Rumpho et al. (2008) discovers that Elysia chlorotica, a sea slug living in Atlantic seaboard of The United States, possesses the ability to undergo photosynthesis. Interestingly, this ability is not hereditarily determined but rather acquired from its natural food source- Vaucheria litorea, a multicellular eukaryotic alga ( Rumpho et al., 2008). This also proves the theory that DNA is capable of being shifted from one species to another. Though most of the times, this foreign DNA be destroyed by host’s enzymes, in this case, transferred DNA from alga can function in host’s organism.
Current experimental evidences have proved that the exchange genetic information between two eukaryotes is uncommon ( Rumpho et al., 2008). Insight into how gene exchange occurs between these two species is revealed by carefully observing their genomes. There are two experimental evidences that support this gene exchanging assumption. Firstly, chloroplasts obtained through heterotrophic feeding cannot process photosynthesis successfully alone. Chloroplasts are the sites that photosynthesis occurs to convert sun light into usable energy ( Rumpho et al., 2008) . In this case, the chloroplast’s gene was sequenced and found to be lack of some genes required for photosynthesis ( Rumpho et al., 2008) . Therefore, some essential protein for water-splitting is absent to generate oxygen for photosynthesis ( Rumpho et al., 2008). Secondly, sequenced DNA of sea slugs contains one of the algal gene, which matches exactly the alga’s DNA sequence coding for oxygenated photosynthesis ( Rumpho et al., 2008).
The integration of genes has the same mechanism as phagocytic relations ( Rumpho et al., 2008). After being consumed, algal chloroplasts and the algal nuclei go directly into the slug’s body and contact with the slug’s digestive epithelium ( Rumpho et al., 2008). The chloroplasts are incorporated to the digestive epithelium ( Rumpho et al., 2008). After that, the algal nuclei are broken down, the agal chromosomes will be inserted into some specific organelles of the slug ( Rumpho et al., 2008). The two sites that are likely to attract the foreign genes are nuclear and mitochondrial genes ( Rumpho et al., 2008).
|Figure 1. A sea slug feeding on alga has the|
green-coloured body due to chloroplasts.
There are two other primary explanations are previously proposed for this phenomenon. Firstly, alga develops in sea slugs’ gut so the whole algal DNA as well as chloroplasts are included in the sea slugs’ cells ( Brahic, 2008). Therefore, the genes are then integrated to sea slug’ DNA to enable the organism to produce primary proteins for chloroplasts to process ( Brahic, 2008). Nevertheless, this hypothesis is then eliminated after experiments on slug’s eggs whose pure DNA and RNA are not contaminated by the alga ( Brahic, 2008). Besides, another hypothesis that a certain virus is responsible for the transport of the DNA between the two unrelated species seems to attract a lot of concerns ( Brahic, 2008). Nonetheless, no evidence has found to support that idea ( Brahic, 2008). As both of the above assumptions are rejected, the explanation related to heterotrophic feeding appears to be credible.
In conclusion, sea plugs receive a transferred gene as well as chloroplasts from its prey to acquire an ability of photosynthesis. Thus, slugs can attain new ability to survive for the rest of its life even in the absence of the prey. Rumpho et al. ( 2008) has found that slugs are able to live independently away from alga for at least five months. Besides, the questions of how the new gene is activated and how the proteins in chloroplasts work remain unanswered.
Rumpho, ME, Worful, JM, Lee, J, Kannan, K, Tyler, MS, Bhattacharya, D, Moustafa, A & Manhart, JR 2008, ‘Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica’, The National Academy of Sciences, vol. 105, no.46, pp. 17867-17871, viewed 20 March 2012, < http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2584685/?tool=pmcentrez&rendertype=abstract>
Brahic, C 2008, ‘Solar-powered sea slug harnesses stolen plant genes’, The scientist, 24 November, viewed 19 March 2012, < http://www.newscientist.com/article/dn16124-solarpowered-sea-slug-harnesses-stolen-plant-genes.html>