Thursday, December 18, 2008

December 2008


DECEMBER 18, 2008
YOU ARE WHAT YOU EAT
I love how science and myth resonate; here is a little examplar on the interaction between the eaters and the eaten. Mixed creatures, particularly the "human animal", captured the imagination of storytellers and artists of the ancient world and the modern world from the half-horse-figured centaurs to the Twentieth Century Bat Boy.

I have always been a fan of the more imaginary side of that little phrase: "You are what you eat." Eat a horse, and let your imagination roam. Little did I realize a sea slug eating algae would bring the whole shebang into the real. 

The Agri-Science industry has frequently rolled out the argument that eating genetically manipulated (GM) plants or animals runs no risk of a gene transfer from the eaten to the eater. This is an old model relying on the comfort in the integrity of the (human) body. With respect to integrity, a paradigm shift may be nearing. There will occasionally be a science article that foretells of a shift, the below article is of the necessary caliber.

The original observation is of an algae-eating-mollusc (a sea slug) acquiring a gene(s) of the algae and that said gene(s) being expressed in the said mollusc. This event is known in the art as horizontal gene transfer, it is fairly common in lower organisms, e.g., bacteria, but amongst sophisticated creatures (eukaryotes) like algae and molluscs it is an extraordinarily rare event and gets itself on to the front page of the Proceedings of the National Academy of Science (November).

Further, the algae-mollusc construction is a step up the genus ladder for horizontal gene transfer, it might even be regarded as a justification to re-examine human eaters and their eaten.

The Article

Rumphoa et al., Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. PNAS November 18, 2008 vol. 105 no. 46 17867-17871

Abstract

The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria litorea. Organelles are sequestered in the mollusc's digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Two possible explanations for the persistence of photosynthesis in the sea slug are (i) the ability of V. litorea plastids to retain genetic autonomy and/or (ii) more likely, the mollusc provides the essential plastid proteins. Under the latter scenario, genes supporting photosynthesis have been acquired by the animal via horizontal gene transfer and the encoded proteins are retargeted to the plastid. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3Åå flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty ("green animals") and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.

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