| The spatial patterns that emerge during early embryonic development of Drosophila melanogaster are governed by gradients in the concentration of specific molecules. While much is known about the identity of these molecules, relatively little work has addressed how these signals can provide the precision that is observed in developmental processes given the high variability in the underlying molecular events. Combining tools from biophysics and from biology, this thesis presents a quantitative analysis of the Bicoid gradient; which is the earliest detectable gradient in the Drosophila embryo.; Bicoid regulates the expression of several other molecules, including hunchback. The input/output relation between Bicoid and Hunchback is measured, analyzing both the mean response of Hunchback to the Bicoid gradient and its noise within a single embryo. This noise corresponds to a precision of ∼10% in reading out the Bicoid concentration. Theory suggests that the observed precision is difficult to achieve by temporal averaging over the available developmental time scales. A mechanism is proposed in which neighboring nuclei average their concentration measurements in order to reduce noise.; Bicoid gradients are measured in D. melanogaster and in embryos of other dipterans, such as D. busckii, Lucilia sericata , and Calliphora vicina, whose egg lengths differ by a factor of 5. The observed characteristic length constants of the Bicoid gradients scale with the average egg sizes of the different species, while developmental time scales are conserved across species. In vivo measurements of diffusion constants demonstrate that the physical properties of the cytoplasm cannot explain the scaling of the Bicoid gradient in eggs of varying sizes.; To analyze the dynamics of the Bicoid gradient a fly construct expressing an N-terminal Bicoid-eGFP fusion was designed. The overall shape of the Bicoid gradient is shown to be stable during syncytial blastoderm stages within a given embryo; whereas Bicoid-eGFP gradients across different embryos exhibit a strong positional variability during early cell cycle 14. Cytoplasmic Bicoid-eGFP diffusion constants are estimated from photobleaching experiments.; These results strongly challenge both the validity of the local concentration readout model and a purely diffusion-driven mechanism for the establishment of the Bicoid gradient. |
