Transcriptomic landscape of the kleptoplastic sea slug <i>Elysia viridis</i>

The longevity of kleptoplasts and sea slugs during starvation may be mediated by multiple factors, including the recognition of the plastids during feeding by multiple receptors (e.g. PRRs, CTLRs and SRs) and ROS-quenching proteins using enzymatic and nonenzymatic mechanisms. In particular, in the transcriptome of E. viridis we found that the presence of CDSs corresponds to multiple PRRs that may be involved in the plastid-recognition process; this is despite the fact that this species has a low receptor richness in comparison with other elysoids (Melo Clavijo et al., 2020). In addition, we also detected multiple enzymatic families involved in the ROS-quenching response. In contrast, the production of antioxidant compounds may contribute in only a minor way to the control of oxidative stress. A further enriched GO category in species that sequester chloroplasts corresponded to G protein-coupled receptors, which suggests that these receptors may be required for plastid recognition in Sacoglossan sea slugs, paralleling their role in other symbioses, such as the mutualism between cnidarians and dinoflagellates (Rosset et al., 2020). Sacoglossan sea slugs may also require the presence of iron ions to reduce the oxidative stress generated after plastid acquisition. All this evidence, derived from the transcriptome analysis of E. viridis, sheds interesting new light on the possible mechanisms used by sea slugs to recognize and establish kleptoplasts within their bodies.