Selection and characterization of spinetoram resistance in field collected Drosophila melanogaster

Insecticides are commonly employed in vineyards to control vinegar flies and limit sour rot disease. Widespread resistance to available insecticides is having a negative impact on managing Drosophila melanogaster populations, rendering control of sour rot more difficult. An insecticide registered for use in vineyards to which resistance is not yet widespread (at least in New York and Missouri) is spinetoram. Spinetoram targets the nicotinic acetylcholine receptor α6, and mutations in α6 have been associated with resistance in some insects. Our goals were to select for a spinetoram resistant strain of D. melanogaster (starting with field collected populations), characterize the resistance, and identify the mutation responsible. After five selections a strain (SpinR) with >190-fold resistance was obtained. Resistance could not be overcome by insecticide synergists, suggesting an altered target site was involved. We cloned and sequenced the α6 allele from the spinetoram resistant strain and identified a mutation causing a glycine to alanine change at amino acid 301 (equivalent position to the G275E mutation found in some spinosad/spinetoram resistant insects). This mutation was found at low levels in field populations, but increased with each selection until it became homozygous in SpinR. We discuss how the identification of the spinetoram resistance mutation can be used for resistance management.

Frequencies and distribution of kdr and Ace alleles that cause insecticide resistance in house flies in the United States

House flies (Musca domestica L) are nuisances and vectors of pathogens between and among humans and livestock. Population suppression has been accomplished for decades with pyrethroids and acetylcholinesterase (AChE) inhibitors, but recurrent selection has led to increased frequency of alleles conferring resistance to those two classes of active ingredients (Geden et al., 2021). A common mechanism of resistance to both classes involves an altered target site (mutations in Voltage gated sodium channel (Vgsc) for pyrethroids or in Ace for AChE inhibitors). As part of ongoing efforts to understand the origin, spread and evolution of insecticide resistance alleles in house fly populations, we sampled flies in 11 different US states, sequenced, and then estimated frequencies of the Vgsc and Ace alleles. There was substantial variation in frequencies of the four common knockdown resistance alleles (kdr (L1014F), kdr-his (L1014H), super-kdr (M918T + L10414F) and 1B (T929I + L1014F) across the sampled states. The kdr allele was found in all 11 states and was the most common allele in four of them. The super-kdr allele was detected in only six collections, with the highest frequencies found in the north, northeast and central United States. The kdr-his allele was the most common allele in PA, NC, TN and TX. In addition, a novel super-kdr-like mutation in mutually exclusive exon 17a was found. The overall frequencies of the different Ace alleles, which we name based on the amino acid present at the mutation sites (V260L, A316S, G342A/V and F407Y), varied considerably between states. Five Ace alleles were identified: VAGF, VAVY, VAGY, VAAY and VSAY. Generally, the VSAY allele was the most common in the populations sampled. The susceptible allele (VAGF) was found in all populations, ranging in frequency from 3% (KS) to 41% (GA). Comparisons of these resistance allele frequencies with those previously found suggests a dynamic interaction between the different alleles, in terms of levels of resistance they confer and likely fitness costs they impose in the absence of insecticides.

Selection for, and characterization of, malathion and zeta-cypermethrin resistance in vineyard-collected Drosophila melanogaster

BACKGROUND

Drosophila melanogaster is a pest in vineyards because of its role in sour rot disease. Insecticides are commonly used, particularly late in the season, to control D. melanogaster and thus sour rot. Use of insecticides in vineyards and neighboring fruit production systems has led to the evolution of insecticide resistance in D. melanogaster, which is now widespread to commonly used insecticides like zeta-cypermethrin and malathion. Implementation of resistance management strategies is facilitated by an understanding of the mechanisms and genetics underlying the resistance.

RESULTS

Starting with a vineyard-collected strain of D. melanogaster (NY18), we selected for a strain that was 1100-fold resistant to zeta-cypermethrin and one that was 40-fold resistant to malathion. Resistance was inherited as an incompletely dominant trait for zeta-cypermethrin. Resistance to malathion was inherited differently between reciprocal crosses. Insecticide bioassays using insecticide synergists found resistance to zeta-cypermethrin was partly suppressible with either piperonyl butoxide or S,S,S-tributylphosphorotrithionate, while resistance to malathion was unchanged by the synergists and mutations in Ace associated with the resistance were found.

CONCLUSIONS

Resistance to zeta-cypermethrin is most likely due to enhanced detoxification, while the results with malathion were associated with two Ace alleles. How the newly selected strains can facilitate diagnostic tools for the identification of the mutations causing the resistance is discussed. © 2022 Society of Chemical Industry.

Insecticide resistance in Drosophila melanogaster in vineyards and evaluation of alternative insecticides

BACKGROUND

Cultivation of grapes is a major crop globally, particularly in support of the wine production industry which has significant economic impact in numerous countries. Sour rot is an economically important disease of grapes. It is caused by an interaction of yeast + acetic acid bacteria, and vectored by Drosophila spp. Substantial control of sour rot in wine grape vineyards has been achieved by control of Drosophila using insecticides such as zeta-cypermethrin. An outbreak of sour rot and high populations of Drosophila melanogaster were observed in 2018 in a vineyard in New York (Finger Lakes region), USA. Flies from this population were found to be resistant to zeta-cypermethrin (the active ingredient in Mustang Maxx®), but whether or not this was a widespread problem was not known. To determine if resistance was geographically limited, we surveyed populations of D. melanogaster collected from 11 vineyards across New York State and one in Missouri (USA). We also evaluated 19 alternative insecticides for their potential use for control of D. melanogaster, by determining their toxicity to a susceptible strain and by examining cross-resistance using a field-collected population.

