Chromosome-level genome assembly of the sacoglossan sea slug Elysia atroviridis.

Abstract

Some sacoglossan sea slugs (Gastropoda:Heterobranchia) possess the remarkable ability to sequester functional chloroplasts into digestive cells. Elysia atroviridis harvests chloroplasts from its algal prey, which are maintained for a relative long period in cells of the digestive tract. We successfully assembled a high-quality chromosome-level genome of E. atroviridis using PacBio and Hi-C sequencing technologies. The final assembly spans 829.0 Mb with contig and scaffold N50 length of 2.0 Mb and 43.9 Mb, respectively, 89.3% of the assembled sequences were anchored to 15 pseudochromosomes. We found no evidence for horizontal gene transfer (HGT), specifically, no photosynthetic genes encoded in the E. atroviridis nucleus genome. A total of 16,472 protein-coding genes were predicted, of which 96.0% were functionally annotated. Phylogenetic analysis indicated E. atroviridis and E. timida formed a clade and their divergence time was estimated approximately 33 million years ago (Mya). Collinearity analysis revealed a high level of synteny with the genome of both species. This genome provides a valuable resource for further investigation into the evolution and mechanisms of kleptoplasty in sacoglossa.