Abstract
Three Japanese natural populations of Drosophila melanogaster were transferred to laboratory cages and kept at three temperature conditions (25°C constant, 20-30°C fluctuating and 18°C constant). Their chromosomal variation was examined after about 20 months. Polymorphic inversions found at high frequency in nature were almost eliminated from the cage populations at 25°C and 20-30°C condition, and were in the process of being eliminated at 18°C. The elimination of the polymorphic inversions was also confirmed in the periodical monitor of cage populations. Inversions were disadvantageous compared to the standard chromosomes in laboratory cages, and the selection coefficient of the average inversion was found to be 0.05 or 0.06 for the heterozygotes and 0.10 or 0.12 for the homozygotes. The direct estimates of fitness components showed that the inversion chromosomes had various selective values in productivity but they had relatively constant selective values in viability which might result in the elimination of inversions. The frequency of unique (newly occurring) inversions in cage populations was equal to that found in nature, 0.0026 per major autosome arm per generation.
