Dominance of insecticide resistance presents a plastic response

Frequency and spatial distribution of knock-down resistance (kdr) to pyrethroids in multiple oilseed rape pest species of the genus Ceutorhynchus

BACKGROUND

The protection of European oilseed rape (OSR) from damaging insects relies on pyrethroid insecticides, but the development of resistance in key coleopteran pests such as the pollen beetle (Brassicogethes aeneus) and the cabbage stem flea beetle (Psylliodes chrysocephala) has resulted in reduced effectiveness of these insecticides. The sodium channel gene mutation L1014F knock-down resistance (kdr) is a contributing factor in resistance to pyrethroids in B. aeneus and P. chrysocephala, but little is known about the status of resistance in weevils of the genus Ceutorhynchus (Coleoptera: Curculonidae). Therefore, the present study investigated pyrethroid susceptibility and the presence of the kdr mutation in four Ceutorhynchus species.

RESULTS

The kdr mutation in either its heterozygous or homozygous form was found in all investigated Ceutorhynchus species (C. picitarsis, C. pallidactylus, C. napi and C. obstrictus). Samples where pyrethroids in bioassays still provided control at 100% field rate or below contained kdr at frequencies of ≤12.5%, whilst bioassays using 100% field rate that did not control Ceutorhynchus populations contained homozygous resistant individuals at frequencies of greater than 55%. Field sampling demonstrated that kdr frequencies in populations of C. picitarsis and C. obstrictus collected from across France and Germany ranged from 0 to 100%.

CONCLUSION

The present study demonstrated the potential of all four Ceutorhynchus species tested to develop pyrethroid resistance via the L1014F (kdr) mutation. Although kdr frequency varies among species and geographic locations, the risk of loss of pyrethroid insecticide effectiveness is high. Integration of other control tools for resistance management is therefore needed. © 2023 Society of Chemical Industry.

Dominance and multi-locus interaction

Dominance is usually considered a constant value that describes the relative difference in fitness or phenotype between heterozygotes and the average of homozygotes at a focal polymorphic locus. However, the observed dominance can vary with the genetic background of the focal locus. Here, alleles at other loci modify the observed phenotype through position effects or dominance modifiers that are sometimes associated with pathogen resistance, lineage, sex, or mating type. Theoretical models have illustrated how variable dominance appears in the context of multi-locus interaction (epistasis). Here, we review empirical evidence for variable dominance and how the observed patterns may be captured by proposed epistatic models. We highlight how integrating epistasis and dominance is crucial for comprehensively understanding adaptation and speciation.

Confirmation of the Y215H mutation in the β2 -octopamine receptor in Varroa destructor is associated with contemporary cases of amitraz resistance in the United States

BACKGROUND

The parasitic mite, Varroa destructor (Anderson and Trueman), is a leading cause of honey bee colony losses around the world. Application of miticides such as amitraz are often the primary method of Varroa control in commercial beekeeping operations in the United States. It is likely that excessive and exclusive amitraz application has led to the development of amitraz resistance in Varroa. A mutation of tyrosine at amino acid position 215 to histidine (Y215H) in the β₂ -octopamine receptor was identified in putatively amitraz-resistant Varroa in the United States. This research investigated the presence of the Y215H mutation in quantitatively confirmed amitraz-resistant Varroa from the United States.

RESULTS

There was a strong association of susceptible and resistant phenotypes with the corresponding susceptible and resistant genotypes respectively, and vice versa. The resistance bioassay may understate resistance levels because of the influence of environmental conditions on the outcome of the test, whereby Varroa with an amitraz-resistant genotype may appear with a susceptible phenotype.

CONCLUSION

Confirmation of the Y215H mutation in the β₂ -octopamine receptor of amitraz-resistant Varroa encourages the development and validation of low-cost, high-throughput genotyping protocols to assess amitraz resistance. Resistance monitoring via genotyping will allow for large-scale passive monitoring to accurately determine the prevalence of amitraz resistance rather than directed sampling of apiaries with known resistance issues. Genotyping of Varroa for amitraz resistance early in the beekeeping season may predict late-season resistance at the colony level and provide beekeepers with enough time to develop an effective Varroa management strategy. © 2023 Society of Chemical Industry. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Hormetic and transgenerational effects in spotted-wing Drosophila (Diptera: Drosophilidae) in response to three commonly-used insecticides

Although insecticide formulations and spray rates are optimized to achieve lethal exposure, there are many factors in agricultural settings that can reduce the effective exposure of insect pests. These include weather patterns, timing of application, chemical degradation/volatilization, plant structural complexity, and resistant populations. While sub-lethal exposure to insecticides can still have negative impacts on pest populations, they can also lead to stimulatory, or hormetic, responses that can increase the fitness of surviving insects. Sub-lethal concentrations may also produce increased tolerance in the offspring of surviving adults through transgenerational effects. Sub-lethal effects are pertinent for the invasive fruit pest, spotted-wing Drosophila, Drosophila suzukii (Matsumura), because its small size, diurnal movement patterns, and utilization of hosts with complex plant structures, such as caneberries and blueberries, make effective insecticide applications tenuous. In this study, we measured spotted-wing Drosophila survivorship, reproductive performance, and offspring tolerance in flies exposed to sub-lethal concentrations of three commonly-used insecticides (zeta-cypermethrin, spinetoram, and pyrethrin). We found some evidence for hormesis, with survival effects being sex- and concentration-dependent for all insecticides. Males were far more susceptible to insecticides than females, which in some cases exhibited higher eclosion success and reproductive rates when exposed to sub-lethal doses. We did not observe significant transgenerational effects at sub-lethal concentrations, despite trends of increased offspring viability for zeta-cypermethrin and spinetoram. More research, however, is needed to fully understand the role that sub-lethal effects may play in pest population dynamics, insecticide efficacy, and the development of genetic resistance.

Inheritance of polygenic but stable pyriproxyfen resistance in a bio-control agent Chrysoperla carnea (Neuroptera: Chrysopidae): cross-resistance and realized heritability

BACKGROUND

Chrysoperla carnea (Neuroptera: Chrysopidae) is a voracious predator frequently used in biological control programs to suppress pest populations of economic importance. However, it performs its duty in a challenging environment where various stress factors such as non-target effects of insecticides limit expected outcomes. A study providing details of genetics, cross-resistance, realized heritability, and stability of insect growth regulators (IGRs) resistance such as pyriproxyfen in this bio-control agent is essential.

RESULTS

Selection with pyriproxyfen, an IGR, resulted in 3092.10-fold and 39.60-fold resistance when judged against Susceptible and Field Pop, respectively. Very low cross-resistance to buprofezin while no cross-resistance to acetamiprid and spinosad was observed. Incompletely dominant, autosomal and polygenic resistance was also associated with high realized heritability (h² = 0.35). Furthermore, resistance to pyriproxyfen was stable in this bio-control agent.

CONCLUSION

These findings make Chrysoperla carnea an ideal fit in integrated pest management (IPM) programs where biological control approaches are employed in combination with IGRs sprays to control various insect pests especially Whitefly, Bemisia tabaci. Releasing pyriproxyfen-resistant Chrysoperla carnea in a multi-sprayed cropping environment would help to keep pest population below economic threshold level. It would also minimize risk of insecticide resistance development in pests surviving even after several insecticide applications. © 2020 Society of Chemical Industry.

