Interlocus nonrandom association of polymorphisms in Drosophila chemoreceptor genes.

Some forms of multilocus selection with epistasis, such as truncation selection, can effectively reduce the mutation load [Kondrashov, A. S. (1988) Nature 336, 435-440]. Many quantitative characters, including complex genetic diseases, are likely to be subject to these types of selection. However, direct measurement of selection in natural populations is difficult and the effect of epistasis on within-species variations remains unclear. Epistatic interaction in the fitness effect can generate linkage disequilibrium (LD). Therefore, we may detect the action of natural selection from its amount and pattern. Here, we report a large number of interlocus nonrandom associations between polymorphisms in 98 Drosophila chemoreceptor genes. LD was examined in two fly samples collected at the same location, but in different seasons. The amount of LD was much larger in the spring sample than in the autumn one. The between-sample difference was much more striking for the replacement polymorphisms than for the silent polymorphisms. This difference between the replacement and silent polymorphisms could not be attributed to differences in the mean marker distances. We also found a significant excess of associations between one frequent and one less common allele for the replacement polymorphisms, but not for the silent polymorphisms. It is unlikely that a simple seasonal bottleneck could explain all these differences in the scale of LD between the samples and between the replacement and silent polymorphisms. Natural selection is suggested to play a significant role in shaping the pattern of LD observed in this study.