Hox genes of the direct-type developing sea urchin Peronella japonica

Peronella japonica is a direct-type developing sea urchin that metamorphoses in 3 days without feeding, providing a good system to analyze developmental mechanisms especially at later larval stages to metamorphosis. We surveyed the Hox genes of P. japonica by PCR and RT-PCR, and isolated 13 Hox-type sequences. Phylogenetic analysis and database searches resulted in identification of all the P. japonica orthologs of the Hox genes that have been reported in other sea urchins, plus 3 Hox-type genes: the Evx-type, the Xlox-type genes, and an additional posterior Hox gene. This is the first report to show the Evx in echinoderms. The present results revealed the putative Hox gene cluster of P. japonica, which contains three anterior, four medial, and four posterior genes plus the Evx. The organization is essentially similar to the vertebrate Hox cluster except for only a single gene for Hox4-5 types. The C-terminal amino acid sequence outside the homeodomain of Hox4/5 suggested that loss of Hox4 might have occurred in the echinoderm lineage