An ectopic expression approach to gastrulation. R. Hoang , T. Blankenship , J. Grosshans , P. Sung , J. Thomas , E. Wieschaus. Dept Molecular Biology, Princeton University, NJ.

   Gastrulation is the process by which an embryo internalises its mesodermal and endodermal precursors and thus transforms its entire shape and structural complexity. Drosophila gastrulation takes less than 1hr and involves multiple morphogenetic processes. Loss of function approaches have been highly successful in identifying patterning genes required for these processes but have been less successful in identifying genes that more directly control the underlying cell behaviors. To identify additional genes of this latter class we have therefore undertaken a screen based on ectopic expression in which we also followed gastrulation directly in order to identify potentially subtle and non-lethal defects.
   Using this approach we have identified the alpha integrin subunit a-PS3 as showing a strong phenotype in which the embryo is thrown into folds as it gastrulates (which surprisingly is not necessarily lethal). This phenotype superficially resembles that of the previously identified gastrulation mutant fog. However we have shown fog expression to be unperturbed in this line and a closer examination of the two phenotypes also reveals differences in their underlying causes: the early events disrupted in fog mutants occur normally in the a-PS3 misexpression line. Time-lapse GFP microscopy shows that a-PS3 does however disrupt multiple processes later in gastrulation. These include germband extension, ventral furrow and posterior midgut invaginations but not cephalic furrow formation. Current work is therefore focussed on understanding the primary basis and shared elements of these phenotypes and uncovering the molecular nature of a-PS3’s action. We hope that this will eventually lead to a greater understanding of how the individual processes of gastrulation both differ from and impact upon one another.