A FUNCTIONALLY CONSERVED HOMOLOGY OF THE DROSOPHILA SEGMENT POLARITY GENE-HH IS EXPRESSED IN TISSUES WITH POLARIZING ACTIVITY IN ZEBRAFISH EMBRYOS

The segment polarity gene hedgehog (hh) encodes a novel signaling prot ein that mediates local cell-cell interactions in the developing Droso phila embryo. Here we describe the existence of an hh-related gene fam ily in the zebrafish, Brachydanio rerio. One of these genes, sonic hed gehog (shh), is expressed in the notochord, floor plate, and posterior fin mesoderm, tissues associated with polarizing activities in variou s vertebrate embryos. The pattern of shh expression in zebra-fish muta nts affecting axial structures, together with the consequences of its ectopic expression in normal embryos, is consistent with a role for sh h in floor plate induction. By expressing shh in transgenic Drosophila embryos, we also demonstrate a strong functional conservation between the fish and fly hh genes.