Patterning the ascidian nervous system: structure, expression and transgenic analysis of the CiHox3 gene.

Hox genes play a fundamental role in the establishment of chordate body plan, especially in the anteroposterior patterning of the nervous system. Particularly interesting are the anterior groups of Hox genes (Hox1-Hox4) since their expression is coupled to the control of regional identity in the anterior regions of the nervous system, where the highest structural diversity is observed. Ascidians, among chordates, are considered a good model to investigate evolution of Hox gene, organisation, regulation and function. We report here the cloning and the expression pattern of CiHox3, a Ciona intestinalis anterior Hox gene homologous to the paralogy group 3 genes. In situ hybridization at the larva stage revealed that CiHox3 expression was restricted to the visceral ganglion of the central nervous system. The presence of a sharp posterior boundary and the absence of transcript in mesodermal tissues are distinctive features of CiHox3 expression when compared to the paralogy group 3 in other chordates. We have investigated the regulatory elements underlying CiHox3 neural-specific expression and, using transgenic analysis, we were able to isolate an 80 bp enhancer responsible of CiHox3 activation in the central nervous system (CNS). A comparative study between mouse and Ciona Hox3 promoters demonstrated that divergent mechanisms are involved in the regulation of these genes in vertebrates and ascidians.