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Mutations of the myosin heavy-chain (MHC) gene ofDrosophila melanogaster were identified among a group of dominant flightless and recessive lethal mutants (map position 2-52, 36A8-B1,2). One mutation is a 0.1-kilobase deletion in the 5' region of the MHC gene and reduces MHC protein in the leg and thoracic muscles of heterozygotes to levels found in 36AC haploids. Three mutations are insertions of 8-to 10-kilobase DNA elements within the MHC gene and produce truncated MHC transcripts. Heterozygotes of these insertional mutations possess levels of MHIC intermediate between those of haploids and diploids. An additional mutation has no gross alteration of the MHC gene or its RNA transcripts. Although leg and larval muscles function normally in each mutant heterozygote, indirect flight muscles are defective and possess disorganized myofibrils. Homozygous mutants die during embryonic or larval development and display abnormal muscle function prior to death. These flndings provide direct genetic evidence that the MHC gene at 36B (2L) is essential for both larval and adult muscle development and function. The results are consistent with the previous molecular evidence that Drosophila, unlike other organisms, has only a single muscle MHC gene per haploid genome. Quantitative expression of both copies of the MHC gene is required for function of indirect flight muscle, whereas expression of a single MHC gene is sufficient for function of larval muscles and adult tubular muscles.Analysis of muscle mutants of the fruit fly Drosophila melanogaster and the small soil nematode Caenorhabditis elegans offers a unique approach to understanding the genetic regulation of myogenesis and muscle function (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). In C. elegans, mutations of myosin heavy-chain (MHC) (9-11), paramyosin (12)

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Preliminary observations on the embryonic lethal, lethal myospheroid ( I ( l ) m y s ) , in Drosophila melanogaster showed that it reached a late stage in embryogenesis and had abnormal somatic muscles, abnormal segmentation, an incompletely developed midgut, and a mass of dorsally herniated tissue (Poulson, unpublished). In particular, the abnormalities in the muscles of this lethal made a more detailed analysis of development of the lethal syndrome seem profitable. This paper is a report of the genetic, developmental, and histochemical analysis of lethal myospheroid. MATERIALS AND METHODSA specially derived stock of M5/Emys X M5/Y (page SO) was the source of all lethal rnyospheroid chromosomes used in this work. The Oregon R stock kept at New Haven, Conn. was used as the wild type strain.In the viability experiments eggs were collected from young females (Cross A and B-4 days old; crosses C through E-3-10 days old; tables l and 2 ) and were transferred with a blunt needle to small squares of black blotting paper moistened with Drosophila Ringer's solution. Each square carrying 25-30 eggs was immediately placed in a small yeasted culture bottle containing approximately 15 cm3 of cornmeal-molasses-agar food. After 48 hours at 25°C all empty egg cases were counted and unhatched eggs were collected from the papers. Their chorions were treated with 95% alcohol until they became transparent so that eggs could be scored as "unfertilized," mys-type, or other abnormal eggs. (Any eggs which showed no development upon gross examination were scored as "unfertilized." This is not a completely valid procedure, for eggs which stop development within the first hour and a half cannot be distinguished from unfertilized eggs by this method. ) Pupae in each creamer were counted, and on hatching, the sex of the adults was determined and they were counted.Carefully timed eggs for both the developmental and histochemical work were collected by the following method. M5/1 m y s $2 were mated to Ore R $8. The Ore R/E mys virgins collected from this cross were aged for 2-3 days and then 10-15 of them were mated to an equal number of young Ore R males in empty half pint milk bottles. (These two crosses eliminated the necessity of considering the effect of the M5 chromosome on the sibling control embryos.) Paper spoons carrying a small amount of Cream of Wheat-molasses food coated with a mixture of yeast and honey were used to maintain the Aies in the bottles and to collect the eggs. Eggs were not collected from females more than 9 days old to eliminate the effects of aging on oogenesis. To reduce fluctuations in temperature spoons were always warmed to 25°C before use, and the first collection at any particular time of day was discarded. Spoons were placed in laying bottles in the 25°C incubator for exactly one half hour and were then transferred to petri dishes and kept at 25" 2 1" until about one half hour before the time of fixation. Just prior to fixation the eggs were transferred from the spoons into Drosophila Ringer's solution in ...