GENETIC-ANALYSIS OF THE DROSOPHILA BETA-3-TUBULIN GENE DEMONSTRATES THAT THE MICROTUBULE CYTOSKELETON IN THE CELLS OF THE VISCERAL MESODERMIS REQUIRED FOR MORPHOGENESIS OF THE MIDGUT ENDODERM

We have investigated the cellular basis for lethality of mutant allele s of the Drosophila melanogaster beta 3-tubulin gene, beta Tub60D. Let hal beta 3 mutations can be grouped into two classes: the most severe mutations (Class I alleles) cause death during the first larval instar , while weaker alleles (Class II) cause death in later larval stages o r in early pupal development. Since beta 3 is not expressed during lar val development, lethality of the Class I mutations must reflect essen tial functions of beta 3 in embryogenesis. beta 3-tubulin is zygotical ly expressed during midembryogenesis in the developing mesoderm, and t he major site of beta 3 accumulation is in the developing muscles duri ng myogenesis. We show that the embryonic pattern of beta 3 expression , including accumulation in the developing musculature, is conserved i n other Drosophila species. However, we found that loss of beta 3 func tion does not cause discernible defects in either the ultrastructure o r function of the larval muscle. Thus beta 3-tubulin is dispensable in its highest site of accumulation. Rather, the essential site of funct ion of beta 3 in embryos is in cells of the visceral mesoderm. Lethali ty of Class I alleles is caused by defects in midgut morphogenesis and failure of gut function. Although the folding pattern is irregular an d the gut is smaller than normal, a complete folded gut forms in mutan t larvae, and the visceral muscle functions normally to move food thro ugh the gut. However, mutant larvae cannot absorb nutrients across the gut wall. Thus loss of beta 3 function in the mesoderm results in def ects in the underlying endodermally derived layer of the gut. Our data provide an assay for cellular interactions between mesoderm and endod ermal tissues and reveal a role for the microtubule cytoskeleton of th e visceral mesodermal cells in differentiation of the endodermal cell layer of the larval gut. (C) 1996 Academic Press, Inc.