Evolution of Vanadium-Binding Protein (Vanabin) Gene Structure in Phlebobranchia and Aplousobranchia Ascidian Lineages

Ascidians within order Phlebobranchia can accumulate extremely high levels of vanadium (V) in their blood cells. Several V-related proteins, including V-binding proteins (vanabins), have been isolated from V-accumulating ascidians. Vanabins were first isolated from a V-rich ascidian, Ascidia sydneiensis samea, belonging to Phlebobranchia. In this study, we searched for vanabin gene homologs in all available public databases containing tunicate genomic data and the draft genome database of A. sydneiensis samea constructed in this study. We found one to six vanabin genes per genome among all available genomes of Phlebobranchia species. We also found a vanabin gene in the genome of an Aplousobranchia species. Thus, we identified vanabins in seven ascidian species, and aligned these according to the C–Xn–C pattern common to the vanabin core domain. Based on a molecular phylogenetic tree derived from 18S ribosomal DNA and natural resistance-associated macrophage protein sequences, we propose a model for the evolution of vanabin gene structure via the following pathway: acquisition of ancestral vanabin by the common ancestor of Phlebobranchia and Aplousobranchia, followed by vanabin gene duplication; nonsynonymous mutation, insertion, and positive selection to create amino acid variation among vanabins; and extension of the vanabin gene cluster. The selective force acting within this pathway may have been V toxicity under high-V conditions.