Jeannette Charlton

The domineering non-autonomy of frizzled and van Gogh clones in the Drosophila wing is a consequence of a disruption in local signaling.

The frizzled (fz) gene is required for the development of distally pointing hairs on the Drosophila wing. It has been suggested that fz is needed for the propagation of a signal along the proximal distal axis of the wing. The directional domineering non-autonomy of fz clones could be a consequence of a failure in the propagation of this signal. We have tested this hypothesis in two ways. In one set of experiments we used the domineering non-autonomy of fz and Vang Gogh (Vang) clones to assess the direction of planar polarity signaling in the wing. prickle (pk) mutations alter wing hair polarity in a cell autonomous way, so pk cannot be altering a global polarity signal. However, we found that pk mutations altered the direction of the domineering non-autonomy of fz and Vang clones, arguing that this domineering non-autonomy is not due to an alteration in a global signal. In a second series of experiments we ablated cells in the pupal wing. We found that a lack of cells that could be propagating a long-range signal did not alter hair polarity. We suggest that fz and Vang clones result in altered levels of a locally acting signal and the domineering non-autonomy results from wild-type cells responding to this abnormal signal.

The flare Gene, Which Encodes the AIP1 Protein of Drosophila, Functions to Regulate F-Actin Disassembly in Pupal Epidermal Cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

Cell size and the morphogenesis of wing hairs in Drosophila.

Summary: Almost all epidermal cells on the Drosophila wing produce a single cuticular hair. This is formed in the pupae from a microvillus‐like cell projection called the prehair. Previous experiments have shown the existence of two mechanisms that ensure that only a single hair is made. One is the restriction of prehair initiation to a small subregion of the cell by the action of the frizzled tissue polarity pathway. The second is a system that ensures the integrity of the prehair. Mutations and drugs that inhibit the actin cytoskeleton lead to the splitting of a single prehair into multiple smaller hairs. We report that large polyploid cells produce multiple hairs both because they form multiple independent prehair initiation centers and because the larger than normal hairs these cells produce have a tendency to split. We show that reducing cell size by starvation partially suppresses the phenotype seen in polyploid cells and that increasing apical cell surface area by mechanical stretching also results in the formation of multiple prehair initiation centers. We also show that the frizzled tissue polarity pathway is functional in large polyploid cells even if it is unable to restrict prehair initiation to a small region of the cell. We conclude that both of these cellular systems are limited in their ability to scale to accommodate larger cell size. genesis 28:82–91, 2000.

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.