Rumpho et al. 2008 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.
http://www.pnas.org/content/105/46/17867


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 3
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.