Abstract
Using two different kinds of experiment, spontaneous mutation at two esterase loci in D. virilis was investigated with special reference to heterozygosity and sex. Twelve newly-arisen variants at the two loci have been obtained among approximately 800, 000 genes examined in cross experiments, and eight variants from cage experiments. The experiments have revealed the following features which appear to be unique to the present study. (1) In the cross experiments only female heterozygotes produced new variants, at a rate of 0.5-1.0×10-4/α-locus/generation. (2) The α-esterase alleles appear to mutate in a particular direction. (3) New variants (α8+ and βD+) which have not been observed previously have been obtained. (4) The Est-α locus coding for a monomer esterase appears to be more mutable than the Est-β locus coding for a dimer enzyme. (5) In the cage experiments, the gene frequencies of newly arisen variants, except for α8+, remained nearly constant. Intracistronic recombination has been suggested as the mechanism responsible for these phenomena.
