Metabolic co-dependence drives the evolutionary ancientHydra-Chlorellasymbiosis

AbstractMany multicellular organisms rely on symbiotic associations for support of metabolic activity, protection, or energy. Understanding the mechanisms involved in controlling such interactions remains a major challenge. In an unbiased approach we identified key players that control the symbiosis betweenHydra viridissimaand its photobiontChlorellasp. A99. We discovered significant upregulation ofHydragenes encoding a phosphate transporter and glutamine synthetase suggesting regulated nutrition supply between host and symbionts. Interestingly, supplementing the medium with glutamine temporarily supports in vitro growth of the otherwise obligate symbioticChlorella, indicating loss of autonomy and dependence on the host. Genome sequencing ofChlorellaA99 revealed a large number of amino acid transporters and a degenerated nitrate assimilation pathway, presumably as consequence of the adaptation to the host environment. Our observations portray ancient symbiotic interactions as a codependent partnership in which exchange of nutrients appears to be the primary driving force.