Tunicates are filter feeding 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. Currently published tunicate genomes include three Phlebobranchiae (sessile solitary benthic), one Thaliacean (planktonic pelagic solitary), one Larvacean (planktonic pelagic solitary) and one Stolidobranchian (sessile colonial benthic). To gain further insights into evolution of the tunicate phylum, we have sequenced the genome of the colonial ascidian Botrylloides leachii.
The predicted B. leachii genome size of ~194 Mb is much smaller than the Botryllus schlosseri genome, estimated to be ~750 Mb, but is similar to Ciona intestinalis (160 Mb). This difference is largely due to an increase in repetitive DNA content between B. leachii (17%) and B. schlosseri (65%). Analysis of homeobox genes typically found in gene clusters, identified many examples of multiple cluster breaks and gene dispersion, suggesting several lineage-specific genome rearrangements occurring during tunicate evolution. These findings further support that this subphylum undergoes uniquely rapid genetic evolution.
In addition, we have found lineage-specific gene gain and loss within the Wnt, Notch and retinoic acid pathways. These diverse changes to key conserved biological pathways are major examples of the genetic strategies yielding the diverse biology observed in the tunicate phylum. Altogether, the B. leachii genome provides is a new resource for the understanding of chordate evolution.