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 pla n, especially in the anteroposterior patterning of the nervous system, Part icularly interesting are the anterior groups of Hox genes (Hox1-Hox4) since their expression is coupled to the control of regional identity in the ant erior regions of the nervous system, where the highest structural diversity is observed. Ascidians, among chordates, are considered a good model to in vestigate evolution of Hox gene, organisation, regulation and function. We report here the cloning and the expression pattern of CiHox3, a Ciona intes tinalis anterior Hox gene homologous to the paralogy group 3 genes. In situ hybridization at the larva stage revealed that CiHox3 expression was restr icted to the visceral ganglion of the central nervous system. The presence of a sharp posterior boundary and the absence of transcript in mesodermal t issues are distinctive features of CiHox3 expression when compared to the p aralogy group 3 in other chordates, We have investigated the regulatory ele ments underlying CiHox3 neural-specific expression and, using transgenic an alysis, we were able to isolate an 80 bp enhancer responsible of CiHox3 act ivation in the central nervous system (CNS), A comparative study between mo use and Ciona Hox3 promoters demonstrated that divergent mechanisms are inv olved in the regulation of these genes in vertebrates and ascidians.