Relationship between amount of esterase and gene copy number in insecticide-resistant Myzus persicae (Sulzer)

Overproduction of the insecticide-degrading esterases, E4 and FE4, in peach -potato aphids, Myzus persicae (Sulzer), depends on both gene amplification and transcriptional control, the latter being associated with changes in D NA methylation. The structure and function of the aphid esterase genes have been studied but the determination of their copy number has proved difficu lt, a common problem with gene amplification. We have now used a combinatio n of pulsed-field gel electrophoresis and quantitative competitive PCR to d etermine relative esterase gene copy numbers in aphid clones with different levels of insecticide resistance (R-1, R-2 and R-3). There are approx. 4-f old increases between susceptible, R-1, R-2 and R-3 aphids, reaching a maxi mum of approx. 80 times more genes in R-3; this gives proportionate increas es in esterase protein relative to susceptible aphids. Thus there is no ove rexpression of the amplified genes, in contrast with what was thought previ ously. For E4 genes, the loss of 5-methylcytosine is correlated with a loss of expression, greatly decreasing the amount of enzyme relative to the cop y number.