ANALYSIS OF AMPLICONS CONTAINING THE ESTERASE GENES RESPONSIBLE FOR INSECTICIDE RESISTANCE IN THE PEACH-POTATO APHID MYZUS-PERSICAE (SULZER)

The amplification of genes encoding an insecticide-detoxifying esteras e (E4) in the peach-potato aphid Myzus persicae is one of the few exam ples where this genetic phenomenon has been shown to be involved in th e response of an intact higher organism to artificial selection. Here we report quantitative and qualitative studies of the repeat units (am plicons) containing the E4 genes in a highly resistant aphid clone. In itial studies to quantify esterase sequences showed a 5-11-fold increa se in resistant aphids compared with susceptible aphids, suggesting th e presence of 10-22 gene copies per diploid genome. A more incisive an alysis by pulsed-field gel electrophoresis confirmed the presence of a bout 12 copies of the E4 gene and showed them to be on about 24 kb amp licons, arranged as a tandem array of direct repeats. This, together w ith previous results from crossing experiments and with recent in situ hybridization studies, confirms that the E4 gene amplification in thi s aphid clone is heterozygous at a single locus. However, these data s how that the gene amplification alone cannot account for the approx. 6 0 times higher levels of E4 protein and its mRNA present in this aphid clone, and therefore resistance must involve changes in both esterase gene copy number and gene expression.