Basal cell protrusive activity is a primary determinant of follicle cell planar polarity. Maureen P. Cetera¹, Lindsay Lewellyn¹, Michael J. Fairchild², Guy Tanentzapf², Sally Horne-Badovinac¹. 1) Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL; 2) Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver, BC.

   Coordinated cell migration is critical for organ formation. We are using the Drosophila egg chamber as a highly tractable model to investigate morphogenetic mechanisms during organogenesis. The initially spherical egg chamber elongates along its A-P axis to form an elliptical egg. This morphogenesis depends on an unusual form of planar polarity at the basal surface of the egg chambers outer follicle cell (FC) layer. Here, linear actin filaments and fibril-like structures in the basement membrane (BM) both align perpendicular to the elongation axis, where they are thought to function as a molecular corselet that restricts egg chamber growth to the A-P axis. Recently, follicle cell planar polarity has been shown to correlate with a dramatic collective migration of the FCs, a phenomenon that causes the entire egg chamber to rotate within the external BM. However, the relationship between the planar polarity and the FC migration is unknown. By temporally manipulating migration either before or after planar polarity establishment we have determined their dependence on one another. The Arp2/3 activator Scar/WAVE is planar polarized at the front of the migrating FCs where it is cell-autonomously required for formation of basal protrusions at the leading edge of each FC. Reducing Arp2/3 activity in the entire epithelium before planar polarity is established blocks FC migration and planar polarity is never established. Conversely, culturing egg chambers with an Arp2/3 inhibitor after polarity establishment also blocks migration but does not influence the tissue-level alignment of basal actin filaments. These data suggest that Arp2/3s ability to drive FC migration is a primary determinant in the establishment, but not the maintenance, of basal actin filament planar polarity.