EXPRESSION OF AXIAL AND SONIC HEDGEHOG IN WILDTYPE AND MIDLINE DEFECTIVE ZEBRAFISH EMBRYOS

We present a description of the expression of the HNF-3 beta homolog a xial (axil in the developing zebrafish up to larva stages and compare it with that of sonic hedgehog (shh). Both genes are expressed in deri vatives of all three germ layers in dynamic patterns that show substan tial overlap, consistent with mutual regulatory interactions between t he two genes. However. we also describe unique sites of expression of both axl and shh indicating that some aspects of their regulation are independent of one another. In no tail (ntl, zebrafish Brachyuryl and floating head (flh, zebrafish Xnot1) mutants, both of which affect not ochord development, early expression of axl in the organizer is unaffe cted, excluding a function for these genes in establishment of axl exp ression. At later stages, ntl and flh mutants show different effects o n the expression of both axl and shh in the neuroectoderm of the trunk and tail reflecting their distinct contributions to the development o f the midline mesoderm; in contrast to flh and ntl mutations whose eff ects are restricted to the trunk and tail. mutation of cyclops (cyc) a ffects axl and shh expression along the entire midline of the neuroect oderm. Endodermal expression of axl and shh is not affected by the mut ations showing that development of the endoderm is under distinct cont rol.