Photo-autotrophy is restricted to bacteria, algae, and the plant kingdom. Yet, several animals are able to establish a symbiosis with unicellular photo-autotrophic organisms to benefit from photosynthesis. Photosymbiosis is found among sponge, cnidarians, flatworms, mollusks and even a vertebrate. In sea slugs (Gastropoda) two different photosymbiotic associations are found: (1) The Nudibranchia harbor Symbiodinium that they ‘steal’ from corals. (2) The Sacoglossa ‘steal’ only the chloroplasts of their macroalgae food, but keep them active in their own cytosol a unique phenomenon across the animal kingdom called functional kleptoplasty. Some species of these two gastropod taxa are able to overcome starvation periods of several weeks or months. During this time they benefit from their photosymbiont or the kleptoplasts in yet elusive ways. Studies suggest the ability to harbor a functional symbiont in Nudibranchia and functional kleptoplasty in Sacoglossa evolved multiple times, Yet, we lack knowledge on what genomics adaptions are needed that enables these slugs to initially establish the symbiosis or the ‘functional kleptoplasty’. In corals the recognition of the symbiont relies on the presence of specific receptors of the innate immune system. Chemically blocking of these receptors significantly reduces the uptake of the algae symbiont (Symbiodinium) into the corals? cell. In the available transcriptomic datasets of one photosymbiotic nudibranch and three kleptoplastic sacoglossans we were able to identify a set of receptors related to those found in symbiotic corals (appendix). This points towards a similar mechanism in symbiont and/or chloroplast recognition among Nudibranchia, Sacoglossa and corals. Yet, transcriptome sequencing is not necessarily suitable for searching genes: the absence of a transcript in an assembled transcriptome is no proof of absence of the gene in the respective genome. This project aims to understand what genomic adaptions are needed to establish photosymbiosis in Nudibranchia and functional kleptoplasty in Sacoglossa, respectively. We will sequence the genomes of selected members of both taxa using the Oxford Nanopore MinION technique. Comparative analysis of these genomes will help to identify shared adaptions between those taxa and among other animals. Gene clustering analyses including members of several other taxa will be performed to specifically search for factors involved in symbiont recognition such as the mentioned receptors. We will also experimentally test if the chemical blocking of some of these receptors results in reduced symbiont recognition. This project is the first comparative genome-based study on Nudibranchia and Sacoglossa sea slugs and will give us a detailed insight if the genome adaptions for photosymbiosis and functional kleptoplasty are similar among the slugs and with other animals, or if the according genome adaptions evolved convergent.