Algal genes in Elysia crispata

May 2, 2003
From: Skip Pierce

Note added 30 May 2006: This is most probably Elysia clarki, a new species which was previously considered a colour form of E. crispata

NOTE: the paper referred to below is now published:
• Pierce, S.K., Massey, S.E., Hanten, J.J. &. Curtis, N.E. (2003). Horizontal Transfer of Functional Nuclear Genes Between Multicellular Organisms. Biological Bulletin, 204: 237-240.

Hi Bill
A while back you invited me to say a word about my research program. I didn't answer because we were sort of in mid experiment. You might be interested to know that, after a few years of searching, we (well, my post-doc Steve Massey and a graduate student, Nick Curtis, who get to work in the lab while I do administrative duties) have developed some pretty solid molecular evidence showing at least one gene that codes for a chloroplast protein is located in the genomic DNA of a sacoglossan slug. The paper reporting the results has been accepted and will be out in the June issue of the Biological Bulletin. As you know many species of Elysia eat particular species of coenocytic algae and certain cells that line the digestive diverticula capture large numbers of intact chloroplasts as the eaten algae passes through the gut. So many plastids are captured that the slugs are green—in fact the density of chloroplasts in the slug is often greater than in the algae. The plastids reside in the cytoplasm of the slug cell, in some species for months, continuing to photosynthesize. So, if you shine light on the slug, it fixes carbon and produces oxygen as if it were a plant. To make a long story short, continued photosynthesis requires the synthesis of certain chloroplast proteins, many of which are coded in the algal nuclear genome. We had obtained a bunch of circumstantial evidence that genes for some of the light harvesting proteins of chloroplasts from the chromophytic alga, Vaucheria litorea, were being synthesized on cytoplasmic ribosomes in Elysia chlorotica, implying that the genes were in the genomic DNA of the host cell. Since finding the genes for those particular proteins was proving problematical for a variety of reasons (lack of algal sequence data, severe lack of sequence conservation, no amino acid sequence data, etc), we began looking at other species, in particular, Elysia crispata. There are several reports in the literature about what alga this species eats, all of which are wrong, I think (we'll publish on that issue later). However, after doing a variety of experiments, which are described in the Biol. Bull. paper, we were able to locate a gene for a chloroplast protein, fucoxanthine-chlorophyll binding protein, in the E. crispata DNA. Some species of Elysia, like E. rufescens, for example, do not seem to be able to maintain the plastids for more than a few days—so you might not expect that algal genes have been transferred in them—but species like E. chlorotica, E. viridis, E. crispata among others, that keep the plastids active for months may have a variety of algal nuclear genes present in the slug DNA. If that turns out to be the case, these slugs may turn out to be important models of not only endosymbiosis, but for things like genetic therapy (although so far, NIH has shown no interest) and mechanisms of evolution. We have some ideas about the mechanism of gene transfer-but no data.

I have included an image of E. crispata that was collected in the Florida Keys, approx. 3cm long.

Skip

pierce@chuma1.cas.usf.edu

Pierce, S.K., 2003 (May 2) Algal genes in Elysia crispata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/9830

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