De novo draft assembly of the Botrylloides leachii genome provides further insight into tunicate evolution.

Abstract

Tunicates are marine invertebrates that compose the closest phylogenetic group to the vertebrates. This chordate subphylum contains a particularly diverse range of reproductive methods, regenerative abilities and life-history strategies. Consequently, tunicates provide an extraordinary perspective into the emergence and diversity of chordate traits. To gain further insights into the evolution of the tunicate phylum, we have sequenced the genome of the colonial Stolidobranchian Botrylloides leachii.\n\nWe have produced a high-quality (90 % BUSCO genes) 159 Mb assembly, containing 82 % of the predicted total 194 Mb genomic content. The B. leachii genome is much smaller than that of Botryllus schlosseri (725 Mb), but comparable to those of Ciona robusta and Molgula oculata (both 160 Mb). We performed an orthologous clustering between five tunicate genomes that highlights sets of genes specific to some species, including a large group unique to colonial ascidians with gene ontology terms including cell communication and immune response.\n\nBy analysing the structure and composition of the conserved gene clusters, we identified many examples of multiple cluster breaks and gene dispersion, suggesting that several lineage-specific genome rearrangements occurred during tunicate evolution. In addition, we investigate lineage-specific gene gain and loss within the Wnt, Notch and retinoic acid pathways. Such examples of genetic change within these highly evolutionary conserved pathways commonly associated with regeneration and development may underlie some of the diverse regenerative abilities observed in the tunicate subphylum. These results supports the widely held view that tunicate genomes are evolving particularly rapidly.