Spontaneous mutation rates at enzyme loci in Drosophila melanogaster.

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Abstract
In a marked-inversion-balanced lethal system, mutations were accumulated at a minimum pressure of natural selection on 2,000 second chromosomes of D. melanogaster that originated for 4 stem chromosomes. Five enzyme loci were tested: .alpha.-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), malate dehydrogenase (Mdh, EC 1.1.1.37), alcohol dehydrogenase (EC 1.1.1.1), hexokinase-C (Hex-C tentative name) and .alpha.-amylase (Amy, EC 3.2.1.1). Band-morph mutants, 1 at the Mdh locus, at the Hex-C locus and 1 at the Amy locus were detected out of 1,658,308 allele replications. Null mutants (17) were found. Accepting that the number of structural genes is the same as that of bands in the salivary gland chromosomes, the total mutation rate per generation for all the structural genes in the 2nd chromsomes is 0.008-0.040, which is much smaller than that estimated for viability polygenes (0.12-0.17). Most viability and other fitness polygenes are probably located in controlling regions outside structural genes.