| Gradients of chemical substances called morphogens underlie the differentiation of naive cells into the various tissue types of a mature organism. By measuring the concentration of a given morphogen in their local area, cells obtain information about their position relative to some axis of the developing embryo, and control the expression of distinct genetic elements accordingly.;An important question is how organisms establish these gradients early in life, with sufficient precision and reproducibility to accurately execute the developmental program. It has long been thought that morphogens were produced at various extrema of the embryo during development, and that long-range gradients were formed by diffusion of the morphogen away from the source. On this model, gradient length is determined by competition between the rates of diffusion and degradation of the morphogen. However, this idea has recently come into controversy, with some suggesting that the diffusion-degradation model does not explain gradient shape, and that gradients must be largely preformed prior to fertilization.;In this thesis, we study the formation of the gradient of Bicoid, a morphogen that controls anterior-posterior axis specification in the fruit fly Drosophila melanogaster. We use a fusion of Bicoid and the photoswitchable fluorescent protein Dronpa to measure and optically modulate the degradation rate of the morphogen. We find that decreasing the lifetime of the morphogen shortens the spatial extent of the gradient, providing support for the diffusion-degradation model of gradient formation.;Additionally, we present experimental results suggesting that previous measurements underestimated the Bicoid diffusion coefficient, as a result of incorrect assumptions of isotropic diffusion in the embryo cortex. We extend our measurements to examine Bicoid gradient formation in unfertilized eggs, finding that the diffusion-degradation model works equally well in this system, and that the only parameter needing modification is the morphogen degradation rate. |
