Isolation of Drosophila flightless mutants which affect myofibrillar proteins of indirect flight muscle

Abstract

A large number of dominant flightless mutants of Drosophila were chemically induced, and their thorax proteins were examined by chemically induced, and their thorax proteins were examined by means of two-dimensional gel electrophoresis (O'Farrell 1975). Among them, 26 lines were found to have deficiency or reduction of some of myofibrillar proteins in indirect flight muscle (IFM). The gel patterns of the mutants could be classified into eleven groups. In general, more than a few polypeptides were either absent or reduced in each mutant line. Although the mutations affect myofibrillar proteins in apparently complex and diverse ways, logical correlations were found among the changes. There are pairs of proteins which always change together when a number of mutants are compared. There are also many pairs in which presence of one protein is necessary, but not sufficient for presence of the other. This suggests that absence of one component leads to disappearance or reduction of others which are either spatially or functionally related to the former. The correlation is possibly due to a hierarchy of the proteins in the myofibrillar assembly processes. Chromosomal loci of eleven typical mutants were examined, and it was found that most of them are located in two small regions of the second and the third chromosomes. IFM myofibrils of these mutants are either abnormal or absent in homozygotes as well as in heterozygotes.

Ultrastructural and molecular analyses of homozygous-viable Drosophila melanogaster muscle mutants indicate there is a complex pattern of myosin heavy-chain isoform distribution.

“…The absence of myosin light chains in these mutants is consistent with a defect in a thick filament protein that results in the turnover of other thick filament-asso ciated proteins, due to lack of thick filament assembly [for a discussion of pleiotropic effects of contractile pro tein gene mutations, see Mogami and Hotta (1981)]. The failure to accumulate known thick filament proteins, in conjunction with the genetic recombination and mutant rescue data, strongly suggested that the mutations are defects within the MHC gene that result in the under production of MHC protein.…”

“…This suggests that the mutations are hypomorphic and do not result in the production of a neomorphic gene product that blocks muscle function in the presence of two wild-type alleles. Mogami and Hotta (1981) previously examined protein accumulation in thoraces of Mhc^ [formerly called Ifm(2)2] by two-dimensional gel electrophoresis. They found that two myosin light chains (spots 138, 185), as well as additional proteins (spots 158, 159), were missing and proteins 33 and 74 were reduced.…”

“…This is because only mutations that affect all isoforms of MHC (the homozygous-lethal class) or mu tations in isoforms that accumulate in the IFM (the homozygous-viable class) were isolated when screening for dominant flightless mutants (Mogami and Hotta 1981). A number of isoforms specific to larval muscle or to other muscles of the adult may exist, and mutations in these isoforms would not be recovered as they do not affect flight muscle function.…”

“…MHC content then was calculated as a percentage of wild-type levels. Analysis of proteins by two-dimensional electrophoresis was as described previously (Mogami and Hotta 1981).…”

“…melanogaster is a useful organism in which to study muscle assembly^ development^ and function be cause physiological and molecular analyses can be aided by the relative ease of mutant isolation and manipula tion. Mutations that affect muscle structure and func tion are isolated readily (Deak 1977;Koana and Hotta 1978;Mogami and Hotta 1981) because the fimction of the indirect flight muscle (IFM) and the jump muscle or tergal depressor of the trochanter muscle (TDT) are not essential to viability or fertility. We showed previously that two wild-type copies of the MHC gene are required for IFM and TDT function, whereas other muscles fimc tion adequately with a single copy (Mogami et al 1986;O'Donnell and Bemstein 1988).…”