Biochemical mechanism, inheritance and cross-resistance to cyromazine in a non-target Chrysoperla carnea: A potential predator of whiteflies and aphids

Developing an eco-friendly Integrated Pest Management (IPM) approach is only possible by minimizing the use of insecticides and their effects on non-targets such as natural enemies. Chrysoperla carnea is a potential predator of several economic pests including whiteflies and aphids. C. carnea selected with cyromazine for 15 generations indicated a great increase in resistance to this Insect Growth Regulator (IGR). There was no cross-resistance to chlorpyrifos, cypermethrin and nitenpyram in Cyro-SEL population. Inheritance to cyromazine was autosomal, incompletely dominant, and polygenic. A high realized heritability (h₂) value (0.37) confirmed more genetic variability. Synergists piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) showed a significant involvement of P-450 monooxygenase and esterase in cyromazine resistance development. These findings are helpful to design a natural enemy-friendly IPM strategy resulting in increased survival and performance of C. carnea controlling economic pests.

Identification and functional characterization of a novel acetyl-CoA carboxylase mutation associated with ketoenol resistance in Bemisia tabaci

Insecticides of the tetronic/tetramic acid family (cyclic ketoenols) are widely used to control sucking pests such as whiteflies, aphids and mites. They act as inhibitors of acetyl-CoA carboxylase (ACC), a key enzyme for lipid biosynthesis across taxa. While it is well documented that plant ACCs targeted by herbicides have developed resistance associated with mutations at the carboxyltransferase (CT) domain, resistance to ketoenols in invertebrate pests has been previously associated either with metabolic resistance or with non-validated candidate mutations in different ACC domains. A recent study revealed high levels of spiromesifen and spirotetramat resistance in Spanish field populations of the whitefly Bemisia tabaci that was not thought to be associated with metabolic resistance. We confirm the presence of high resistance levels (up to >640-fold) against ketoenol insecticides in both Spanish and Australian B. tabaci strains of the MED and MEAM1 species, respectively. RNAseq analysis revealed the presence of an ACC variant bearing a mutation that results in an amino acid substitution, A2083V, in a highly conserved region of the CT domain. F1 progeny resulting from reciprocal crosses between susceptible and resistant lines are almost fully resistant, suggesting an autosomal dominant mode of inheritance. In order to functionally investigate the contribution of this mutation and other candidate mutations previously reported in resistance phenotypes, we used CRISPR/Cas9 to generate genome modified Drosophila lines. Toxicity bioassays using multiple transgenic fly lines confirmed that A2083V causes high levels of resistance to commercial ketoenols. We therefore developed a pyrosequencing-based diagnostic assay to map the spread of the resistance alleles in field-collected samples from Spain. Our screening confirmed the presence of target-site resistance in numerous field-populations collected in Sevilla, Murcia and Almeria. This emphasizes the importance of implementing appropriate resistance management strategies to prevent or slow the spread of resistance through global whitefly populations.

Molecular characterization of Cry1F resistance in fall armyworm, Spodoptera frugiperda from Brazil

Fall armyworm, Spodoptera frugiperda (J.E. Smith) is a major lepidopteran pest of maize in Brazil and its control particularly relies on the use of genetically engineered crops expressing Bacillus thuringiensis (Bt) toxins such as Cry1F. However, control failures compromising the efficacy of this technology have been reported in many regions in Brazil, but the mechanism of Cry1F resistance in Brazilian fall armyworm populations remained elusive. Here we investigated the molecular mechanism of Cry1F resistance in two field-collected strains of S. frugiperda from Brazil exhibiting high levels of Cry1F resistance. We first rigorously evaluated several candidate reference genes for normalization of gene expression data across strains, larval instars and gut tissues, and identified ribosomal proteins L10, L17 and RPS3A to be most suitable. We then investigated the expression pattern of ten potential Bt toxin receptors/enzymes in both neonates and 2nd instar gut tissue of Cry1F resistant fall armyworm strains compared to a susceptible strain. Next we sequenced the ATP-dependent Binding Cassette subfamily C2 gene (ABCC2) and identified three mutated sites present in ABCC2 of both Cry1F resistant strains: two of them, a GY deletion (positions 788-789) and a P799 K/R amino acid substitution, located in a conserved region of ABCC2 extracellular loop 4 (EC4) and another amino acid substitution, G1088D, but in a less conserved region. We further characterized the role of the novel mutations present in EC4 by functionally expressing both wild type and mutated ABCC2 transporters in insect cell lines, and confirmed a critical role of both sites for Cry1F binding by cell viability assays. Finally, we assessed the frequency of the mutant alleles by pooled population sequencing and pyrosequencing in 40 fall armyworm populations collected from maize fields in different regions in Brazil. We found that the GY deletion being present at high frequency. However we also observed many rare alleles which disrupt residues between sites 783-799, and their diversity and abundance in field collected populations lends further support to the importance of the EC4 domain for Cry1F toxicity.

Selection and characterization of the inheritance of resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to chlorantraniliprole and cross-resistance to other diamide insecticides

BACKGROUND

Understanding the genetic basis of insect resistance to insecticides can help to implement insecticide resistance management (IRM) strategies. In this study, we selected a strain of Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) resistant to chlorantraniliprole using the F₂ screen method, characterized the inheritance of resistance and evaluated patterns of cross-resistance to other diamide insecticides.

RESULTS

The chlorantraniliprole-resistant strain (Chlorant-R) was selected from a field-collected population with an estimated allele frequency of 0.1316. The estimated median lethal concentration (LC₅₀ ) values were 0.011 and 2.610 µg a.i. cm^-2 for the susceptible (Sus) and Chlorant-R strains, respectively, resulting in a resistance ratio (RR) of 237-fold. The LC₅₀ values of the reciprocal crosses were 0.155 and 0.164 µg a.i. cm^-2 , indicating that resistance is autosomally inherited. Resistance was characterized as incompletely recessive and monogenic at concentrations close to the recommended field rates of chlorantraniliprole. Survival of the resistant strain and heterozygous larvae in maize plants treated at the field rate was ∼ 60 (similar to the susceptible strain on untreated maize) and 15%, respectively. The Chlorant-R strain showed low cross-resistance to cyantraniliprole (RR ∼ 27-fold) and very high cross-resistance to flubendiamide (RR > 42 000-fold).

CONCLUSIONS

The frequency of chlorantraniliprole resistance allele was moderately high in a field-collected population of S. frugiperda. The inheritance of chlorantraniliprole resistance was characterized as autosomal, incompletely recessive and monogenic. S. frugiperda showed cross-resistance to other diamide insecticides. These results provide essential information for the implementation of IRM programs to preserve the useful life of diamide insecticides for controlling S. frugiperda in Brazil. © 2019 Society of Chemical Industry.

Inheritance and fitness costs of resistance to Bacillus thuringiensis toxin Cry2Ad in laboratory strains of the diamondback moth, Plutella xylostella (L.)

The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), is one of the main pests of Brassica crops worldwide. Management of P. xylostella is particularly challenging, as different field populations have readily acquired resistance to a wide range of insecticides, including Bacillus thuringiensis (Bt) toxins. In this study, a novel strain of P. xyllostela (Fuzhou-R2Ad) with 120-fold resistance to Bt Cry2Ad was selected in the laboratory, after screening for 66 generations from the susceptible strain Fuzhou-S. In the absence of Bt Cry2Ad toxin, the Fuzhou-R2Ad had significantly lower fitness as compared to the susceptible strain, which might be related to induced genetic changes to Bt toxins. We used several models to measure the dominance levels of insecticide resistance among different strains and found an incompletely recessive inheritance pattern of the Fuzhou-R2Ad resistance, which might be controlled by multiple genes. This study constitutes the first report of laboratory-acquired resistance to Cry2Ad toxin in P. xylostella. Our work presents further insights into the mechanism of Bt resistance and has immediate implications for the integrated pest management of P. xylostella globally.

