SPATIAL REGULATION OF FLOATING HEAD EXPRESSION IN THE DEVELOPING NOTOCHORD

The zebrafish homeobox gene floating head (flh) is essential for notoc hord development and is one of the earliest genes to be expressed in n otochord precursors. To understand how flh is regulated during notocho rd development, we compared the wild-type flh expression pattern to th at in embryos mutant for flh and no tail (ntl), the zebrafish homologu e of Brachyury, In the early gastrula, the pattern of flh expression i s not affected in either flh or ntl mutants, implying that the initial establishment of a gastrula notochord domain is independent of the fu nction of these genes. However, flh RNA is expressed at lower levels i n flh mutants, suggesting that flh positively regulates its own expres sion. During gastrulation, flh mutants show an abrupt loss of flh expr ession in cells which have involuted and entered the hypoblast, while the rest of the expression pattern appears normal, thus flh(+) functio n is specifically required to maintain flh expression in hypoblast cel ls. The anterior-most part of the notochord rudiment differentially ma intains flh expression in both wild types and flh mutant embryos, sugg esting that there is unique regulation of flh in this region of the de veloping notochord. In ntl mutants, the spatial pattern of flh express ion is altered as early as the late gastrula stage, becoming broad and diffuse, We hypothesize that ntl(+) is required for the proper conver gence movements of flh(-) expressing cells. (C) 1997 Wiley-Liss, Inc.