Distribution and evolution of the serine/aspartate racemase family in invertebrates. II. Frequent and widespread parallel evolution of aspartate racemase.

Our previous studies showed that invertebrate animal serine racemase (SerR) and aspartate racemase (AspR) evolved from a common ancestral gene and are widely distributed. However, the overall molecular evolutionary background of these genes has remained unclear. In the present study, we have cloned, expressed and characterized five SerR and three AspR genes from six invertebrate species. The coexistence of SerR and AspR paralogs has been observed in some species, and the presence of both SerR and AspR is here confirmed in the flatworm Macrostomum lignano, the feather star Anneissia japonica, the ark shell Anadara broughtonii and the sea hare Aplysia californica. Comparison of the gene structures revealed the evolution of SerR and AspR. The ancestral species of metazoans probably had a single SerR gene, and the first gene duplication in the common ancestor species of the eumetazoans occurred after the divergence of porifera and eumetazoans, yielding two SerR genes. Most eumetazoans lost one of the two SerR genes, while the echinoderm A. japonica retained both genes. Furthermore, it is clear that invertebrate AspR genes arose through parallel evolution by duplication of the SerR gene followed by substitution of amino acid residues necessary for substrate recognition in multiple lineages.