A genomic approach to identify and monitor a novel pyrethroid resistance mutation in the redlegged earth mite, Halotydeus destructor

Resistance mechanisms are typically uncovered by identifying sequence variation in known candidate genes, however this strategy can be problematic for species with no reference data in known relatives. Here we take a genomic approach to identify resistance to pyrethroids in the redlegged earth mite, Halotydeus destructor, a member of the Penthalidae family of mites that are virtually uncharacterized genetically. Based on shallow genome sequencing followed by a genome assembly, we first identified contigs of the H. destructor parasodium channel gene. By linking variation in this gene to known resistant phenotypes, we located a single nucleotide polymorphism in resistant mites. This polymorphism results in a leucine (L) to phenylalanine (F) amino acid substitution in the II6 region (predicted) of the gene (L1024F). This novel mutation has not previously been linked to pyrethroid resistance, although other polymorphisms have been identified in the two-spotted spider mite, Tetranychus urticae at the same locus (L1024V). The sequencing approach was successful in generating a candidate polymorphism that was then validated using laboratory bioassays and field surveys. A high throughput Illumina-based sequencing diagnostic was developed to rapidly assess resistance allele frequencies in pools of mites sourced from hundreds of populations across Australia. Resistance was confirmed to be widespread in the southern wheatbelt region of Western Australia. Two different resistance mutations were identified in field populations, both resulting in the same amino acid substitution. The frequency and distribution of resistance amplicon haplotypes suggests at least two, and probably more independent origins of resistance.

Biotic interactions govern genetic adaptation to toxicants

The genetic recovery of resistant populations released from pesticide exposure is accelerated by the presence of environmental stressors. By contrast, the relevance of environmental stressors for the spread of resistance during pesticide exposure has not been studied. Moreover, the consequences of interactions between different stressors have not been considered. Here we show that stress through intraspecific competition accelerates microevolution, because it enhances fitness differences between adapted and non-adapted individuals. By contrast, stress through interspecific competition or predation reduces intraspecific competition and thereby delays microevolution. This was demonstrated in mosquito populations (Culex quinquefasciatus) that were exposed to the pesticide chlorpyrifos. Non-selective predation through harvesting and interspecific competition with Daphnia magna delayed the selection for individuals carrying the ace-1^R resistance allele. Under non-toxic conditions, susceptible individuals without ace-1^R prevailed. Likewise, predation delayed the reverse adaptation of the populations to a non-toxic environment, while the effect of interspecific competition was not significant. Applying a simulation model, we further identified how microevolution is generally determined by the type and degree of competition and predation. We infer that interactions with other species—especially strong in ecosystems with high biodiversity—can delay the development of pesticide resistance.

Impact of violated high-dose refuge assumptions on evolution of Bt resistance

Transgenic crops expressing Bacillus thuringiensis (Bt) toxins have been widely and successfully deployed for the control of target pests, while allowing a substantial reduction in insecticide use. The evolution of resistance (a heritable decrease in susceptibility to Bt toxins) can pose a threat to sustained control of target pests, but a high‐dose refuge (HDR) management strategy has been key to delaying countervailing evolution of Bt resistance. The HDR strategy relies on the mating frequency between susceptible and resistant individuals, so either partial dominance of resistant alleles or nonrandom mating in the pest population itself could elevate the pace of resistance evolution. Using classic Wright‐Fisher genetic models, we investigated the impact of deviations from standard refuge model assumptions on resistance evolution in the pest populations. We show that when Bt selection is strong, even deviations from random mating and/or strictly recessive resistance that are below the threshold of detection can yield dramatic increases in the pace of resistance evolution. Resistance evolution is hastened whenever the order of magnitude of model violations exceeds the initial frequency of resistant alleles. We also show that the existence of a fitness cost for resistant individuals on the refuge crop cannot easily overcome the effect of violated HDR assumptions. We propose a parametrically explicit framework that enables both comparison of various field situations and model inference. Using this model, we propose novel empiric estimators of the pace of resistance evolution (and time to loss of control), whose simple calculation relies on the observed change in resistance allele frequency.

High chlorpyrifos resistance in Culex pipiens mosquitoes: strong synergy between resistance genes

We investigated the genetic determinism of high chlorpyrifos resistance (HCR), a phenotype first described in 1999 in Culex pipiens mosquitoes surviving chlorpyrifos doses ⩾1 mg l(-1) and more recently found in field samples from Tunisia, Israel or Indian Ocean islands. Through chlorpyrifos selection, we selected several HCR strains that displayed over 10 000-fold resistance. All strains were homozygous for resistant alleles at two main loci: the ace-1 gene, with the resistant ace-1(R) allele expressing the insensitive G119S acetylcholinesterase, and a resistant allele of an unknown gene (named T) linked to the sex and ace-2 genes. We constructed a strain carrying only the T-resistant allele and studied its resistance characteristics. By crossing this strain with strains harboring different alleles at the ace-1 locus, we showed that the resistant ace-1(R) and the T alleles act in strong synergy, as they elicited a resistance 100 times higher than expected from a simple multiplicative effect. This effect was specific to chlorpyrifos and parathion and was not affected by synergists. We also examined how HCR was expressed in strains carrying other ace-1-resistant alleles, such as ace-1(V) or the duplicated ace-1(D) allele, currently spreading worldwide. We identified two major parameters that influenced the level of resistance: the number and the nature of the ace-1-resistant alleles and the number of T alleles. Our data fit a model that predicts that the T allele acts by decreasing chlorpyrifos concentration in the compartment targeted in insects.

The dominance effect of the adaptive transposable element insertion Bari-Jheh depends on the genetic background

Although adaptive mutations are often considered to be dominant, it has been recently shown that a substantial proportion of adaptive mutations should display heterozygote advantage. In this work, we take advantage of a recently characterized transposable element insertion mediating oxidative stress response in Drosophila melanogaster to test the dominance effect of an adaptive mutation. The comparison of the survival curves of heterozygous and the two corresponding homozygous flies indicated that the dominance effect of Bari-Jheh depends on the genetic background. Both in homozygous and in heterozygous flies, Bari-Jheh was associated with upregulation of Jheh1 (Juvenile Hormone Epoxyde Hydrolase 1) and/or Jheh2 genes. Our results add to the limited number of studies in which the dominance effect of adaptive mutations has been empirically estimated and highlights the complexity of their inheritance.

Mapping of single-copy genes by TSA-FISH in the codling moth, Cydia pomonella

Background

We work on the development of transgenic sexing strains in the codling moth, Cydia pomonella (Tortricidae), which would enable to produce male-only progeny for the population control of this pest using sterile insect technique (SIT). To facilitate this research, we have developed a number of cytogenetic and molecular tools, including a physical map of the codling moth Z chromosome using BAC-FISH (fluorescence in situ hybridization with bacterial artificial chromosome probes). However, chromosomal localization of unique, single-copy sequences such as a transgene cassette by conventional FISH remains challenging. In this study, we adapted a FISH protocol with tyramide signal amplification (TSA-FISH) for detection of single-copy genes in Lepidoptera. We tested the protocol with probes prepared from partial sequences of Z-linked genes in the codling moth.

