The genetic structure of natural populations of Drosophila melanogaster. XI. Genetic variability in a local population

Abstract

Six hundred and ninety one second chromosomes were extracted from a sample population of Raleigh, North Carolina. Salivary gland chromosomes of all lines were observed and the number of inversion carrying chromosomes was 130. Viabilities of homozygotes and heterozygotes were examined. The frequency of lethal carrying chromosomes was 275/691 (or 0.398):70/130 (or 0.538) in inversion carrying chromosomes and 205/561 (or 0.365) in inversion free chromosomes. The average homozygote viability was 0.4342 including lethal lines and 0.7163 excluding those, the average heterozygote viability being 1.0000. The average degree of dominance of viability polygenes was estimated to be 0.293 ± 0.071 for all heterozygotes whose component chromosomes had better viabilities than 0.6 of the average heterozygote viability, 0.177 ± 0.077 for inversion free heterozygotes and 0.489 ± 0.082 for inversion heterozygotes. Mutation rates of viability polygenes and lethal genes were estimated on the basis of genetic loads and average degrees of dominance of lethal genes and viability polygenes. Possible overdominance and epistasis were detected, but the magnitude must be very small. The effective size of the population was estimated to be much greater than 10,000 by using the allelism rate of lethal carrying chromosomes and their frequency. On the basis of these findings and the comparison with the predicted result, the mechanisms of the maintenance of genetic variability in the population are discussed.