Methylation and expression of amplified esterase genes in the aphid Myzus persicae (Sulzer)

Most populations of the aphid Myzus persicae have amplified genes (up to 80 copies) encoding the insecticide-detoxifying esterase E4. This paper repor ts the analysis of methylation of the E4 gene and its flanking DNA with the use of methylation-sensitive restriction enzymes, CpG profiling and bisulp hite sequencing. In combination these show that E4 has 5-methyl-cytosine co nfined to CpG doublets, as previously shown for vertebrate genomes; this is the first such report for an insect gene. The methylation is present withi n the gene but absent from upstream regions, including the 5' CpG-rich regi on around the start of transcription, and from 3' flanking DNA. Methylated E4 genes are expressed; loss of the 5-methylcytosine is correlated with a l oss of transcription, although this is not accompanied by a global loss of the 5-methylcytosine present in the aphid genome. These results suggest tha t the methylation of E4 has a positive role in expression, and call into qu estion the widely held view that methylation in invertebrate genomes is con fined to regions that do not contain genes and that methylation is always a ssociated with gene silencing.