EVIDENCE THAT THE E4 AND FE4 ESTERASE GENES RESPONSIBLE FOR INSECTICIDE RESISTANCE IN THE APHID MYZUS-PERSICAE (SULZER) ARE PART OF A GENE FAMILY

The amplification of genes encoding the esterases E4 and FE4 is a wide spread mechanism of insecticide resistance in the peach-potato aphid, Myzus persicae (Sulzer). We present evidence that in susceptible aphid s the two genes are adjacent to each other in a head-to-tail arrangeme nt with E4 upstream of FE4 and with approx. 19 kb of intervening seque nce. There are also at least two other closely related sequences which might come from other members of an esterase gene family, in line wit h reports of other insect gene families encoding detoxifying enzymes. The close identity between E4 and FE4 genes indicates a recent duplica tion and divergence. The subsequent amplifications giving multiple cop ies of either E4 or FE4 must have involved two separate events, each p robably occurring once and then being selected by insecticide exposure and spread by migration. The cloning of sequences upstream of the FE4 gene suggest, by comparison with E4, that the two genes are regulated in different ways. FE4 has sequences corresponding to a conventional promoter (TATA box and CAP site) that are not present in E4; on the ot her hand, FE4 lacks the CpG island present 5' of E4 genes that may con trol expression through changes in DNA methylation. The differences ar e likely to have occurred by the duplication event that gave rise to E 4 and FE4 leading to different 5' sequences.