Speaker: Thomas Gregor, Princeton University, Dept. of Molecular Biology

Title: Establishment of morphogen gradients in early embryonic development

Abstract: During development of an organism, cells differentiate into a variety of classes, laid out in a specific spatial order. Cells in different locations therefore must somehow "know" where they are. In the fly embryo this positional information is provided by a set of specific proteins (called morphogens) that are present in spatially varying concentrations. The mechanisms by which these gradients are established are not completely understood, although diffusion from localized sources, as established by the mother during egg formation, is thought to play an important role. The morphogen Bicoid (Bcd) defines the front-to-back axis in flies. Here we study the formation of the Bicoid gradient in the embryos of several fly species (Drosophila melanogaster, Drosophila busckii, Lucilia, and Calliphora) whose egg lengths differ by a factor of 5. Gradient length constants were measured by immuno-fluorescence staining, and eggs were micro-injected with dextran molecules to measure diffusion constants using 2-photon imaging. We also constructed a fluorescent Bicoid-GFP transformant that allows us to study the dynamics of the gradient in living embryos. The results suggest that a model solely reliant on protein diffusion and protein degradation cannot explain the spatial and temporal distribution of Bicoid in the embryo.

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