Genes controlling cellular polarity in Drosophila.

The control of cellular polarity is one of the least understood aspects of development. Genes have been identified in Drosophila that affect the polarity of embryonic cells in all three axes, apical-basal, proximodistal and dorsoventral. Mutations that affect adult polarity are also known and mutant flies show several types of pattern alteration, including rotations and mirror-image duplications. Imaginal discs are much greater in size, however, than the embryo, and adult structures contain very large numbers of cells, many of which are not visibly differentiated with respect to their immediate neighbours. In regions where neighbouring cells are similar to each other, the imaginal polarity mutants alter the orientation of bristles and hairs, but do not change cellular fate. Other regions, such as the tarsal segments of the legs, the ommatidia of the eye and the bracketed bristle sockets on the tibia, behave as discrete fields. Within these fields, fine-scale mirror-image reversals and pattern duplications are observed, analogous to those caused by the embryonic segment polarity mutants. Thus, the polarised transmission of information can affect either orientation or fate depending on whether cells are differentiated from their immediate neighbours. Cellular polarity will be critically dependent on both the internal cytoskeletal architecture and the spatial organisation of signal transduction molecules within the cell membrane.