Results

Using a modified TSA-FISH protocol we successfully mapped a partial sequence of the Acetylcholinesterase 1 (Ace-1) gene to the Z chromosome and confirmed thus its Z-linkage. A subsequent combination of BAC-FISH with BAC probes containing anticipated neighbouring Z-linked genes and TSA-FISH with the Ace-1 probe allowed the integration of Ace-1 in the physical map of the codling moth Z chromosome. We also developed a two-colour TSA-FISH protocol which enabled us simultaneous localization of two Z-linked genes, Ace-1 and Notch, to the expected regions of the Z chromosome.

Conclusions

We showed that TSA-FISH represents a reliable technique for physical mapping of genes on chromosomes of moths and butterflies. Our results suggest that this technique can be combined with BAC-FISH and in the future used for physical localization of transgene cassettes on chromosomes of transgenic lines in the codling moth or other lepidopteran species. Furthermore, the developed protocol for two-colour TSA-FISH might become a powerful tool for synteny mapping in non-model organisms.

Testing for beneficial reversal of dominance during salinity shifts in the invasive copepod Eurytemora affinis, and implications for the maintenance of genetic variation

Maintenance of genetic variation at loci under selection has profound implications for adaptation under environmental change. In temporally and spatially varying habitats, non-neutral polymorphism could be maintained by heterozygote advantage across environments (marginal overdominance), which could be greatly increased by beneficial reversal of dominance across conditions. We tested for reversal of dominance and marginal overdominance in salinity tolerance in the saltwater-to-freshwater invading copepod Eurytemora affinis. We compared survival of F1 offspring generated by crossing saline and freshwater inbred lines (between-salinity F1 crosses) relative to within-salinity F1 crosses, across three salinities. We found evidence for both beneficial reversal of dominance and marginal overdominance in salinity tolerance. In support of reversal of dominance, survival of between-salinity F1 crosses was not different from that of freshwater F1 crosses under freshwater conditions and saltwater F1 crosses under saltwater conditions. In support of marginal overdominance, between-salinity F1 crosses exhibited significantly higher survival across salinities relative to both freshwater and saltwater F1 crosses. Our study provides a rare empirical example of complete beneficial reversal of dominance associated with environmental change. This mechanism might be crucial for maintaining genetic variation in salinity tolerance in E. affinis populations, allowing rapid adaptation to salinity changes during habitat invasions.

Point mutations associated with organophosphate and carbamate resistance in Chinese strains of Culex pipiens quinquefasciatus (Diptera: Culicidae)

Acetylcholinesterase resistance has been well documented in many insects, including several mosquito species. We tested the resistance of five wild, Chinese strains of the mosquito Culex pipiens quinquefasciatus to two kinds of pesticides, dichlorvos and propoxur. An acetylcholinesterase gene (ace1) was cloned and sequenced from a pooled sample of mosquitoes from these five strains and the amino acids of five positions were found to vary (V185M, G247S, A328S, A391T, and T682A). Analysis of the correlation between mutation frequencies and resistance levels (LC₅₀) suggests that two point mutations, G247S (r² = 0.732, P = 0.065) and A328S (r² = 0.891, P = 0.016), are associated with resistance to propoxur but not to dichlorvos. Although the V185M mutation was not associated with either dichlorvos or propoxur resistance, its RS genotype frequency was correlated with propoxur resistance (r² = 0.815, P = 0.036). And the HWE test showed the A328S mutation is linked with V185M, also with G247S mutation. This suggested that these three mutations may contribute synergistically to propoxur resistance. The T682A mutation was negatively correlated with propoxur (r² = 0.788, P = 0.045) resistance. Knowledge of these mutations may help design strategies for managing pesticide resistance in wild mosquito populations.

Dominant inheritance of field-evolved resistance to Bt corn in Busseolafusca

Transgenic crops expressing Bacillus thuringiensis (Bt) toxins have been adopted worldwide, notably in developing countries. In spite of their success in controlling target pests while allowing a substantial reduction of insecticide use, the sustainable control of these pest populations is threatened by the evolution of resistance. The implementation of the “high dose/refuge” strategy for managing insect resistance in transgenic crops aims at delaying the evolution of resistance to Bt crops in pest populations by promoting survival of susceptible insects. However, a crucial condition for the “high dose/refuge” strategy to be efficient is that the inheritance of resistance should be functionally recessive. Busseolafusca developed high levels of resistance to the Bt toxin Cry 1Ab expressed in Bt corn in South Africa. To test whether the inheritance of B. fusca resistance to the Bt toxin could be considered recessive we performed controlled crosses with this pest and evaluated its survival on Bt and non-Bt corn. Results show that resistance of B. fusca to Bt corn is dominant, which refutes the hypothesis of recessive inheritance. Survival on Bt corn was not lower than on non-Bt corn for both resistant larvae and the F₁ progeny from resistant × susceptible parents. Hence, resistance management strategies of B. fusca to Bt corn must address non-recessive resistance.

Lengthening of insect development on Bt zone results in adult emergence asynchrony: does it influence the effectiveness of the high dose/refuge zone strategy?

The “High Dose/Refuge” strategy (HD/R) is the currently recommended Insect Resistance Management strategy (IRM) to limit resistance development to Bacillus thuringiensis (Bt) plants. This strategy requires planting a “refuge zone” composed of non-Bt plants suitable for the target insect and in close proximity to a “Bt zone” expressing a high toxin concentration. One of the main assumptions is that enough susceptible adults mate with resistant insects. However, previous studies have suggested that the high toxin concentration produced by Bt plants induces slower insect development, creating an asynchrony in emergence between the refuge and the Bt zone and leading to assortative mating between adults inside each zone. Here, we develop a deterministic model to estimate the impact of toxin concentration, emergence asynchrony and refuge zone size on the effectiveness of the HD/R strategy. We conclude that emergence asynchrony only affects resistance when toxin concentration is high and resistance is recessive. Resistance develops more rapidly and survival of susceptible insects is higher at lower toxin concentration, but in such situations, resistance is insensitive to emergence asynchrony.

Effectiveness of the high dose/refuge strategy for managing pest resistance to Bacillus thuringiensis (Bt) plants expressing one or two toxins

To delay resistance development to Bacillus thuringiensis (Bt) plants expressing their own insecticide, the application of the Insect Resistance Management strategy called "High Dose/Refuge Strategy" (HD/R) is recommended by the US Environmental Protection Agency (US EPA). This strategy was developed for Bt plants expressing one toxin. Presently, however, new Bt plants that simultaneously express two toxins are on the market. We used a mathematical model to evaluate the efficiency of the HD/R strategy for both these Bt toxins. As the current two-toxin Bt plants do not express two new Cry toxins but reuse one toxin already in use with a one-toxin plant, we estimated the spread of resistance when the resistance alleles are not rare. This study assesses: (i) whether the two toxins have to be present in high concentration, and (ii) the impact of the relative size of the refuge zone on the evolution of resistance and population density. We concluded that for Bt plants expressing one toxin, a high concentration is an essential condition for resistance management. For the pyramided Bt plants, one toxin could be expressed at a low titer if the two toxins are used for the first time, and a small refuge zone is acceptable.

