NONPERIODIC CUES GENERATE 7 FTZ STRIPES IN THE DROSOPHILA EMBRYO

We have examined the expression pattern of the segmentation gene fushi tarazu (ftz) by in situ hybridization to whole mount embryos using di goxygenin labeled probes. This method has revealed previously undetect ed stages in the development of the ftz RNA pattern. The ftz stripes a rise individually in a distinct, non-linear order along the anterior-p osterior axis of the embryo. In addition, the stripes develop differen tially along the dorsal-ventral axis; most stripes emerge on the ventr al side and then gradually spread dorsally until they surround the ent ire circumference of the embryo. The order of appearance of ftz stripe s is not inversely correlated with the order of appearance of hairy (h ) stripes as would be expected if ftz stripes were generated by h repr ession. Furthermore, the seven ftz stripes are correctly established i n embryos carrying mutations in h, eve or runt, with normal expression patterns decaying only after cellularization. Thus, the so called pri mary pair-rule genes are involved in the refinement rather than establ ishment of the ftz stripes. The contribution of cis-acting regulatory elements to the ftz pattern was examined. The zebra and upstream eleme nts interact to generate seven correctly positioned stripes at the end of cellularization. However, stripe establishment is not correctly mi micked by any ftz/lac fusion gene: stripes arise in an order drastical ly different from the endogenous ftz gene suggesting the existence of ftz regulatory elements outside the 10-kb region examined to date. The se observations suggest that the ftz pattern is directed by at least t wo independent regulatory systems: first, stripe establishment is dire cted by regionally distributed factors that act differentially in indi vidual stripes along both anterior-posterior and dorsal-ventral axes o f the egg and, second, stripe refinement and maintenance are mediated by pair-rule gene products that interact with previously identified ft z regulatory elements. This multi-level regulation provides a back-up system that ensures the development of seven stripes in the blastoderm .