Gradients and the Control of Pattern Formation

New York University, New York NY

Investigators

Abstract

Recent experiments suggest that gradients of regulatory molecules are important for establishing the body plans of multicellular animals. However, little is known about the mechanisms whereby target genes respond to changes in the concentration of regulatory molecules. This application proposes to study these mechanisms in Drosophila, which is ideal for this purpose because the early embryo is a syncytium of nuclei that do not possess cytoplasmic membranes. This permits the study of transcription mechanisms directly, without complications from cell-cell signaling processes. The work will focus on four gap genes, hunchback, giant, Kruppel, and knirps, whose overlapping protein gradients form an integrated system that establishes striped patterns of gene expression in middle regions of the embryo. This work is driven by the hypothesis that these gradients establish positions of gene expression borders by acting as concentration-dependent repressors. For each gradient, the borders of several target genes are set at different positions with respect to the source. This suggests that each target gene responds to a different concentration threshold of repressive activity. To test this hypothesis, previously characterized enhancers and the yeast FLP-FRT recombination system will be used to ectopically express different levels of each gene outside its normal domain. These experiments will test whether expression of a given gap gene is sufficient for a particular response, and identify potential new target responses. The misexpression system will also be used to dissect regions of the gap proteins that are required for repression. Initially, the focus will be on evolutionarily conserved domains that have been previously implicated in repression. Together these experiments will help clarify the genetic circuitry that controls the positioning and spacing of gene expression boundaries in the early embryo, and provide insights into general mechanisms that control how different target genes respond to different combinations and concentrations of regulatory proteins. They will also provide a firm foundation for studies designed to unravel the mechanisms that change body plans during evolution.