Elysia Chlorotica: A Novel System for the Elucidation of Horizontal Gene Transfer, Invertebrate Developmental Biology and Secondary Metabolites
Author(s)Worful, Jared M.
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AbstractInvestigations of new and unusual organisms serve as an opportunity to discover novel biochemical and evolutionary adaptations. Elysia chlorotica is a sacoglossan mollusc (sea slug) which represents a biologically intriguing and scientifically relevant research opportunity. Early studies on Elysia sp. led to the discovery of functional algal (Vaucheria litorea) chloroplasts (kleptoplasts) within the cells lining the digestive track of this unusual organism. The endosymbiotic association between the kleptoplasts and the sea slug has evolved into an obligate, but not hereditary association. Despite the semiautonomous nature of chloroplasts and the absence of any algal nucleo-cytosol in the sea slug, the kleptoplasts remain functional for months providing reduced carbon to the animal from photosynthetic carbon fixation (as if it were a plant). The mystery that surrounds an animal that looks and lives like a plant, has spurred questions from young and old alike. This study sought to remove part of the mystery related to the source of algal nuclear genes encoding essential chloroplast proteins in the sea slug. Here we report the presence of and expression of the algal nuclear gene psbO in E. chlorotica. This work helps set the foundation for the possibility of extensive horizontal gene transfer (HGT) between two very distantly related multi-cellular eukaryotes. Sequencing of an E. chlorotica EST library using 454 GS-FLX technology has provided a reference base of transcribed genes. Bioinformatic analysis has yielded a small set of genes which may have been transferred from a photosynthetic donor, been acquired by E. chlorotica, and targeted to the kleptoplasts within the sea slug. These findings reinforce the necessity for sequencing the E. chlorotica genome to verify putative HGT targets, study the mechanism(s) of integration and expression of the foreign genes, and aid in the characterization of the EST library. A laboratory culturing strategy for E. chlorotica was developed enabling the study of the developmental biology of the association as well as the possibility to examine the mechanisms involved in establishing and maintaining functional kleptoplasty. The increased availability of animals represents an avenue of expanded research and teaching opportunities, and perhaps also a profitable enterprise for a willing entrepreneur. Along these lines, a feasibility study was completed showing that consumers are interested in purchasing cultured E. chlorotica and paying a nominal fee. Finally, Elysia sp. have been the target of secondary metabolite studies due to the presence of kahalalides which exhibit a variety of potent pharmaceutical effects. While E. chlorotica collected from Halifax, NS, was not found to contain any kahalalides in this study, it was shown to contain a known chemical, loliolide, which is a typical constituent of plants and results from the degradation of xanthins. Loliolide may serve as an insect repellent, germination inhibitor and a possible antimicrobial agent. Further studies will aid in the elucidation of the complex biology and evolutionary adaptations of E. chlorotica potentially leading to the evolution of inheritable photosynthesis in a metazoan. This unique organism will undoubtedly provide beneficial insight into new scientific concepts and potential commercial applications.