Metabolic co-dependence drives the evolutionarily ancient <i>Hydra-Chlorella</i> symbiosis.

Abstract

Many 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 between <i>Hydra viridissima</i> and its photosynthetic symbiont <i>Chlorella</i> sp. A99. We discovered significant up-regulation of <i>Hydra</i> genes 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 symbiotic <i>Chlorella</i>, indicating loss of autonomy and dependence on the host. Genome sequencing of <i>Chlorella</i> sp. A99 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.