The role of the atypical cadherin Fat2 in establishing planar cell polarity in the Drosophila follicle epithelium

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The elongation of tissues or body axes is a fundamental process during animal development. Tissue elongation often requires planar polarity information that is used to establish the axis of elongation. Tissue elongation is then executed involving individual cell behaviors including directed cell shape changes, oriented cell divisions, or cell rearrangements. How planar polarity information directs cell behavior is poorly understood. A useful model system to study tissue elongation is the Drosophila ovary, in which the future egg develops as part of an egg chamber. Egg chambers are initially spherical in form, but then elongate to obtain their characteristic elliptical shape. Failure to elongate results in spherical eggs and female sterility. Each egg chamber is composed of germ line cells and a single layer of epithelial follicle cells surrounding the germ line cells. We have shown previously that egg chamber elongation requires the activity of the atypical cadherin Fat2 in follicle cells. Fat2 establishes a planar polarity in the tissue that is used to organize the cytoskeleton and the extracellular matrix of follicle cells. The planar polarized cytoskeleton and extracellular matrix contribute to egg chamber elongation. However, how Fat2 establishes this planar polarity and how the elongation is executed at the level of individual cell behaviors remains poorly understood. This proposal has two aims. First, to reveal the molecular mechanism by which Fat2 establishes planar polarity in the follicle epithelium. Second, to reveal the behavior and dynamics of follicle cells during egg chamber elongation. To address the first aim, we will perform a structure-function-analysis to identify critical regions of the Fat2 protein required for the establishment of planar polarity information and egg chamber elongation. To address the second aim, we have established in toto imaging of living egg chambers using Single Plane Illumination Microscopy (SPIM). We will use this method to visualize and quantitatively analyze the behavior and dynamics of follicle cells. We will determine whether follicle cells undergo directed cell shape changes, oriented cell divisions or cell rearrangements. We will furthermore analyze the contribution of these individual cell behaviors to the overall elongation of the egg chamber. Moreover, to identify the cellular mechanisms by which Fat2 contributes to egg chamber elongation, we will compare cell behaviors in wild-type and fat2 mutant egg chambers.We expect that these studies will provide novel insights into the mechanisms that link planar polarity information to individual cell behaviors to tissue-scale morphogenesis.