THE PRODUCTS OF RIBBON AND RAW ARE NECESSARY FOR PROPER CELL-SHAPE AND CELLULAR-LOCALIZATION OF NONMUSCLE MYOSIN IN DROSOPHILA

Mutations in the genes rib and raw cause defects in the morphology of a number of tissues in homozygous mutant embryos. A variety of tubular epithelial tissues adopt a wide, round shape in mutants and dorsal cl osure fails. Cells of the normal tubular epithelia are columnar and we dge-shaped, and cells of the epidermis become elongated dorsoventrally as dorsal closure occurs. However, the cells of mutants are round or cuboidal in all of the tissues with mutant phenotypes, consistent with the hypothesis that the products of these genes are required for prop er cell shape. Cytoskeletal defects, in particular, defects in myosin- driven contraction of the cortical actin cytoskeleton, could be respon sible for the lack of specific cell shapes in mutant embryos. This pos sibility is supported by our observation that the intracellular locali zation of nonmuscle myosin to the leading edge of the dorsally closing epidermis is absent or reduced in rib and raw mutant embryos. In cont rast, the band of actin that is also located at the leading edge is ne ither eliminated nor interrupted by either rib or raw mutations. Furth ermore, mutations of zipper, the gene encoding the nonmuscle myosin he avy chain, exhibit mutant phenotypes in most of the same tissues affec ted by rib and raw, and many of the phenotypes are similar to those of rib and raw. Therefore, the products of rib and raw may be required f or proper myosin-driven contraction of the actin cytoskeleton. (C) 199 8 Academic Press.