Cell fates in the adult abdomen of Drosophila are determined by wingless during pupal development.

wingless activity is required for the patterning of tergites and sternites in the adult abdomen of Drosophila melanogaster. In the absence of wg, tergite and sternite cuticular differentiation is replaced by that of pleura. Temperature shift analysis of a wg temperature-sensitive allele has shown that sensory bristles and tergite/sternite histotype are determined independently and that wg is required between 15 and 20 hr after pupariation for bristle formation. The determination of sensory mother cells at this stage of development was confirmed by expression of the neuralized gene in a subset of the proliferating histoblasts. Ectopic expression of wg leads to the appearance of ectopic bristles and expanded tergite and sternite, indicating that wg expression is sufficient to promote both bristle formation and tergite/sternite differentiation. wg is expressed in the dividing and spreading histoblast population in a restricted pattern which may determine the spatial arrangement of cuticular elements.

Role of the PS integrins in Drosophila development.

The PS1 and PS2 integrins of Drosophila are heterodimers of alphaPS1betaPS and alphaPS2betaPS subunits, respectively, with very strong structural similarity to vertebrate integrins. Cell transfection experiments indicate that the PS integrins are receptors for extracellular matrix components and are functionally analogous to vertebrate integrins. Matrix ligands from Drosophila tissues have been identified for PS1 and PS2 integrins, using transformed cells and a cell-spreading assay. Mutations in all three subunit genes have been identified, and the phenotypes of mutants indicate that PS integrins are required for the proper morphogenesis of a number of embryonic tissues. Using methods to produce genetic mosaics and transformation of integrin transgenes into whole animals, integrin functions in adult morphogenesis also have been examined. In the pupal wing, integrins are critically required to maintain the connection between dorsal and ventral epithelia, and recent results suggest that in early pupal development, the integrins are acting as specific receptors, as opposed to a non-specific cell-matrix glue.

The cold-sensitive period for frizzled in the development of wing hair polarity ends prior to the start of hair morphogenesis.

The function of the frizzled (fz) gene is essential for the development of the normal pattern of hairs on the Drosophila wing. In the absence of fz function hairs develop, but they display an abnormal polarity. Mutations in fz result in an altered subcellular location for the assembly of the F-actin filled prehair that becomes the adult cuticular hair. This observation led to the suggestion that fz and other tissue polarity genes form a regulatory pathway that controls the initiation of prehairs. We have isolated a cold-sensitive fz allele and found that the cold-sensitive period for fz in the pupal wing starts in the early pupae and ends prior to the first sign of prehair morphogenesis. This cold-sensitive mutation is due to a missense mutation in a putative transmembrane domain. Western blot analysis shows that the accumulation of the mutant protein is not cold sensitive, consistent with the supposition that it is the activity of the mutant protein that is cold sensitive. Our data argue that fz has a regulatory function in specifying where the prehair forms, but no role in the actual morphogenesis of the prehair.

Frizzled gene expression and development of tissue polarity in the Drosophila wing.

Almost every cell in the Drosophila pupal wing forms a single, distally pointing cuticular hair. The function of the frizzled (fz) gene is essential for the elaboration of the normal wing hair pattern. In the absence of fz function hairs develop, but they display an abnormal polarity. We have examined the developmental expression of the fz gene at the RNA level via in situ hybridization and at the protein level via Western blotting. We have found that fz is expressed in all regions of the epidermis before, during, and after the fz cold sensitive period. We have also found that fz function is not required for normal fz expression. We have further found that mutations in several other tissue polarity genes do not noticeably alter the expression or the modification state of the Fz protein.

Tissue polarity genes of Drosophila regulate the subcellular location for prehair initiation in pupal wing cells.

The Drosophila wing is decorated with a regular array of distally pointing hairs. In the pupal wing, the hairs are formed from micro-villus like prehairs that contain large bundles of actin filaments. The distal orientation of the actin bundles reveals the proximal-distal polarity within the pupal wing epithelium. We have used F-actin staining to examine early stages of prehair development in both wild-type and mutant pupal wings. We have found a striking correlation between hair polarity and the subcellular location for assembly of the prehair. In a wild-type wing, all of the distally pointing hairs are derived from prehairs that are formed at the distal vertex of the hexagonally shaped pupal wing cells. Mutations in six tissue polarity genes result in abnormal hair polarity on the adult wing, and all also alter the subcellular location for prehair initiation. Based on their cellular phenotypes, we can place these six genes into three phenotypic groups. Double mutant analysis indicates that these phenotypic groups correspond to epistasis groups. This suggests that the tissue polarity genes function in or on a pathway that controls hair polarity by regulating the subcellular location for prehair formation.