of biological mergers any more, but they are still fascinating and under-documented, in part because they were neglected for so long.
*One reason I write blog entries is to present my research interests and ideas to a potentially broader audience. As such, I like to like papers of mine when possible.
M. E. Rumpho, J. M. Worful, J. Lee, K. Kannan, M. S. Tyler, D. Bhattacharya, A. Moustafa, J. R. Manhart (2008). From the Cover: Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica Proceedings of the National Academy of Sciences, 105 (46), 17867-17871 DOI: 10.1073/pnas.0804968105