Adherens junctions limit the extent of the septate junctions in Drosophila midgut enterocytes but are not required for apical-basal polarity

Adherens junctions formed by E-cadherin adhesion complexes play central roles in the organisation and apical-basal polarisation of both mammalian and insect epithelia. Here we investigate the function of the components of the E-cadherin adhesion complex in the Drosophila midgut epithelium, which establishes polarity by a different mechanism from other fly epithelia and has an inverted junctional arrangement, in which the adherens junctions lie below the septate junctions. Unlike other epithelial tissues, loss of E-cadherin, Armadillo ({beta}-catenin) or -catenin has no effect on the polarity or organisation of the adult midgut epithelium. This is not due to redundancy with N-cadherin, providing further evidence that the midgut polarises in distinct way from other epithelia. However, E-cadherin (shg) and armadillo mutants have an expanded septate junction domain and a smaller lateral domain below the septate junctions. Thus, E-cadherin adhesion complexes limit the basal extent of the septate junctions. This function does not appear to depend on the linkage of E-cadherin to the actin cytoskeleton because -catenin mutants do not significantly perturb the relative sizes of the septate and sub-septate junction domains.