The importance of modelling the spread of insecticide resistance in a heterogeneous environment: the example of adding synergists to bed nets

Background

Insecticides are an effective and practical tool for reducing malaria transmission but the development of resistance to the insecticides can potentially compromise controls efforts. In this study a mathematical model was developed to explore the effects on mosquito populations of spatial heterogeneous deployment of insecticides. This model was used to identify important parameters in the evolution of insecticide resistance and to examine the contribution of new generation long-lasting insecticidal bed nets, that incorporate a chemical synergist on the roof panel, in delaying insecticide resistance.

Methods

A genetic model was developed to predict changes in mosquito fitness and resistance allele frequency. Parameters describing insecticide selection, fitness cost and the additional use of synergist were incorporated. Uncertainty and sensitivity analysis were performed followed by investigation of the evolution of resistance under scenarios of fully effective or ineffective synergists.

Results

The spread of resistance was most sensitive to selection coefficients, fitness cost and dominance coefficients while mean fitness was most affected by baseline fitness levels. Using a synergist delayed the spread of resistance but could, in specific circumstances that were thoroughly investigated, actually increase the rate of spread. Different spread dynamics were observed, with simulations leading to fixation, loss and most interestingly, equilibrium (without explicit overdominance) of the resistance allele.

Conclusions

This strategy has the potential to delay the spread of resistance but note that in an heterogeneous environment it can also lead to the opposite effect, i.e., increasing the rate of spread. This clearly emphasizes that selection pressure acting inside the house cannot be treated in isolation but must be placed in context of overall insecticide use in an heterogeneous environment.

Evolution and the microbial control of insects

Insect pathogens can be utilized in a variety of pest management approaches, from inundative release to augmentation and classical biological control, and microevolution and the consideration of evolutionary principles can potentially influence the success of all these strategies. Considerable diversity exists in natural entomopathogen populations and this diversity can be either beneficial or detrimental for pest suppression, depending on the pathogen and its mode of competition, and this should be considered in the selection of isolates for biological control. Target hosts can exhibit considerable variation in their susceptibility to entomopathogens, and cases of field-evolved resistance have been documented for Bacillus thuringiensis and baculoviruses. Strong selection, limited pathogen diversity, reduced gene flow, and host plant chemistry are linked to cases of resistance and should be considered when developing resistance management strategies. Pre- and post-release monitoring of microbial control programs have received little attention; however, to date there have been no reports of host-range evolution or long-term negative effects on nontarget hosts. Comparative analyses of pathogen population structure, virulence, and host resistance over time are required to elucidate the evolutionary dynamics of microbial control systems.

Blocking the evolution of insecticide-resistant malaria vectors with a microsporidian

Finding a way to block the evolution insecticide resistance would be a major breakthrough for the control of malaria. We suggest that this may be possible by introducing a stress into mosquito populations that restores the sensitivity of genetically resistant mosquitoes and that decreases their longevity when they are not exposed to insecticide. We use a mathematical model to show that, despite the intense selection pressure imposed by insecticides, moderate levels of stress might tip the evolutionary balance between costs and benefits of resistance toward maintaining sensitivity. Our experimental work with the microsporidian parasite Vavraia culicis infecting two lines of resistant mosquitoes and a sensitive line suggests that it may indeed be possible to stress the mosquitoes in the required way. The mortality of resistant mosquitoes 24 h after exposure to the insecticide was up to 8.8 times higher in infected than in uninfected ones; if mosquitoes were not exposed to the insecticide, resistant mosquitoes infected by the microsporidian lived about half as long as uninfected ones and insecticide-sensitive mosquitoes (with or without the parasite). Our results suggest that biopesticides or other insecticides that interfere with the expression of resistance may help to manage insecticide resistance in programs of malaria control.

Fitness landscapes: an alternative theory for the dominance of mutation

Deleterious mutations tend to be recessive. Several theories, notably those of Fisher (based on selection) and Wright (based on metabolism), have been put forward to explain this pattern. Despite a long-lasting debate, the matter remains unresolved. This debate has focused on the average dominance of mutations. However, we also know very little about the distribution of dominance coefficients among mutations, and about its variation across environments. In this article we present a new approach to predicting this distribution. Our approach is based on a phenotypic fitness landscape model. First, we show that under a very broad range of conditions (and environments), the average dominance of mutation of small effects should be approximately one-quarter as long as adaptation of organisms to their environment can be well described by stabilizing selection on an arbitrary set of phenotypic traits. Second, the theory allows predicting the whole distribution of dominance coefficients among mutants. Because it provides quantitative rather than qualitative predictions, this theory can be directly compared to data. We found that its prediction on mean dominance (average dominance close to 0.25) agreed well with the data, based on a meta-analysis of dominance data for mildly deleterious mutations. However, a simple landscape model does not account for the dominance of mutations of large effects and we provide possible extension of the theory for this class of mutations. Because dominance is a central parameter for evolutionary theory, and because these predictions are quantitative, they set the stage for a wide range of applications and further empirical tests.

The relationship between population structure and aluminum tolerance in cultivated sorghum

BACKGROUND

Acid soils comprise up to 50% of the world's arable lands and in these areas aluminum (Al) toxicity impairs root growth, strongly limiting crop yield. Food security is thereby compromised in many developing countries located in tropical and subtropical regions worldwide. In sorghum, SbMATE, an Al-activated citrate transporter, underlies the Alt(SB) locus on chromosome 3 and confers Al tolerance via Al-activated root citrate release.

METHODOLOGY

Population structure was studied in 254 sorghum accessions representative of the diversity present in cultivated sorghums. Al tolerance was assessed as the degree of root growth inhibition in nutrient solution containing Al. A genetic analysis based on markers flanking Alt(SB) and SbMATE expression was undertaken to assess a possible role for Alt(SB) in Al tolerant accessions. In addition, the mode of gene action was estimated concerning the Al tolerance trait. Comparisons between models that include population structure were applied to assess the importance of each subpopulation to Al tolerance.

CONCLUSION/SIGNIFICANCE

Six subpopulations were revealed featuring specific racial and geographic origins. Al tolerance was found to be rather rare and present primarily in guinea and to lesser extent in caudatum subpopulations. Alt(SB) was found to play a role in Al tolerance in most of the Al tolerant accessions. A striking variation was observed in the mode of gene action for the Al tolerance trait, which ranged from almost complete recessivity to near complete dominance, with a higher frequency of partially recessive sources of Al tolerance. A possible interpretation of our results concerning the origin and evolution of Al tolerance in cultivated sorghum is discussed. This study demonstrates the importance of deeply exploring the crop diversity reservoir both for a comprehensive view of the dynamics underlying the distribution and function of Al tolerance genes and to design efficient molecular breeding strategies aimed at enhancing Al tolerance.

Segregating variation for temperature-dependent sex determination in a lizard

Temperature-dependent sex determination (TSD) was first reported in 1966 in an African lizard. It has since been shown that TSD occurs in some fish, several lizards, tuataras, numerous turtles and all crocodilians. Extreme temperatures can also cause sex reversal in several amphibians and lizards with genotypic sex determination. Research in TSD species indicates that estrogen signaling is important for ovary development and that orthologs of mammalian genes have a function in gonad differentiation. Nevertheless, the mechanism that actually transduces temperature into a biological signal for ovary versus testis development is not known in any species. Classical genetics could be used to identify the loci underlying TSD, but only if there is segregating variation for TSD. Here, we use the 'animal model' to analyze inheritance of sexual phenotype in a 13-generation pedigree of captive leopard geckos, Eublepharis macularius, a TSD reptile. We directly show genetic variance and genotype-by-temperature interactions for sex determination. Additive genetic variation was significant at a temperature that produces a female-biased sex ratio (30°C), but not at a temperature that produces a male-biased sex ratio (32.5°C). Conversely, dominance variance was significant at the male-biased temperature (32.5°C), but not at the female-biased temperature (30°C). Non-genetic maternal effects on sex determination were negligible in comparison with additive genetic variance, dominance variance and the primary effect of temperature. These data show for the first time that there is segregating variation for TSD in a reptile and consequently that a quantitative trait locus analysis would be practicable for identifying the genes underlying TSD.

