Induction of anterior neural fates in the ascidian Ciona intestinalis

The sensory vesicle of ascidians is thought to be homologous to the vertebr ate forebrain and midbrain (Development 125 (1998) 1113). Here we report th e isolation of two sensory vesicle markers in the ascidian Ciona intestinal is, which are homologs of vertebrate otx and gsx homeobox genes. By using t hese markers to analyze the induction of anterior neural tissue in Ciona, w e find that the restriction of anterior neural fate to the progeny of the a nterior animal blastomeres is due to a combination of two factors. The vege tal blastomeres show a differential inducing activity along the anterior-po sterior axis, while the competence to respond to this inducing signal is ma rkedly higher in the anterior animal blastomeres than in the posterior anim al blastomeres. This differential competence to respond is also observed in response to bFGF, a candidate neural inducer in ascidians (J. Physiol. 511 .2 (1998) 347) and can be detected by the gastrula stage. Our results, howe ver, indicate that bFGF can only induce a subset of the responses of the en dogenous inducer, suggesting that additional signals in the embryo are nece ssary to induce st fully patterned nervous system. (C) 2001 Elsevier Scienc e ireland Ltd. All rights reserved.