![]() The exchange of genetic material between two eukaryotes is extremely rare, or at least not well documented to date. Most of these latter examples are associated with parasitism or phagotrophy, including the elegant studies of HGT from the α-proteobacteria Wolbachia to insects and nematodes ( 16– 18), and the finding of rhizobial-like genes in plant parasitic nematodes ( 19, 20). As a result of primary and secondary endosymbiosis, plastid genomes (ptDNAs) encode less than 10% of the predicted 1,000 to 5,000 proteins required to sustain the metabolic capacity of the plastid ( 8, 9).Įxamples of HGT between unrelated or nonmating species are abundant among prokaryotes ( 10, 11) but less so between prokaryotes and unicellular ( 12– 14) or multicellular eukaryotes ( 15– 20). In this case, in addition to EGT, transfer of genes between the unrelated organisms by lateral or horizontal gene transfer (HGT) and loss of genes occurred as a result of the “merger” of the two nuclei (host and endosymbiont) ( 7). The diverse group of secondary or “complex” algae (e.g., chromalveolates, euglenids), in turn, arose by secondary endosymbiosis-the uptake of a eukaryotic alga (green or red lineage) by a heterotrophic eukaryotic host. The cyanobacterial genome was greatly reduced by endosymbiotic gene transfer (EGT) to the host nucleus and wholesale gene loss, giving rise to the primary lineages of plants and green algae (streptophytes and chlorophytes), red algae (rhodophytes), and glaucophytes ( 3– 6). This comes from extensive analysis of organelles such as plastids (e.g., chloroplasts) that originated via primary endosymbiosis of a free-living cyanobacterium ( 1, 2). Symbiotic associations and their related gene transfer events are postulated to contribute significantly to evolutionary innovation and biodiversity. 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. 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. litorea because this sequence is identical from the predator and prey genomes. ![]() 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. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. 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. litorea plastids to retain genetic autonomy and/or ( ii) more likely, the mollusc provides the essential plastid proteins. ![]() Two possible explanations for the persistence of photosynthesis in the sea slug are ( i) the ability of V. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Organelles are sequestered in the mollusc's digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. The sea slug Elysia chlorotica acquires plastids by ingestion of its algal food source Vaucheria litorea.
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