The process of epithelial morphogenesis is ubiquitous in animal development, but much remains to be learned about the mechanisms that shape epithelial tissues. The follicle cell (FC) epithelium encapsulating the growing germline of Drosophila is an excellent system to study fundamental elements of epithelial development. During stages 8 to 10 of oogenesis, the FC epithelium transitions between simple geometries - cuboidal, columnar and squamous – and redistributes cell populations in processes described as posterior migration, squamous cell flattening and main body cell columnarization. Here we have carried out a quantitative morphometric analysis of these poorly understood events in order to establish the parameters of and delimit the potential processes that regulate the transitions. Our results compel a striking revision of accepted views of these phenomena, by showing that posterior migration does not involve FC movements, that there is no role for columnar cell apical constriction in FC morphogenesis, and that squamous cell flattening may be a compliant response to germline growth. We utilize mechanical modeling involving finite element computational technologies to demonstrate that time-varying viscoelastic properties and growth are sufficient to account for the bulk of the FC morphogenetic changes.
The armadillo protein of Drosophila and its vertebrate homologues, β-catenin and plakoglobin, are implicated in cell adhesion and wnt signaling. Here, we examine the conservation of these two functions by assaying the activities of mammalian β-catenin and plakoglobin in Drosophila. We show that, in the female germ line, both mammalian β-catenin and plakoglobin complement an armadillo mutation. We also show that shotgun mutant germ cells (which lack Drosophila E-cadherin) have a phenotype identical to that of armadillo mutant germ cells. It therefore appears that armadillo's role in the germ line is solely in a complex with Drosophila E-cadherin (possibly an adhesion complex), and both β-catenin and plakoglobin can function in Drosophila cadherin complexes. In embryonic signaling assays, we find that plakoglobin has no detectable activity whereas β-catenin's activity is weak. Surprisingly, when overexpressed, either in embryos or in wing imaginal disks, both β-catenin and plakoglobin have dominant negative activity on signaling, an effect also obtained with COOH-terminally truncated armadillo. We suggest that the signaling complex, which has been shown by others to comprise armadillo and a member of the lymphocyte enhancer binding factor-1/T cell factor–family, may contain an additional factor that normally binds to the COOH-terminal region of armadillo.
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