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Our work focuses on how asymmetry is initiated and elaborated along the DV axis. This axis is defined by a growth factor, called Gurken, produced by the oocyte. The key to generating asymmetry is that since Gurken is only found on the dorsal side of the oocyte, it activates a receptor tyrosine kinase (the听Drosophila听homolog of the vertebrate EGF receptor [Egfr]) in the adjacent follicle cells (Figure 2).

Receptor activation triggers a signal transduction pathway that directs follicle cell fate and thus distinguishes dorsal cells, which receive the signal, from ventral cells, which do not. This signaling process, which is refined over time by downstream feedback mechanisms, ultimately determines the structure of the eggshell along its DV dimension. In mutants where either the signal or receptor is absent, no follicle cells are signaled to produce dorsal-appropriate eggshell structures and instead only ventral-like eggshell is secreted. What does the eggshell have to do with the embryo?

While the dorsal follicle cells secrete more elaborate eggshell structures, the ventral follicle cells produce an unknown secreted molecule that acts as the spatial cue that defines the DV axis future embryo. Thus the patterning events initiated by Gurken ultimately govern both eggshell and embryonic development.

A mutation affecting eggshell patterning
One of our research goals is to understand the series of events that are triggered by the initial Gurken signal and instruct the dorsal follicle cells to secrete the dorsal side of the eggshell, including its prominent eggshell appendages. To approach this question, we are studying a gene called capicua. Females lacking capicua function produce eggs with dramatic eggshell defects: the dorsal appendages are very broad and shifted toward the ventral side, with a wider space between them (Figure 3).

This phenotype reflects an expansion of the dorsal side of the eggshell and therefore suggests that more follicle cells have adopted a 鈥渄orsal鈥� fate. Increased dorsal follicle cell fate determination suggests an increase in Gurken signaling; indeed, other mutants with similar 鈥渄orsalized鈥� phenotypes exhibit an expanded distribution of Gurken within the oocyte.

Surprisingly, however, Gurken is normal in capicua mutant ovaries, suggesting that loss of capicuafunction affects follicle cell fate at a later step. Since mutations in capicua appear to disrupt follicle cell patterning in a previously undescribed manner, the study of capicua function may yield novel insights into how the pattern of follicle cell fates is determined.

Figure 4a

Figure 4b

These phenotypes resemble the cellular response to small wounds in epithelial monolayers in both vertebrate model systems and Drosophila embryos, where actin accumulates at the edge of each cell facing the wound and forms a 鈥渃ontractile ring鈥� that constricts to close the wound and preserve the integrity of the epithelium. In vertebrate cultured cell systems, cells undergoing programmed cell death are extruded from an epithelial monolayer by a similar mechanism, again maintaining the barrier function of the epithelium.

Are our mutant cells provoking a wound healing response in their neighbors? This general question has two components. First, what is the nature of the defect in our mutant cells? Are they dying and inducing a wound healing response in their neighbors? Though they are able to secrete eggshell and seem to be intact through oogenesis, we are testing for early signs of programmed cell death. We are also working to identify the gene affected by this mutation, which may reveal the underlying cellular defect. Whatever the defect, how do neighboring cells detect and respond to this change? Though recent work has identified some of the molecules that are required for contractile ring assembly, constriction and healing, little is known in Drosophila or other systems about how a cell or group of cells can signal this type of response in adjacent cells. We hope that this work may ultimately allow us to use the genetics of this model organism to address these cell biological questions.