Table 1. Changing frequencies of four kinds of chromosomes in DDT or Dieldrin

Oshima, C. and T. Watanabe. National About forty homozygous strains of four Institute of Genetics, Misima, Japan. kinds of chromosomes, ST, AR, CH and PP, The effect of insectic&de selection on were used in the experiments. These experimental populations of strains, originated from the Mather popDrosophila pseudoobscura. ulation in California, had been established by Dobzhansky in 1963. Heterozygous female flies for ST and AR were crossed with heterozygous male flies for CH and PP. Two initial populations were made with 250 female and 250 male offspring and after two generations (one generation period lasts about 20 days), the salivary gland chromosomes of 150 larvae hatched from sampled eggs were observed for detecting the frequency of each chromosome. In the F 2 generation of both populations, the frequencies of the four kinds of chromosomes should be theoretically equal, i.e. 25 per cent of each. However, the observed frequencies were ST:33.3, AR:26.3, CH:22.7, PP:17.7 per cent in Population I and ST:26.3, AR:28.0, CH:26.3, PP:19.3 per cent in Population II. Further, each population was divided into two populations A and B. Flies in Population A were exposed to insecticide test paper (DDT 17., 27. or Dieldrin 0.17.) for one hour and transferred into a new cage in each new generation. Flies in Population B were transferred into a new cage without exposure. The test papers and test kits were prepared by WHO in Geneva and sent to us. These populations are being maintained in a constant temperature room (25 ° C) and the varying frequencies of chromosomes from F 2 to F12 in each population were observed as shown in Table 1.