ECDYSONE SIGNALING PLAYS KEY ROLES IN FOLLICULAR EPITHELIUM MORPHOGENESIS AND FOLLICLE CELL SURVIVAL

In Drosophila, the ecdysone signaling is required for normal oogenesis and is controlled by a heteromeric receptor composed of the Ecdysone Receptor (EcR) and Ultraspiracle (USP) nuclear receptor proteins. In order to investigate the role of Ecdysone signaling in follicular epithelium development we analyzed the effect of EcR or USP loss of function using either reverse genetic or classical genetic approaches. We used transgenic RNA interference (RNAi) coupled with UAS/Gal4 technique to knock down the levels of EcR in somatic follicle cells. We found that targeting RNAi of all EcR isoforms or silencing EcR-B1 isoform by using the ubiquitous Tubulin-Gal4 driver causes severe alteration in egg chamber development. Silencing of EcR-B1 isoform in flp-out clones causes apoptotic follicle cell death and affects follicular epithelium monolayer structure. Multilayered follicle cells were also detected knocking down EcR-B1 isoform by using enhancer trap lines that drive Gal4 expression at mid-oogenesis. The analysis of epithelial junction proteins (Armadillo, DE-cadherin alpha-spectrin and Discs-large) reveals their altered distribution. The actin cytoskeleton was strongly affected and the follicle cell nuclei show strong nuclear staining. Conversely, knock out of EcR-A isoform alters oogenesis only upon the ubiquitous expression of the silencing transgene which causes production of eggs with short and flattened dorsal appendages. Interestingly, we found that mosaic follicle cells homozigous for the usp3 mutation that interferes with USP repressor activity dye by apoptosis at early stages of oogenesis and show only slightly increased Discs-large expression levels but not alteration in DE-cadherin expression levels.