Abstract
BACKGROUND: Morphogen gradients are thought to create concentration thresholds that differentially position the expression boundaries of multiple target genes. Despite intensive study, it is still unclear how the concentration profiles within gradients are spatially related to the critical patterning thresholds they generate. RESULTS: Here we use a combination of quantitative measurements and ectopic-misexpression experiments to examine the transcriptional-repression activities of the Hunchback (Hb) protein gradient in Drosophila embryos. Our results define five expression boundaries that are set primarily by differences in Hb concentration and two boundaries that are set by combinatorial mechanisms involving Hb and at least one other repressor. CONCLUSIONS: Hb functions as a repressive morphogen, but only within a specific range of concentrations ( approximately 40% to approximately 4.4% of maximum Hb concentration), within which there are at least four distinct concentration thresholds. The lower limit of the range reflects a position where the slope of the gradient becomes too shallow for resolution by specific target genes. Concentrations above the upper limit do not contribute directly to differential-repression mechanisms, but they provide a robust source that permits proper functioning of the gradient in heterozygous embryos that contain only one functional hb gene.