THE VENTRAL AND POSTERIOR EXPRESSION OF THE ZEBRAFISH HOMEOBOX GENE EVE1 IS PERTURBED IN DORSALIZED AND MUTANT EMBRYOS

We have identified and characterized zebrafish eve1, a novel member of the Drosophila even-skipped (eve) gene family. eve1 RNAs are expresse d initially in late blastulae with a peak during the gastrula stage, a t which time expression is confined to ventral and lateral cells of th e marginal zone of the zebrafish embryo. Later, eve1 transcripts are l ocated in the most posterior part of the extending tail tip. We show t hat LiCl, known to dorsalize Xenopus embryos, has the same effect in z ebrafish, resulting in embryos with exaggerated dorsoanterior structur es. In LiCl-treated embryos, eve1 transcripts are completely absent. e ve1 is therefore a marker of ventral and posterior cells. In the light of its ventroposterior expression domain, the localization of eve1 tr anscripts was analysed in spadetail (spt) and no tail (ntl), two mutan ts with abnormal caudal development. In spt(b140) homozygous mutants, there is an accumulation of cells in the tail region, resulting from i nadequate migratory behaviour of precursors to the trunk somites. Thes e cells, in their abnormal environment, express eve1, emphasizing the correlation between ventroposterior position and eve1 expression. In h omozygous mutant embryos for the gene ntl (the homologue of mouse Brac hyury, originally called Zf-T), posterior structures are missing (M. E . Halpern, C. B. Kimmel, R. K. Ho and C. Walker, 1993; Cell In press). While mutant and wild-type embryos do not differ-in their eve1 transc ript distribution during gastrulation, eve1 expression is absent in th e caudal region of mutant ntl embryos during early somitogenesis, indi cating a requirement for ntl in the maintenance of eve1 expression dur ing tail extension. Our findings suggest that eve1 expression is corre lated with a ventral and posterior cell fate, and provide first insigh ts into its regulation.