Genetic Coadaptation of the Amylase Gene System in Drosophila melanogaster: Evidence for the Selective Advantage of the Lowest AMY Activity and of Its Epistatic Genetic Background

In natural populations of Drosophila melanogaster, an amylase isozyme with the lowest [alpha]-amylase activity (AMY[superscript 1,1]) is predominant. To evaluate the selective significance of AMY[superscript 1,1] and its regulatory factor(s), we examined selection experiments in laboratory populations on two distinct food environments. After 300 generations, AMY[superscript 1,1] became predominant (89%) in a glucose (a product of AMY)-rich environment, while an isozyme with higher [alpha]-amylase activity, AMY[superscript 1,6], became predominant (83%) in a starch (substrate)-rich environment. We found that the identical alleles of the amylase (Amy) gene, which encodes each of AMY[superscript 1,1] and AMY[superscript 1,6], were shared between the two populations in the different food environments, employing the nucleotide sequencing of the duplicated Amy genes. Nevertheless, AMY[superscript 1,6] homozygotes selected in the starch-rich environment had a twofold higher AMY enzyme activity than those selected in the glucose-rich environment, suggesting a coadaptation of the coding region and its regulatory factor(s) on the genetic background. Such a difference in AMY enzyme activity was not detected between AMY[superscript 1,1] homozygotes, suggesting that the effect of the genetic background is epistatic. Our results indicate that natural selection is working on the Amy gene system as a whole for flies to adapt to the various food environments of local populations.