Environment-dependent inbreeding depression: its ecological and evolutionary significance

Inbreeding depression is a major evolutionary and ecological force that influences population dynamics and the evolution of inbreeding-avoidance traits such as mating systems and dispersal. There is now compelling evidence that inbreeding depression is environment-dependent. Here, we discuss ecological and evolutionary consequences of environment-dependent inbreeding depression. The environmental dependence of inbreeding depression may be caused by environment-dependent phenotypic expression, environment-dependent dominance, and environment-dependent natural selection. The existence of environment-dependent inbreeding depression challenges classical models of inbreeding as caused by unconditionally deleterious alleles, and suggests that balancing selection may shape inbreeding depression in natural populations; loci associated with inbreeding depression in some environments may even contribute to adaptation to others. Environment-dependent inbreeding depression also has important, often neglected, ecological and evolutionary consequences: it can influence the demography of marginal or colonizing populations and alter adaptive optima of mating systems, dispersal, and their associated traits. Incorporating the environmental dependence of inbreeding depression into theoretical models and empirical studies is necessary for understanding the genetic and ecological basis of inbreeding depression and its consequences in natural populations.

Dominance and the maintenance of polymorphism in multiallelic migration-selection models with two demes

The maintenance of genetic variation in a spatially heterogeneous environment has been one of the main research themes in theoretical population genetics. Despite considerable progress in understanding the consequences of spatially structured environments on genetic variation, many problems remain unsolved. One of them concerns the relationship between the number of demes, the degree of dominance, and the maximum number of alleles that can be maintained by selection in a subdivided population. In this work, we study the potential of maintaining genetic variation in a two-deme model with deme-independent degree of intermediate dominance, which includes absence of G x E interaction as a special case. We present a thorough numerical analysis of a two-deme three-allele model, which allows us to identify dominance and selection patterns that harbor the potential for stable triallelic equilibria. The information gained by this approach is then used to construct an example in which existence and asymptotic stability of a fully polymorphic equilibrium can be proved analytically. Noteworthy, in this example the parameter range in which three alleles can coexist is maximized for intermediate migration rates. Our results can be interpreted in a specialist-generalist context and (among others) show when two specialists can coexist with a generalist in two demes if the degree of dominance is deme independent and intermediate. The dominance relation between the generalist allele and the specialist alleles play a decisive role. We also discuss linear selection on a quantitative trait and show that G x E interaction is not necessary for the maintenance of more than two alleles in two demes.

Costs of insensitive acetylcholinesterase insecticide resistance for the malaria vector Anopheles gambiae homozygous for the G119S mutation

BACKGROUND

The G119S mutation responsible for insensitive acetylcholinesterase resistance to organophosphate and carbamate insecticides has recently been reported from natural populations of Anopheles gambiae in West Africa. These reports suggest there are costs of resistance associated with this mutation for An. gambiae, especially for homozygous individuals, and these costs could be influential in determining the frequency of carbamate resistance in these populations.

METHODS

Life-history traits of the AcerKis and Kisumu strains of An. gambiae were compared following the manipulation of larval food availability in three separate experiments conducted in an insecticide-free laboratory environment. These two strains share the same genetic background, but differ in being homozygous for the presence or absence of the G119S mutation at the ace-1 locus, respectively.

RESULTS

Pupae of the resistant strain were significantly more likely to die during pupation than those of the susceptible strain. Ages at pupation were significantly earlier for the resistant strain and their dry starved weights were significantly lighter; this difference in weight remained when the two strains were matched for ages at pupation.

CONCLUSIONS

The main cost of resistance found for An. gambiae mosquitoes homozygous for the G119S mutation was that they were significantly more likely to die during pupation than their susceptible counterparts, and they did so across a range of larval food conditions. Comparing the frequency of G119S in fourth instar larvae and adults emerging from the same populations would provide a way to test whether this cost of resistance is being expressed in natural populations of An. gambiae and influencing the dynamics of this resistance mutation.

Fitness costs of insect resistance to Bacillus thuringiensis

Evolution of resistance by insect pests threatens the continued effectiveness of Bacillus thuringiensis (Bt) toxins in sprays and transgenic crops. Fitness costs of Bt resistance occur when, in the absence of Bt toxins, fitness is lower for resistant insects than for susceptible insects. Modeling results show that fitness costs can delay resistance by selecting against Bt-resistant genotypes in refuges where insects are not exposed to Bt toxins. In 77 studies including 18 species, fitness costs were detected in 62% of experiments testing for declines in resistance and in 34% of fitness component comparisons. Mean fitness costs were 15.5% for survival, 7.4% for development time, and 2.5% for mass. Although most fitness costs were recessive, nonrecessive costs can select more strongly against resistance. Because fitness costs vary with ecological conditions, refuges designed to increase the dominance or magnitude of fitness costs could be especially useful for delaying pest resistance.

Host-plant effects the expression of resistance to Bacillus thuringiensis kurstaki in Trichoplusia ni (Hubner): an important factor in resistance evolution

Pathogens are thought to exert strong selection on their hosts leading to increased host resistance. Bacillus thuringiensis kurstaki (Bkt) is a ubiquitous entomopathogen that has become the mainstay of nonchemical control of Lepidopteran pests and thus, the potential exists for the evolution of resistance in targeted host insects. We have studied the expression of Btk resistance in the cabbage looper, Trichoplusia ni (Hubner). For this generalist insect herbivore, three common host plants, tomato, pepper and cucumber, vary in suitability for larval growth and development. Here we show that the host plant also affects the overall toxicity of Btk, the relative expression of resistance between a resistant and a susceptible line and their F(1) reciprocal crosses, and importantly, the dominance of the resistance trait. This study demonstrates that tri-trophic interactions involving an insect, host plants and a pathogen have the potential to strongly influence the evolutionary response of an insect host to a pathogen.

Unresolved boundaries of evolutionary theory and the question of how inheritance systems evolve: 75 years of debate on the evolution of dominance

One of the key issues in the evolution of life is the evolution of inheritance systems. In population genetics, the earliest attempt at addressing the latter problem revolved around Fisher's theory on the evolution of dominance. Fisher's hypothesis was that inheritance systems could be modified during the evolutionary process in such a way that wild-type phenotypes could become dominant with respect to mutant phenotypes. This would result in the buffering of a population against the deleterious effects of mutations. The debate that ensued on this topic has been one of the most longstanding in evolutionary theory. At present, the prevalent view is that dominance cannot evolve as a direct result of selection. Furthermore, it has been argued that due to inherent constraints in biochemical systems, the manifestation of dominance is a default expectation and hence evolutionary explanations are not necessary. This has led to the position that the subject is generally resolved and no further debate is necessary. However, there are also several studies indicating that dominance levels can be modified as a result of changes in the genetic background. Furthermore, other studies have indicated that dominance selection is possible in certain circumstances. To a large degree, conclusions from both of the latter types of studies have been ignored. In this article, the history of several intellectual and methodological traditions that have contributed to this debate are traced, including experimental genetics, theoretical population genetics and theoretical biochemistry. In the light of both old and contemporary works on this topic, it is argued that contrary to the prevalent view, the evolution of dominance is not a resolved issue. A re-examination of this issue is essential, given that dominance evolution is likely to be an important stepping stone towards understanding the evolution of inheritance systems.

