Throughout the biosphere organisms form intimate relationships with each other, and symbiosis occurs when such relationships are essential for the persistence of both partners. Symbiosis has been one of the fundamental guiding principles of life for a billion years or more, but the mechanisms by which these relationships form and persist are barely known. Marine sponges are ancient multicellular organisms that form symbiotic relationships with microorganisms. When this symbiosis fails, for example due to environmental stress, then the host sponge dies. This project will investigate a proposed new molecular mechanism, which has allowed for the evolution of this ancient symbiosis. We postulate that microbial symbionts have in the past acquired genes from their sponge host and now use these genes to avoid their own digestion by the sponge. This process would thus allow for the symbiosis to persist. The results of this project will not only provide a mechanistic explanation for the symbiosis between marine sponges and their resident microorganisms, but will also provide insight into the evolution of symbiotic systems in general.