Salivary gland formation in the developing Drosophila embryo provides a excellent system for understanding the molecular and cellular interactions required for tissue morphogenesis. Salivary gland cells participate in many of the movements characteristic of tissue differentiation, including nuclear repositioning, cell shape changes, invagination and tube formation. Only two major cell types comprise the Drosophila specialized for secretion, and the salivary duct cells, which are squamous epithelial cells organized into a polarized epithelium specialized for the passage of secretions. Once cells are specified to become salivary gland, they differentiate without further cell division; growth occurs simply by increasing the volume of individual cells. Many of the genes that control salivary gland fate determination and early morphogenesis have been identified and characterized. In previous work, we identified trachea-less (trh), a gene encoding a transcription factor required for tube formation in the salivary gland duct, trachea and filzkorper. In trh mutant embryos, each of these group of cells fails to invaginate to form their characteristic tubes. We have shown that trh is expressed in these tube-forming cells and have identified the molecules that control trh expression in the salivary gland duct. The goal of the research proposed here is to understand how the TRH transcription factor controls tube formation not only in the salivary duct but in other cells that also depend on its activity. We will first describe the cellular changes that occur during normal tube formation as a basis for assessing the relative contribution of trh and its downstream target genes in forming the tubes of the salivary duct and trachea. Second, we will identify these downstream genes and characterize their corresponding mutant phenotypes. Finally, we will study the molecular interactions between the TRH and the genes it regulates. The proposed work will link developmental genetics to the cell biology of organ assembly.
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