Investigation of the role of programmed cell death in pattern formation in the Drosophila embryo

Pattern formation in Drosophila embryogenesis can be defined as the processes which transform a single-celled, fertilized egg into a multi-tissued larva. An essential element of Drosophila embryonic pattern formation is cell number regulation. Specific numbers of cells are needed to form individual embryonic tissues. Required cell numbers are obtained through a balance of cell division, cell allocation and cell death. Cell division and allocation often result in more than the requisite number of cells in some tissues. Excess cells may be removed by the genetically controlled process of programmed cell death or apoptosis. Although programmed cell death occurs in numerous embryonic Drosophila tissues, its role in abdominal development has not been investigated. We found that cell death is important for normal germband retraction and patterning of epidermal fates in the abdomen. In another set of experiments, regulation was examined in embryos with expansions or compressions of normal cell numbers. An increase in cell removal was observed in expanded regions, as evidenced by a normalization of cell numbers and an increase in apoptosis. However, compressed regions were not as efficient in normalizing cell numbers as seen by defects in tissues derived from condensed regions.;Identifying the mechanisms that specify which cells live or die in a particular tissue is paramount to understanding how regulation occurs in wild-type and mispatterned embryos. A model tissue to study the events which singled-out doomed cells is the embryonic epidermis. Apoptosis is relatively abundant in the epidermis and numerous mutants exist that affect normal patterning of this tissue. In order to facilitate cell death analyses, we developed a procedure to generate spatial and temporal maps of apoptotic cells throughout development. Comparison of cell death maps between wild-type and epidermal mutant embryos revealed that segment polarity genes are essential for viability of rows of cells in the developing embryonic epidermis. The loss of cells in segment polarity mutants may contribute to segment polarity phenotypes. The outcome from this work provides a novel cell death mapping procedure, sheds new light on the role of cell death in embryonic regulation, provides a starting point to understanding the determinants of the cell death fate in the epidermis and demonstrates that apoptosis is essential for normal embryonic abdominal development.