COMPLEX EXPRESSION OF THE ZP-50 POU GENE IN THE EMBRYONIC ZEBRAFISH BRAIN IS ALTERED BY OVEREXPRESSION OF SONIC HEDGEHOG

We report the characterization of the zebrafish zp-50 class III POU do main gene. This gene is first activated in the prospective diencephalo n after the end of the gastrula period. During somitogenesis, zp-50 is expressed in a very dynamic and complex fashion in all major subdivis ions of the central nervous system. After one day of development, zp-5 0 transcripts are present in the fore- and midbrain in several distinc t cell clusters. In the hindbrain, zp-50 expression is found in two ty pes of domains. Correct zp-50 expression in the ventral fore- and midb rain requires genes known to be involved in dorsoventral patterning of the zebrafish CNS. Transcripts of the sonic hedgehog (shh) gene encod ing an intercellular signaling molecule are detected in the forming di encephalon shortly prior to the appearance of zp-50 mRNA. Correct expr ession in this region of both shh, and zp-50, requires a functional cy clops (cyc) locus: shh and zp-50 transcripts are likewise absent from the ventral rostral brain of mutant cyc(-/-) embryos. Injection of syn thetic shh mRNA into fertilized eggs causes ectopic zp-50 expression a t more dorsal positions of the embryonic brain. The close spatial and temporal coincidence of expression in the rostral brain, the similar r esponse to the cyc(-) mutation, and the ectopic zp-50 expression in th e injection experiments all suggest that zp-50 may directly respond to the reception of the Shh signal.