Vaucheria litorea coevolution with sacoglossans

July 16, 2001
From: Matthew Crouch

Dear Dr. Rudman,
First, I would like to thank you for the site. It is very thorough and presents the material in a format useful to all interested parties, from seasoned researchers to the curious high school student.

I cannot believe that the presence of symbiotic chloroplasts in sacoglossans and similar organisms is not routinely discussed in relation to endosymbiotic theory (at least not in texts I have read). It would seem to be rather ...compelling... evidence supporting the theory. Second, although I am relatively new to this fascinating topic, I offer a few thoughts/questions on the evolution of the algae V. litorea. [see Liz Summer's message for photo.]

The Sacoglossans clearly benefit from the ability to incorporate chloroplasts into their metabolic pathways. Peculiarities in certain of the food species raise the possibility that the plants have been selected to facilitate this symbiosis as well. Dr. Rumpho has presented evidence concerning the unusual durability of the chloroplasts of V. litorea within the body of Elysia chlorotica with respect to other plant species. As described in a recent publication, the chloroplasts remained physically intact and continued to translate proteins for a rather unusual amount of time. This seems rather curious -- why should this particular plant species produce chloroplasts that are so much more stable in hostile environments (like those found in the interior of the sacoglossans)?

It is possible that the sacoglossans have taken advantage of the plant that produces the chloroplasts ideally suited for uptake and symbiosis. However, it also seems possible that the plants have seen selected to produce durable chloroplasts. My reasoning is that the feeding behavior of local slugs might be greatly reduced after they had incorporated symbiotic chloroplasts. Of course, the reverse could be true as well, but IF the plants could sate their predators and then survive, they would do much better than their neighbors who were being devoured by much more hungry Sacoglossans. Because of the benefits to the predators, the adaptation would be driven by pressure form both sides. The adaptation may have been forced by selective pressure from both species.

Anybody here know the relevant information concerning the feeding behavior of the sacoglassans before and after incorporating symbiotonts? It seems like feeding behavior would be wasteful after it was no longer needed (Being ignorant about sea slug physiology, I am assuming that the animal raches a maximum size and that its nutritional requirements therefore peak).

Most predictions I can think of would be the same no matter which way you look at it (one would expect the other prey plants to produce similarly durable chloroplasts etc). However, considering the evolution of the symbiosis an adaptation of the plant or multiple species (slug and prey) rather than merely one of the slug might lead to novel predictions or questions, especially concerning gene transfer from the plants to the slugs.

Would chloroplasts be coded to produce additional copies of the genes in the plant nucleus required for the operation and survival of the chloroplasts? (perhaps in a manner similar to plasmids in bacteria?) Perhaps the plants contain copies of the necessary genes or large quantities of very stable proteins coded by said genes in the cytoplasm itself , which are absorbed along with the chloroplasts? (evidence in Green et al, 2000). Would they be built to better resist digestive behaviour - a weird alteration of the exterior membrane maybe? Could genes be isolated in the plant which give such protection? Above all else, are there any other ways in which chloroplasts isolated from the prey species of the sacoglossans are different from other chloroplasts?

Again, my ignorance should be obvious. I doubt any of the above suggestions sounds feasible to the expert ear, but I wanted to offer the thought just in case it might be valuable and maybe stimulate a little thought about possibilities which might not be so farfetched.

Sincerely,
Matt Crouch

esperandus@hotmail.com

Crouch, M., 2001 (Jul 16) Vaucheria litorea coevolution with sacoglossans. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4814

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