Dominance variation across six herbicides of the Arabidopsis thaliana csr1-1 and csr1-2 resistance alleles

Dominance of a resistance trait can be defined as a measure of the relative position of the phenotype of the heterozygote RS compared with the phenotype of the two corresponding homozygotes, SS and RR. This parameter has been shown to have primary importance in the dynamics of pesticide resistance evolution. Literature on insecticide resistance suggests that dominance levels in the presence of insecticide vary greatly from completely recessive to completely dominant. With insecticides, both the chemical applied and the dosages used have been demonstrated to affect the dominance. By contrast, almost all herbicide resistances have been found to be inherited as partially to totally dominant traits. This discrepancy between weeds and insects may partly result from the methodologies applied to measure the dominance, ie a single dose for herbicide versus several doses for insecticide. Using two well-known resistances (csr1-1 and csr1-2) to acetolactate synthase (ALS) inhibitors in Arabidopsis thaliana (L) Heynh (mouse-ear cress), we used several herbicide doses to determine the dominance level to six ALS-inhibiting herbicides. The dominance level in the presence of herbicide varied from completely dominant to completely recessive, depending on the resistance allele and the herbicide tested. The dominance of the csr1-1 and csr1-2 resistance alleles ranged from 0 (completely recessive) to 1.1 (dominant) and from 0 to 0.3 (partially dominant), respectively. The recessivity of some resistance alleles in the presence of herbicide could lead to the development of improved resistance management in order to delay or avoid herbicide resistance evolution, especially in the control of outcrossing weed species.

Evolution of dominance in metabolic pathways

Dominance is a form of phenotypic robustness to mutations. Understanding how such robustness can evolve provides a window into how the relation between genotype and phenotype can evolve. As such, the issue of dominance evolution is a question about the evolution of inheritance systems. Attempts at explaining the evolution of dominance have run into two problems. One is that selection for dominance is sensitive to the frequency of heterozygotes. Accordingly, dominance cannot evolve unless special conditions lead to the presence of a high frequency of mutant alleles in the population. Second, on the basis of theoretical results in metabolic control analysis, it has been proposed that metabolic systems possess inherent constraints. These hypothetical constraints imply the default manifestation of dominance of the wild type with respect to the effects of mutations at most loci. Hence, some biologists have maintained that an evolutionary explanation is not relevant to dominance. In this article, we put into question the hypothetical assumption of default metabolic constraints. We show that this assumption is based on an exclusion of important nonlinear interactions that can occur between enzymes in a pathway. With an a priori exclusion of such interactions, the possibility of epistasis and hence dominance modification is eliminated. We present a theoretical model that integrates enzyme kinetics and population genetics to address dominance evolution in metabolic pathways. In the case of mutations that decrease enzyme concentrations, and given the mechanistic constraints of Michaelis-Menten-type catalysis, it is shown that dominance of the wild type can be extensively modified in a two-enzyme pathway. Moreover, we discuss analytical results indicating that the conclusions from the two-enzyme case can be generalized to any number of enzymes. Dominance modification is achieved chiefly through changes in enzyme concentrations or kinetic parameters such as k(cat), both of which can alter saturation levels. Low saturation translates into higher levels of dominance with respect to mutations that decrease enzyme concentrations. Furthermore, it is shown that in the two-enzyme example, dominance evolves as a by-product of selection in a manner that is insensitive to the frequency of heterozygotes. Using variation in k(cat) as an example of modifier mutations, it is shown that the latter can have direct fitness effects in addition to dominance modification effects. Dominance evolution can occur in a frequency-insensitive manner as a result of selection for such dual-effects alleles. This type of selection may prove to be a common pattern for the evolution of phenotypic robustness to mutations.

The dominance of the herbicide resistance cost in several Arabidopsis thaliana mutant lines

Resistance evolution depends upon the balance between advantage and disadvantage (cost) conferred in treated and untreated areas. By analyzing morphological characters and simple fitness components, the cost associated with each of eight herbicide resistance alleles (acetolactate synthase, cellulose synthase, and auxin-induced target genes) was studied in the model plant Arabidopsis thaliana. The use of allele-specific PCR to discriminate between heterozygous and homozygous plants was used to provide insights into the dominance of the resistance cost, a parameter rarely described. Morphological characters appear more sensitive than fitness (seed production) because 6 vs. 4 differences between resistant and sensitive homozygous plants were detected, respectively. Dominance levels for the fitness cost ranged from recessivity (csr1-1, ixr1-2, and axr1-3) to dominance (axr2-1) to underdominance (aux1-7). Furthermore, the dominance level of the herbicide resistance trait did not predict the dominance level of the cost of resistance. The relationship of our results to theoretical predictions of dominance and the consequences of fitness cost and its dominance in resistance management are discussed.

Delaying evolution of insect resistance to transgenic crops by decreasing dominance and heritability

The refuge strategy is used widely for delaying evolution of insect resistance to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Farmers grow refuges of host plants that do not produce Bt toxins to promote survival of susceptible pests. Many modelling studies predict that refuges will delay resistance longest if alleles conferring resistance are rare, most resistant adults mate with susceptible adults, and Bt plants have sufficiently high toxin concentration to kill heterozygous progeny from such matings. In contrast, based on their model of the cotton pest Heliothis virescens, Vacher et al. (Journal of Evolutionary Biology, 16, 2003, 378) concluded that low rather than high toxin doses would delay resistance most effectively. We demonstrate here that their conclusion arises from invalid assumptions about larval concentration-mortality responses and dominance of resistance. Incorporation of bioassay data from H. virescens and another key cotton pest (Pectinophora gossypiella) into a population genetic model shows that toxin concentrations high enough to kill all or nearly all heterozygotes should delay resistance longer than lower concentrations.

Complex genetic architecture of population differences in adult lifespan of a beetle: nonadditive inheritance, gender differences, body size and a large maternal effect

Evolutionary responses to selection can be complicated when there is substantial nonadditivity, which limits our ability to extrapolate from simple models of selection to population differentiation and speciation. Studies of Drosophila melanogaster indicate that lifespan and the rate of senescence are influenced by many genes that have environment- and sex-specific effects. These studies also demonstrate that interactions among alleles (dominance) and loci (epistasis) are common, with the degree of interaction differing between the sexes and among environments. However, little is known about the genetic architecture of lifespan or mortality rates for organisms other than D. melanogaster. We studied genetic architecture of differences in lifespan and shapes of mortality curves between two populations of the seed beetle, Callosobruchus maculatus (South India and Burkina Faso populations). These two populations differ in various traits (such as body size and adult lifespan) that have likely evolved via host-specific selection. We found that the genetic architecture of lifespan differences between populations differs substantially between males and females; there was a large maternal effect on male lifespan (but not on female lifespan), and substantial dominance of long-life alleles in females (but not males). The large maternal effect in males was genetically based (there was no significant cytoplasmic effect) likely due to population differences in maternal effects genes that influence lifespan of progeny. Rearing host did not affect the genetic architecture of lifespan, and there was no evidence that genes on the Y-chromosome influence the population differences in lifespan. Epistatic interactions among loci were detectable for the mortality rate of both males and females, but were detectable for lifespan only after controlling for body size variation among lines. The detection of epistasis, dominance, and sex-specific genetic effects on C. maculatus lifespan is consistent with results from line cross and quantitative trait locus studies of D. melanogaster.

