Draft hybrid genome assembly of the photosynthetic sacoglossan Elysia crispata provides insights into sacoglossan biology

We present here a draft genome of the photosynthetic sacoglossan sea slug, Elysia crispata (ecotype crispata ) generated with a reference guided approach using Illumina short reads and long PacBio HiFi reads. The genome size is approximately 671 Mb with an N50 of 460 kb. BUSCO genome completeness was 98.2%, comprising 97.5% single-copy complete and 0.7% duplicated complete metazoan core genes, with an additional 0.7% partial core genes. This genome provided interesting observations such as the possible remnants of an endogenous retrovirus and the presence of polyketide synthesis proteins thought to protect the slug from photosynthetic damage, based on homology with characterized sacoglossan PKS proteins. Few genomes of photosynthetic sacoglossans are available and this draft genome of the E. crispata sea slug will serve as a valuable dataset to study sacoglossan biology, genetics, evolution, and photosymbiosis. In addition, our assembly strategy may serve as a guide to other researchers who also do not have Hi-C data to assemble their genomes, though this approach relies on the availability of a closely related reference genome for scaffolding.