The Hox gene cluster controls spatial patterning mechanisms along the anterior/posterior axis of bilateral metazoans. There exists a co-linearity between the order of the Hox genes in the genome and the spatial order of their domains of expression during development. Echinoderms have pentamerous radial body plans that are different from those of other deuterostomes. Thus expression patterns of echinoderm Hox genes in the adult rudiment might illuminate their evolutionary transformations from bilateral to radial structures. Since Peronella japonica is a direct-type developer that metamorphoses in 3 days without feeding, it provides a good system in which to analyse Hox gene expression in the rudiment.

We PCR-amplified the Hox-type sequences of P. japonica using genome DNA and cDNA prepared from larval RNAs as a template. We used two pairs of degenerated primers that corresponded to the first and third helices of the homeodomain of the Hox genes. As a result we obtained 13 Hox-type sequences, named Pj1–Pj13 in order of their determination. All the sequences were detected by RT-PCR, suggesting that they are being expressed in larvae (Table 1).