Phylogenetic correspondence of the body axes in bilaterians is revealed by the right-sided expression of Pitx genes in echinoderm larvae.

Chordates and echinoderms are two of the three major deuterostome phyla and show conspicuous left-right (LR) asymmetry. The establishment of LR asymmetry has been explored in vertebrates, but is largely unknown in echinoderms. Here, we report the expression pattern of genes that are orthologous to the chordate left-side specific gene Pitx, cloned from the sea urchin Hemicentrotus pulcherrimus (HpPitx) and the starfish Asterina pectinifera (ApPitx). HpPitx transcripts were first detected bilaterally in one cell of the ventrolateral primary mesenchyme-cell aggregate of early prism larvae. New expression was detected asymmetrically in the right counterpart of a bilateral pair of mesodermal coelomic pouches and in the right ectoderm. In starfish bipinnaria larvae, the ApPitx signal was detected in the right coelomic pouch and in the right half of the ectoderm along the ciliary bands. These results suggest that the function of Pitx in establishing LR asymmetry was introduced in the last common ancestor of echinoderms and chordates. However, the right-side specific expression in echinoderm larvae is inverted compared to chordate embryos. This indicates that the LR axis is inversely represented between echinoderms and chordates, which supports the scenario that dorsoventral axis inversion was introduced into the chordate lineage by turning upside down.