There are some well known examples of horizontal gene transfer. Retroviruses have inserted massive amounts of their genes into host genomes and bacteria of different species are well know for swapping genes. Similarly, mitochondria and chloroplasts in eukaryotic cells, which are likely the result of ancient endosymbiosis, retain a small independent genome, but have transferred the majority of their genes to the host nucleus. However, this process was previously unheard of in multicellular organisms. Enter the Gastropod Mollusk, Elysia chlorotica, which lends biological truth to its nickname, “the crawling leaf”.
Many elysiid slugs, like E. chlorotica, feed predominately on a single species of algae (this is a important). As they feed, they use their specialized radula mouthparts to split open the algae filaments and suck the contents into their digestive system. There, the chloroplasts from the algae are phagocytized (enveloped) by the slug’s stomach cells and stored in vesicles. Now within the slug’s cells, the chloroplasts actually continue to carry out photosynthesis, sometimes for the rest of the slug’s life. It has even been shown that the slugs, laden with chloroplasts, can survive only on carbon dioxide and light.
The question here immediately becomes; how is the slug maintaining the chloroplasts? In plants, chloroplasts were independent photosynthetic cyanobacteria taken up by the original plant cells. As endosymbiosis evolved between the eukaryotic plant cells and the cyanobacteria (which became chloroplasts), most of the cyanobacteria’s genes were relocated to the plant cell’s nucleus. Therefore, though chloroplasts retain a few crucial genes in their own genome, most of the genes required for their functionality and maintenance currently reside in the plant cell nucleus. They should not be able to survive for long periods within the slug’s cells because they no longer have access to their care-taking genes in the algae genome.
One hypothesis was that the slug also absorbed a supply of materials for upkeeping the chloroplasts when it ate the algae. However, researchers have surprisingly been able to sequence several plant genes from within the slug’s genome. Most recently, they have even shown that the slugs are able to independently synthesize chlorophyll-a, a crucial component of photosynthesis in the chloroplasts. Amazingly, these slugs have somehow incorporated some of the genes necessary for chloroplast upkeep from algae into their own genomes. This is a completely stupefying discovery, and sorting out the mechanism of this horizontal gene transfer should keep researchers hard at work for a while.
Oh yeah, remember how I mentioned the importance of these slugs eating only one species of algae? Well, that is probably because the slug has only incorporated the chloroplast upkeep genes form that one species of algae into its genome. Therefore, in order to take advantage of this evolutionary adaption, they need to consume chloroplasts form exactly the same species of algae.
Pierce, S.K. et al., 2003. Horizontal Transfer of Functional Nuclear Genes Between Multicellular Organisms. Biol Bull, 204(3), 237-240.
Pierce, S.K. et al., 2007. Transfer, integration and expression of functional nuclear genes between multicellular species. Symbiosis, 43, 57–64.