Factors influencing the toxicity of xenobiotics against larval mosquitoes

In order to examine the factors influencing xenobiotic toxicity against larval mosquitoes, the larvicidal performances of two conventional insecticides (temephos and Bacillus thuringiensis var. israelensis: Bti) and a new potential phyto-insecticide (decomposed leaf litter) were compared under different conditions against three detritivorous larval mosquito types. Bioassays performed under standard conditions indicated differential tolerance levels according to the xenobiotic and the larval type. Bioassays performed under different conditions of xenobiotic dose and geometry of the water column indicated differential effects of those parameters on mortality rates. This allowed us to distinguish the performances of temephos versus those of Bti and leaf litter. These toxicological performances were examined as indicators for analysis of xenobiotic bioavailability for mosquito larvae in environmental water, and also for their comparative interest in field mosquito control.

The complex genetic architecture of Drosophila life span

Continuous phenotypic variation in life span results from segregating genetic variation at multiple loci, the environmental sensitivity of expression of these loci, and the history of environmental variation experienced by the organism throughout its life. We have mapped quantitative trait loci (QTL) that produce variation in the life span of mated Drosophila melanogaster using a panel of recombinant inbred lines (RIL) that were backcrossed to the parental strains from which they were derived. Five QTL were identified that influence mated life span, three were male-specific, one was female-specific, and one affected life span in both sexes. The additive allelic effects and dominance of QTL were highly sex-specific. One pair of QTL also exhibited significant epistatic effects on life span. We summarize all of the QTL mapping data for Drosophila life span, and outline future prospects for disentangling the genetic and environmental influences on this trait.

Differential expression and induction of two Drosophila cytochrome P450 genes near the Rst(2)DDT locus

Previous studies have shown that the major metabolic resistance locus in the insecticide-resistant Drosophila line Rst(2)DDT(Wisconsin) maps between the markers cn and vg on chromosome 2. Six cytochrome P450 genes exist in this region. We investigated the expression levels of these P450 genes in DDT-resistant and -susceptible fly lines. We report: (i) DDT resistance is significant (> 30-fold) and dominant, (ii) resistance is reduced by the cytochrome P450 inhibitor PBO, (iii) there is constitutive over-expression relative to susceptible flies of two genes encoding cytochrome P450 enzymes within the cn-vg region (CYP6G1 = 4.3-fold; CYP12D1 = 6.0-fold), and (iv) exposure to DDT results in an increased expression of only one of these two P450 genes (CYP12D1 > or = 6-fold above constitutive resistant fly baselines).

Resistência a inseticidas em populações de Simulium (Diptera, Simuliidae)

Populações de Simulium (Chirostilbia) pertinax Kollar, 1832 do Sul e Sudeste do Brasil, foram analisadas quanto à susceptibilidade ao Temephos, considerando-se os históricos de controle e possível resistência. Bioensaios in situ foram realizados para populações dos estados do Paraná (Tibaji e Rolândia), Rio de Janeiro (Muriqui) e São Paulo (Barra do Una, Ilhabela, e Morungaba). As populações foram caracterizadas como susceptíveis (S) ou resistentes (R) submetendo-se larvas nos últimos estádios a uma concentração operacional (0,1ppm i.a./10min) de Temephos (Abate 500E) como diagnóstica. Os possíveis mecanismos para o desenvolvimento de resistência ao organofosforado são discutidos considerando-se antigas e novas estratégias de controle.

High genetic variability for resistance to Bacillus thuringiensis toxins in a single population of diamondback moth

The long-term benefit of insecticidal products based on Cry toxins, either in sprays or as transgenic crops, is threatened by the development of resistance by target pests. The models used to predict evolution of resistance to Cry toxins most often are monogenic models in which two alleles are used. Moreover, the high-dose/refuge strategy recommended for implementation with transgenic crops relies on the assumption that the resistance allele is recessive. Using selection experiments, we demonstrated the occurrence in a laboratory colony of diamondback moth of two different genes (either allelic or nonallelic) that confer resistance to Cry1Ab. At the concentration tested, resistance was dominant in one selection line and partially recessive in the other. Resistant insects from the two selection lines also differed in their cross-resistance patterns. The diamondback moth colony was derived from a field population from the Philippines, which originally showed a different resistance phenotype. This is the first time that an insect population has been directly shown to carry more than one gene conferring resistance to the same Cry toxin.

The evolution of recombination in a heterogeneous environment

Most models describing the evolution of recombination have focused on the case of a single population, implicitly assuming that all individuals are equally likely to mate and that spatial heterogeneity in selection is absent. In these models, the evolution of recombination is driven by linkage disequilibria generated either by epistatic selection or drift. Models based on epistatic selection show that recombination can be favored if epistasis is negative and weak compared to directional selection and if the recombination modifier locus is tightly linked to the selected loci. In this article, we examine the joint effects of spatial heterogeneity in selection and epistasis on the evolution of recombination. In a model with two patches, each subject to different selection regimes, we consider the cases of mutation-selection and migration-selection balance as well as the spread of beneficial alleles. We find that including spatial heterogeneity extends the range of epistasis over which recombination can be favored. Indeed, recombination can be favored without epistasis, with negative and even with positive epistasis depending on environmental circumstances. The selection pressure acting on recombination-modifier loci is often much stronger with spatial heterogeneity, and even loosely linked modifiers and free linkage may evolve. In each case, predicting whether recombination is favored requires knowledge of both the type of environmental heterogeneity and epistasis, as none of these factors alone is sufficient to predict the outcome.

Quantitative trait loci for life span in Drosophila melanogaster: interactions with genetic background and larval density

The genetic architecture of variation in adult life span was examined for a population of recombinant inbred lines, each of which had been crossed to both inbred parental strains from which the lines were derived, after emergence from both high and low larval density. QTL affecting life span were mapped within each sex and larval density treatment by linkage to highly polymorphic roo-transposable element markers, using a composite interval mapping method. We detected a total of six QTL affecting life span; the additive effects and degrees of dominance for all were highly sex- and larval environment-specific. There were significant epistatic interactions between five of the life span QTL, the effects of which also differed according to genetic background, sex, and larval density. Five additional QTL were identified that contributed to differences among lines in their sensitivity to variation in larval density. Further fine-scale mapping is necessary to determine whether candidate genes within the regions to which the QTL map are actually responsible for the observed variation in life span.

The character or the variation: the genetic analysis of the insecticide-resistance phenotype

In this critique it is argued that the genetic basis of the evolution of resistance is dependent on how the phenotypic, and underlying genotypic, variation is channelled during a selective response. A polygenic response is preferentially favoured if selection acts within the phenotypic distribution of susceptibles; a monogenic response is predicted if selection screens rare mutations with phenotypes outside that susceptible distribution. The relevance of this model to the method of genetic analysis, the prediction of resistance mechanisms to novel insecticides, the generation of resistant beneficial insects and the development of the most effective resistance and integrated pest management programmes is discussed.