FORK HEAD DOMAIN GENES IN ZEBRAFISH

Nine members of the fork head domain gene family (fkd1-fkd9) were isol ated from early cDNA libraries in the zebrafish. They show unique expr ession patterns in whole-mount RNA in situ hybridization during the fi rst 24 h of embryonic development. These fkd genes fall into three of ten classes, based on sequence similarities within the DNA-binding dom ain, whereas members for the other seven classes described in other ve rtebrates were not found. In addition to conserved residues at certain positions in the fork head domain, characteristic transcription activ ation domains as well as similarities in expression patterns were foun d for members of the different classes. Members of class I (fkd1/axial , fkd2/Zffkh1, fkd4 and fkd7) are differentially transcribed in unsegm ented dorsal axial structures such as the floor plate, the notochord, the hypochord and, in addition, the endoderm. Transcripts of fkd3 and fkd5 (class II) are mainly detected in the cells of the ectoderm which form neural tissues, as is the case for genes of this class in other species. RNAs of the three members of class V (fkd6, fkd8 and fkd9) ar e expressed in the paraxial mesoderm and transiently in the neuroectod erm. fkd6 is strongly expressed in neural crest cells from early stage s on, whereas fkd2 and fkd7 are transcribed in individual neural crest cells in the pharyngula period.