Transcriptome landscape of kleptoplastic sea slug <i>Elysia viridis</i>

Certain sacoglossan sea slugs can sequester and maintain photosynthetically active chloroplasts through algae feeding, a phenomenon called kleptoplasty. The period while these plastids remain active inside the slug’s body is species- and environment-dependent and can span from a few days to more than three weeks. Here we report for the first time the transcriptome of sea slug Elysia viridis (Montagu, 1804), which can maintain kleptoplasts for more than two weeks and is distributed along all the Atlantic European coastline. The obtained transcriptome of E. viridis comprised 12,884 protein-coding sequences (CDS). The shortest one was 261bp, and the longest 8,766bp; the whole transcriptome has a total length of 9.3Mb (Table S4 and Fig. S2). Analysing these CDS, we identified 9,422 different proteins, with best hits mainly from two genera: Elysia (87.2%), and Plakobranchus (11.0%) (Fig. S2); the other 2.3% corresponded to multiple genera of sea slugs and snails (Tectipleura) (Kano et al., 2016). We got the functional annotation (Gene Ontologies, GO) corresponding to 9,333 CDS: 4,755 CDS associated with 2,583 Biological Process (BP); 5,466 CDS linked to 683 Cellular Components (CC); and 6,693 related to 1,606 Molecular Functions (MF). We identified 201 CDS related to response to stress (GO:0006950) and 10 CDS associated with the regulation of response to stress (GO:0080134). Focussing on the ROS-quenching toolkit, we found 24 CDS related to oxidoreductase complex (GO:1990204) and 560 annotated with oxidoreductase activity (GO:0016491) acting in a large number of donors, e.g., CH-OH, CH=O, C=O, CH and CH2. In addition, we found 39 CDS with antioxidant activity (GO:00162099) and other CDS with ROS-quenching function: superoxide dismutase (GO:0004784), peroxidase (GO:0004601), glutathione oxidoreductase (GO:0097573) and peroxidase (GO:0004602); and thioredoxin peroxidase (GO:0008379) activity. Furthermore, we found 8 CDS related to the symbiont response (GO:0140546) and nine related to the pattern recognition receptor signalling pathway (GO:0002221).