Homeobox genes exhibit evolutionary conserved regionalization in the central nervous system of an ascidian larva

Animals in each subgroup of the phylum Chordata exhibit a similar process by which they form a tubular central nervous system (CNS). However, little is known about spatial relationship among the CNSs of chordates; vertebrates, cephalochordates and urochordates (tunicates). Ascidians constitute a major animal group in the subphylum Urochordata. In the present study, we examined the expression patterns of labial and orthodenticle related genes of the ascidian, Halocynthia roretzi, in the developing larval CNS. These homeobox genes exhibited region-specific expression patterns that are strikingly similar to those of murine Hoxb-1 and Otx2. The regionalization as characterized by the expression of these genes supports the division of the ascidian larval CNS suggested by the previous morphological studies. Furthermore, conservation of the expression pattern of the homeobox genes suggests that such regionalization occurred in the CNS of a putative common ancestor of chordates.