The Drosophila 36B muscle myosin heavy chain (MHC) gene has five sets of alternatively spliced exons that encode functionally important domains of the MHC protein and provide a combinatorial potential for expression of as many as 480 MHC isoforms. In this study, in situ hybridization analysis has been used to examine the complexity and muscle specificity of MHC isoform expression in the fibrillar indirect flight muscle (IFM), the tubular direct flight muscles (DFM) and tubular tergal depressor of the trochanter muscle (TDT), and the visceral esophageal muscle in the adult thorax. Our results show that alternative splicing of the MHC gene transcripts is precisely regulated in these thoracic muscles, which express three MHC isoforms. Individual thoracic muscles each express transcripts of only one isoform, as detectable by in situ hybridization. An apparently novel fourth MHC isoform, with sequence homology to the rod but not to the head domain of the 36B MHC, is expressed in two direct flight muscles. These findings form a basis for transgenic experiments designed to analyze the muscle-specific functions of MHC domains encoded by alternative exons.

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