Transient induction of 43-kD SPDO during myofibril assembly irreversibly prevents thin filament elongation. Transient induction of 43-kD SPDO during myofibril assembly irreversibly prevents thin filament elongation. Thin filament lengths (A) and myofibril diameters (B) were measured as indicated in the diagram (C) using Deltavision software, from deconvolved images of Bodipy- phallacidin stained IFM myofibrils isolated from either pupal flies (1 PM D4 APF), D1 adult hs-43 kD spdo flies heat-shocked at 12 PM D4 APF, or untreated D2 adult flies. Error bars, SEM. (C) Schematic diagram of longitudinal and cross sections of an IFM myofibil from an adult hs-43 kD spdo fly that was heat shocked during pupation. Light gray shading indicates Bodipy-phallacidin stained “core” of the myofibril with shorter thin filaments and dark gray shading indicates Bodipy-phallacidin stained “peripheral” thin filaments. Michelle Mardahl-Dumesnil, and Velia M. Fowler J Cell Biol 2001;155:1043-1054 © 2001 Rockefeller University Press