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 0.51.0 X 10-4/a-locus/ generation.(2) The a-esterase alleles appear to mutate in a particular direction. (3) New variants (a8+ and i ) which have not been observed previously have been obtained. (4) The Est-a locus coding for a monomer esterase appears to be more mutable than the Est-j3 locus coding for a dimer enzyme. (5) In the cage experiments, the gene frequencies of newly arisen variants, except f or a8+, remained nearly constant.Intracistronic recombination has been suggested as the mechanism responsible for these phenomena.
SummaryWith 12 alleles at the Est-α locus of Drosophila virilis, 44 genotypes of females heterozygous for a pair of them were constructed, and about 4 × 104 of their F1 progenies per genotype were electrophoretically examined. Eighty-three variants different from both of their parental alleles were obtained from a total of 2·0 × 106 progenies. Statistical analysis suggested most of their variants to be recombinants. And no null or inactive allele, of which frequency is several percent in natural populations, appeared in 11·6 × 104 progenies from females heterozygous for active alleles. These results suggest that the high variation of the Est-α locus in this species is generated by recombinants from female heterozygotes.
Reexamination of the electrophoretic mobilities of esterases encoded by the Est-alpha and the Est-beta alleles of Drosophila virilis was carried out in detail using both thin-layer agar gel and polyacrylamide slab gel electrophoresis. Many allelic products with fine differences in their electrophoretic mobilities were found and designated by a new system. Some esterases separable by the agar gel method were indistinguishable using the polyacrylamide gel method. But the polyacrylamide gel method uncovered two multiband homozygotes, alpha(d).77 and beta(d) 1.28. Some allelic frequencies on the basis of the new designation were estimated in two natural populations. As a result, it is proposed that the total scope of allelic variation at the two esterase loci of Drosophila virilis is composed of discrete distribution patterns of gene frequencies, each histogram of which shows a bell-shaped pattern.
In order to explore the mechanism of electrophoretic mutation due to intragenic recombination at the Est-a locus of Drosophila virilis, some selected alleles were repeatedly used in cross experiments.Some genotypes of female heterozygotes could produce several mutants, whereas others generated no significant number of mutants. Moreover, the directional nature of mutant occurrence was also confirmed as seen in the previous studies (Tsuno 1981). When newly arisen mutants obtained in the previous study were used in cross experiments, no increase in mutation rate was observed. From these observations, it is suggested that the majority of the mutations is due to intragenic recombination within introns rather than gene conversion. Thus, a general model for the Est-a locus is proposed.
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