Population Genetical Studies on Insecticide Resistance of Insects

Abstract

The resistance to DDT and nicotine sulfate in the Hikone strain of D. melanogaster and D. virilis, and the Canton S strain of D. melanogaster inbred for many generations, was analyzed genetically. Flies of these strains were crossed with flies having recessive marker genes on each autosome individually and the resistance of F1 hybrids and F3 flies having various genotypes was estimated. DDT resistance in D. melanogaster was assumed to be controlled by dominant genes on both the 2nd and the 3rd chromosomes. DDT and nicotine sulfate resistance in D. virilis were ascertained to be controlled by dominant genes on both the 2nd and the 5th chromosomes. The levels of DDT and dieldrin resistance of 18 strains of D. melanogaster collected from 10 localities and 11 strains collected from 5 localities in Europe, Africa, Asia and North and South America and cultured at Biological Lab. of Cold Spring Harbor were estimated respectively by using 4% DDT test-paper, and using 0.8 and 0.4% dieldrin test-papers prepared by the WHO in Geneva. The total average mortalities of these strains 25 hrs. after exposure for various times were almost linear, when plotted on logarithmic paper. The expression of resistance to DDT and dieldrin is, therefore, normally distributed in populations. Significant differences in resistance to both insecticides and also to dehydration and starvation were observed between strains, but not between localities. The killing effect of dieldrin for Drosophila was about 100 times as strong as that of DDT. The DDT resistance of the population having a coadaptation system formed by both DDT and natural selection appeared to be maintained in DDT free environment. An artificial population which did not have a coadaptation system was produced and the change of its DDT resistance has been followed under natural selection. The resistance manifested by resistance genes located on the 2nd chromosome of Hikone strain was shown to have been linearly reduced during 2 years. This finding seems to show that highly resistant genes would be at some disavantage in such a DDT free environment. It could be ascertained that DDT selection established not only resistant genes, but also accelerated the effect of natural selection accumulating viability genes in artificial populations.