Larval salivary glands of Drosophila are well known to exhibit polytene salivary glands formed via endoreplication and have been long used to study this process. In contrast, little is known of the development of adult Drosophila salivary glands except that they consist of a single layer, tubular epithelium that originates from a population of diploid cells found as an imaginal ring near larval salivary gland ducts. We have shown that the adult salivary glands contain three distinct epithelial domains, two of which are comprised of cuboidal epithelial cells and one of squamous epithelial cells. These cell types develop during the pupal period and after eclosion secretory cells develop extensive apical membrane invaginations. The junctional polarity of the epithelial cells exhibits an unusual change soon after eclosion as adherens junctions migrate from a position apical to the septate junction to a more basal position. The polarity factors aPKC, Baz, Crb, Dlg1 and Scrib, as well as members of the integrin adhesion complex protein localise to discrete sub-cellular domains within cuboidal epithelial cells. Knockdown of scrib within these cells results in a loss of apicobasal polarity.
Adult salivary gland epithelial cells are polyploid and increase in number immediately post-eclosion yet do not undergo mitosis. By using genetic tools designed for the MARCM lineage tracing technique we have shown that the polyploid cells lose chromosomes during the division period and appear to be using amitosis as a mechanism to increase cell number. We have identified the first evidence for amitosis involvement in primary formation of a tissue and the adult Drosophila salivary gland will serve as a model for genetic analysis of this mode of division, and for study of junctional reorganisation within epithelia.