Non-periodic cues generate seven 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 digoxygenin labeled probes. This method has revealed previously undetected stages in the development of the ftz RNA pattern. The ftz stripes arise individually in a distinct, non-linear order along the anterior-posterior axis of the embryo. In addition, the stripes develop differentially along the dorsal-ventral axis; most stripes emerge on the ventral side and then gradually spread dorsally until they surround the entire circumference of the embryo. The order of appearance of ftz stripes is not inversely correlated with the order of appearance of hairy (h) stripes as would be expected if ftz stripes were generated by h repression. Furthermore, the seven ftz stripes are correctly established in embryos carrying mutations in h, eve or runt, with normal expression patterns decaying only after cellularization. Thus, the so called primary pair-rule genes are involved in the refinement rather than establishment of the ftz stripes. The contribution of cis-acting regulatory elements to the ftz pattern was examined. The zebra and upstream elements interact to generate seven correctly positioned stripes at the end of cellularization. However, stripe establishment is not correctly mimicked by any ftz/lac fusion gene: stripes arise in an order drastically different from the endogenous ftz gene suggesting the existence of ftz regulatory elements outside the 10-kb region examined to date. These observations suggest that the ftz pattern is directed by at least two independent regulatory systems: first, stripe establishment is directed by regionally distributed factors that act differentially in individual stripes along both anterior-posterior and dorsal-ventral axes of the egg and, second, stripe refinement and maintenance are mediated by pair-rule gene products that interact with previously identified ftz regulatory elements. This multi-level regulation provides a back-up system that ensures the development of seven stripes in the blastoderm.