Bristle growth in the abscence of actin bundles. R. Hopmann , K. Miller. Dept Biol, Washington Univ, St Louis, MO.

   The adult bristle of the fly is part of a 4 cell sensory organ that forms during metamorphosis. The bristle proper is elaborated as a long cytoplasmic extension that becomes encased in cuticle. Large, membrane associated actin bundles run longitudinally along the length of the growing bristle; these bundles are thought to be important for the normal growth and morphology of the bristle shaft. When bristle growth is complete, the actin bundles break down and the cell regresses, leaving a hollow tube with surface grooves that are the "footprints" of the actin bundles. We have studied the effects of actin capping protein mutations on actin bundle formation and bristle growth. In the adult cuticle, homozygous clones of a null allele of capping protein b subunit (cpb) cause a variety of effects on bristles and trichomes (non-sensory hairs). Macrochaete within the clone are strongly affected: the normal surface grooves are missing, and the bristle is flattened and bent. Examination of null clones in pupae yields the surprising finding that although the growing bristle shaft is filled with filamentous actin, it is not organized in bundles. Thus, the organization of actin into longitudinal bundles does not appear to be essential for extension of the bristle shaft.