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We used null mutations of Drosophila actin and myosin genes to investigate two aspects of myofibril assembly.First, we eliminated all actin or myosin in flight muscles to evaluate contributions of thick and thin filaments to sarcomere formation. Results demonstrate that thick and thin filament arrays can assemble independently but that both are essential for sarcomeric order and periodicity. Second, we examined how filament stoichiometry affects myofibril assembly. We find that heterozygotes for actin (Act88F) or myosin heavy chain (Mhc36B) null alleles have complex myofibrillar defects, whereas Mhc36B-/+ ; Act88F-/+ double heterozygotes have nearly normal myofibrils. These results imply that most defects observed in single heterozygotes are due to filament imbalances, not deficits, and suggest that thick and thin filament interactions regulate myofibrillar growth and alignment.

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