RESULTS

There were high levels of resistance to zeta-cypermethrin, malathion, and acetamiprid found in all populations sampled. Resistance to zeta-cypermethrin and malathion was stable over 33 months. Results from two vineyards also suggested that resistance to spinetoram was starting to evolve. The alternative insecticides we evaluated had LC₅₀ values to the susceptible strain ranging from 0.65 to 15 000 ng·cm^-2 .

CONCLUSION

Resistance to zeta-cypermethrin, malathion, and acetamiprid is geographically widespread and the levels of resistance are similar between early season and late season collections. Cross-resistance was detected against all the insecticides tested, with the lowest levels seen for broflanilide, fipronil, and flumethrin. These patterns of resistance/cross-resistance/multiple resistance are discussed in terms of selection within and outside of vineyards. The implications of these results to insecticide resistance monitoring and management are discussed.

Transcriptomic and proteomic analysis of pyrethroid resistance in the CKR strain of Aedes aegypti

Aedes aegypti is an important vector of human viral diseases. This mosquito is distributed globally and thrives in urban environments, making it a serious risk to human health. Pyrethroid insecticides have been the mainstay for control of adult A. aegypti for decades, but resistance has evolved, making control problematic in some areas. One major mechanism of pyrethroid resistance is detoxification by cytochrome P450 monooxygenases (CYPs), commonly associated with the overexpression of one or more CYPs. Unfortunately, the molecular basis underlying this mechanism remains unknown. We used a combination of RNA-seq and proteomic analysis to evaluate the molecular basis of pyrethroid resistance in the highly resistant CKR strain of A. aegypti. The CKR strain has the resistance mechanisms from the well-studied Singapore (SP) strain introgressed into the susceptible Rockefeller (ROCK) strain genome. The RNA-seq and proteomics data were complimentary; each offering insights that the other technique did not provide. However, transcriptomic results did not quantitatively mirror results of the proteomics. There were 10 CYPs which had increased expression of both transcripts and proteins. These CYPs appeared to be largely trans-regulated, except for some CYPs for which we could not rule out gene duplication. We identified 65 genes and lncRNAs as potentially being responsible for elevating the expression of CYPs in CKR. Resistance was associated with multiple loci on chromosome 1 and at least one locus on chromosome 3. We also identified five CYPs that were overexpressed only as proteins, suggesting that stabilization of CYP proteins could be a mechanism of resistance. Future studies to increase the resolution of the resistance loci, and to examine the candidate genes and lncRNAs identified here will greatly enhance our understanding of CYP-mediated resistance in A. aegypti.

CYP-mediated resistance and cross-resistance to pyrethroids and organophosphates in Aedes aegypti in the presence and absence of kdr

Aedes aegypti thrives in urban environments and transmits several debilitating human viral diseases. Thus, our ability to control this mosquito species in endemic areas is of utmost importance. The use of insecticides, mostly pyrethroids and organophosphates (OPs), has long been the primary means of controlling A. aegypti, but widespread insecticide resistance has emerged. The two main mechanisms of pyrethroid resistance in A. aegypti are CYP-mediated detoxification and mutations in the target site, voltage-sensitive sodium channel (Vssc), referred to as knockdown resistance (kdr). Knowledge about the contributions and interactions of these mechanisms to resistance is important for the understanding of the molecular and evolutionary basis of insecticide resistance, and to determine the effectiveness of insecticides. In this study, we address two aims: 1) determine the patterns of CYP-mediated cross-resistance to pyrethroid and OP insecticides, both in the presence and absence of kdr (S989P + V1016G), and 2) determine whether the interaction between the two mechanisms yields a greater than, less than, or additive effect on resistance. We tested seven pyrethroids and four OPs against three congenic strains of A. aegypti: ROCK (susceptible), CYP:ROCK (CR) (resistant due to CYP-mediated detoxification without kdr), and CYP + KDR:ROCK (CKR) (resistant due to both CYPs and kdr), and compared these to the congenic KDR:ROCK strain that was previously reported. We found that resistance ratios (RRs) were variable between pyrethroids and strains, ranging from 6.2- to 42-fold for CR, and 70- to 261-fold for CKR. In general, we found that CYP-mediated resistance alone contributes less to resistance than kdr. The effect of the combined mechanisms on resistance was significantly greater than additive for all pyrethroids except (1R)-trans-fenfluthrin. CYP-mediated pyrethroid resistance conferred cross-resistance to both methyl paraoxon and fenitrothion, and negative cross-resistance to methyl parathion and naled. Based on our results, we recommend that etofenprox and cyfluthrin be avoided for A. aegypti control in areas where these two resistance mechanisms are prevalent.

Janus: prediction and ranking of mutations required for functional interconversion of enzymes

Identification of residues responsible for functional specificity in enzymes is a challenging and important problem in protein chemistry. Active-site residues are generally easy to identify, but residues outside the active site are also important to catalysis and their identities and roles are more difficult to determine. We report a method based on analysis of multiple sequence alignments, embodied in our program Janus, for predicting mutations required to interconvert structurally related but functionally distinct enzymes. Conversion of aspartate aminotransferase into tyrosine aminotransferase is demonstrated and compared to previous efforts. Incorporation of 35 predicted mutations resulted in an enzyme with the desired substrate specificity but low catalytic activity. A single round of DNA back-shuffling with wild-type aspartate aminotransferase on this variant generated mutants with tyrosine aminotransferase activities better than those previously realized from rational design or directed evolution. Methods such as this, coupled with computational modeling, may prove invaluable in furthering our understanding of enzyme catalysis and engineering.