Genome assembly of the acoel flatworm Symsagittifera roscoffensis, a model for research on photosymbiosis

Symsagittifera roscoffensis is a well-known member of the order Acoela that lives in symbiosis with the algae Tetraselmis convolutae during its adult stage. Its natural habitat is the eastern coast of the Atlantic, where at specific locations thousands of individuals can be found lying in large pools on the surface of sand at low tide and in the sandy interstitial web at high tide. As a member of the Acoela it has been used as a proxy for early bilaterian animals; however, its phylogenetic position remains debated. In order to understand the basic structural characteristics of the acoel genome, we sequenced and assembled the genome of aposymbiotic S. roscoffensis. The size of S. roscoffensis genome was measured to be in range 910 - 940 Mb. Sequencing of the genome was performed using PacBio Hi-Fi technology. Hi-C and RNA-seq data were also generated to scaffold and annotate the genome. The resulting assembly is 1.1 Gb large (covering 118% of the estimated genome size) and highly continuous, with N50 scaffold size of 1.04 Mb. The repetitive fraction of the genome is 61%, of which 85% (half of the genome) are LTR retrotransposons. Genome-guided transcriptome assembly identified 34,493 genes, of which 29,351 are protein coding (BUSCO score 97.6%), and 30.2% of genes are spliced leader (SL) trans-spliced. The completeness of this genome suggests that it can be used extensively to characterize gene families and conduct accurate phylogenomic reconstructions. SignificanceSymsagittifera is a representative of the phylum Acoela, the first offshoot of bilaterian animals. This key phylogenetic position adds an extra value to the knowledge of its genome, since it will inform us on how the genome of a bilaterian ancestor might have looked like. Moreover, Symsagittifera roscoffensis is a model organism used in symbiogenesis research. Host and algae can be cultured independently and, after mixing, the symbiosis can be followed. Symbiogenesis was established early on during the evolution of Metazoa. In spite of its biological relevance, very little is known on the molecular mechanisms that control it. Here the genome of the acoel host should provide us with insights on the first adaptations to symbiogenesis occurring in bilateral animals.