Copy number variation and transposable elements feature in recent, ongoing adaptation at the Cyp6g1 locus.

The increased transcription of the Cyp6g1 gene of Drosophila melanogaster, and consequent resistance to insecticides such as DDT, is a widely cited example of adaptation mediated by cis-regulatory change. A fragment of an Accord transposable element inserted upstream of the Cyp6g1 gene is causally associated with resistance and has spread to high frequencies in populations around the world since the 1940s. Here we report the existence of a natural allelic series at this locus of D. melanogaster, involving copy number variation of Cyp6g1, and two additional transposable element insertions (a P and an HMS-Beagle). We provide evidence that this genetic variation underpins phenotypic variation, as the more derived the allele, the greater the level of DDT resistance. Tracking the spatial and temporal patterns of allele frequency changes indicates that the multiple steps of the allelic series are adaptive. Further, a DDT association study shows that the most resistant allele, Cyp6g1-[BP], is greatly enriched in the top 5% of the phenotypic distribution and accounts for approximately 16% of the underlying phenotypic variation in resistance to DDT. In contrast, copy number variation for another candidate resistance gene, Cyp12d1, is not associated with resistance. Thus the Cyp6g1 locus is a major contributor to DDT resistance in field populations, and evolution at this locus features multiple adaptive steps occurring in rapid succession.

Cis-regulatory elements in the Accord retrotransposon result in tissue-specific expression of the Drosophila melanogaster insecticide resistance gene Cyp6g1.

Transposable elements are a major mutation source and powerful agents of adaptive change. Some transposable element insertions in genomes increase to a high frequency because of the selective advantage the mutant phenotype provides. Cyp6g1-mediated insecticide resistance in Drosophila melanogaster is due to the upregulation of the cytochrome P450 gene Cyp6g1, leading to the resistance to a variety of insecticide classes. The upregulation of Cyp6g1 is correlated with the presence of the long terminal repeat (LTR) of an Accord retrotransposon inserted 291bp upstream of the Cyp6g1 transcription start site. This resistant allele (DDT-R) is currently at a high frequency in D. melanogaster populations around the world. Here, we characterize the spatial expression of Cyp6g1 in insecticide-resistant and -susceptible strains. We show that the Accord LTR insertion is indeed the resistance-associated mutation and demonstrate that the Accord LTR carries regulatory sequences that increase the expression of Cyp6g1 in tissues important for detoxification, the midgut, Malpighian tubules, and the fat body. This study provides a significant example of how changes in tissue-specific gene expression caused by transposable-element insertions can contribute to adaptation.

Cyp12a4 confers lufenuron resistance in a natural population of Drosophila melanogaster.

Lufenuron is an insect growth regulator insecticide mainly used for the control of the cat flea. To understand mechanisms of resistance to lufenuron, we have characterized lufenuron resistance in a natural population of Drosophila melanogaster. In this study we have used precise genetic mapping to identify a mechanism of lufenuron resistance: the overexpression of the cytochrome P450 gene Cyp12a4. Cyp12a4 is predicted to encode a mitochondrial cytochrome P450 enzyme. Expression of Cyp12a4 in D. melanogaster third-instar larvae was detected in the midgut and Malpighian tubules of both lufenuron-resistant and wild-type strains. The level of Cyp12a4 expression in the midgut is higher in the lufenuron-resistant strain than in wild-type strains. Driving the expression of Cyp12a4 in the midgut and Malpighian tubules by using the GAL4/UAS gene expression system results in lufenuron resistance, but it does not result in resistance to three other insecticide classes. Transgenic expression of Cyp12a4 in a ubiquitous expression pattern results in late embryonic lethality, suggesting that high-level ectopic expression of Cyp12a4 is detrimental to development.

The genetic basis of resistance to diazinon in natural populations of Drosophila melanogaster.

Isofemale strains of Drosophila melanogaster were established from single inseminated females collected from populations along the east coast of Australia. Strains were tested for resistance to the organophosphorus insecticide diazinon at larval and/or adult stages of the life cycle. Considerable phenotypic variation was observed within and between population samples but there was no association between collection site of a sample and resistance status. Adult and larval resistance levels were uncorrelated. Resistance levels in adults were low (2-fold) and polygenically based. Larval resistance levels, due to single genes (or gene complexes) on chromosomes II and III, were significant (15-fold). Evidence indicates that the gene on chromosome II is Cyp6g1.