Functional and quantitative analysis of morphogen thresholds in the early Drosophila embryo

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. In Drosophila, proteins in the segmentation cascade form gradients to direct the patterning of anterior-posterior (AP) axis in the early embryo. Several segmentation proteins have been proposed to function as morphogens. However it is not clear whether threshold concentrations within these gradients are responsible to establish the relative positioning of gene expression boundaries. This thesis focuses on one activation morphogen Bicoid (Bcd) and one previously hypothesized repression morphogen Hunchback (Hb).;A combination of quantitative measurements and genetic manipulations was used to examine the transcriptional activation activities of Bcd and the repression activities of Hb. A general correlation was found between the concentration of Bcd protein required for target gene activation and the positioning of these targets along the AP axis. However, concentrations required for activation in embryos with flattened Bcd gradients were consistently lower than the concentrations at the boundaries of these genes in wild type embryos, suggesting that Bcd target gene expression patterns are not primarily positioned by the wild type Bcd gradient. Although Bcd is required for target gene activation, the patterning function of Bcd is partially redundant with a patterning gradient provided by the Torso-dependent terminal system. In contrast, Hb seems to function as a concentration-dependent repressor morphogen, which differentially establish five expression boundaries. These boundaries correspond to a specific range of Hb concentrations (∼40% - ∼4.4% of maximum Hb concentration). Interestingly, Hb is also required for positioning two gene expression boundaries outside of this range, but these are established by combinatorial mechanisms involving Hb and at least one other repressor.;In summary, the work in thesis supports a model whereby activator morphogens set general trend for target gene activation, while the preciseness of expression boundaries is primarily achieved by repressor mechanisms.