Genetic analysis of permethrin resistance in the house fly,Musca domestica L

The evolving puzzle of autosomal versus Y-linked male determination in Musca domestica

Sex determination is one of the most rapidly evolving developmental pathways, but the factors responsible for this fast evolution are not well resolved. The house fly, Musca domestica, is an ideal model for studying sex determination because house fly sex determination is polygenic and varies considerably between populations. Male house flies possess a male-determining locus, the M factor, which can be located on the Y or X chromosome or any of the five autosomes. There can be a single M or multiple M factors present in an individual male, in heterozygous or homozygous condition. Males with multiple copies of M skew the sex ratio toward the production of males. Potentially in response to these male-biased sex ratios, an allele of the gene transformer, Md-tra(D), promotes female development in the presence of one or multiple M factors. There have been many studies to determine the linkage and frequency of these male determining factors and the frequency of Md-tra(D) chromosomes in populations from around the world. This review provides a summary of the information available to date regarding the patterns of distribution of autosomal, X-linked and Y-linked M factors, the relative frequencies of the linkage of M, the changes in frequencies found in field populations, and the fitness of males with autosomal M factors vs. Y-linked M. We evaluate this natural variation in the house fly sex determination pathway in light of models of the evolution of sex determination.

Cross-resistance, genetics, and realized heritability of resistance to fipronil in the house fly, Musca domestica (Diptera: Muscidae): a potential vector for disease transmission

Houseflies, Musca domestica (L.), are ubiquitous pests that have the potential to spread a variety of pathogens to humans, poultries, and dairies. Pesticides are commonly used for the management of this pest. Fipronil is a GABA-gated chloride channel-inhibiting insecticide that has been commonly used for the management of different pests including M. domestica throughout the world. Many pests have developed resistance to this insecticide. A field-collected strain of M. domestica was selected with fipronil for continuous 11 generations to assess the cross-resistance, genetics, and realized heritability for designing a resistance management strategy. Laboratory bioassays were performed using the feeding method of mixing insecticide concentrations with 20% sugar solutions and cotton soaks dipped in insecticide solutions were provided to tested adult flies. Bioassay results at G12 showed that the fipronil-selected strain developed a resistance ratio of 140-fold compared to the susceptible strain. Synergism bioassay with piperonyl butoxide (PBO) and S,S,S,-tributyl phosphorotrithioate (DEF) indicated that fipronil resistance was associated with microsomal oxidase and also esterase. Reciprocal crosses between resistant and susceptible strains showed an autosomal and incompletely dominant resistance to fipronil. The LC50 values of F1 and F'1 strains were not significantly different and dominance values were 0.74 and 0.64, respectively. The resistance to fipronil was completely recessive (D(ML) = 0.00) at the highest dose and incompletely dominant at the lowest dose (D(ML) = 0.87). The monogenic resistance based on chi-square goodness of fit test and calculation of the minimum number of segregating genes showed that resistance to fipronil is controlled by multiple genes. The fipronil resistance strain confirmed very low cross-resistance to emamectin benzoate and spinosad while no cross-resistance to chlorpyrifos and acetamiprid when compared to that of the field population. The heritability values were 0.112, 0.075, 0.084, 0.008, and 0.052 for fipronil, emamectin benzoate, spinosad, acetamiprid, and chlorpyrifos, respectively. It was concluded that fipronil resistance in M. domestica was autosomally inherited, incompletely dominant, and polygenic. These findings would be helpful for the better and successful management of M. domestica.

Investigations of the constitutive overexpression of CYP6D1 in the permethrin resistantLPR strain of house fly (Musca domestica)

House fly (Musca domestica) CYP6D1 is a cytochrome P450 involved in metabolism of xenobiotics. CYP6D1 is located on chromosome 1 and its expression is inducible in response to the prototypical P450 inducer phenobarbital (PB) in insecticide susceptible strains. Increased transcription of CYP6D1 confers resistance to permethrin in the LPR strain, and this trait maps to chromosomes 1 and 2. However, the constitutive overexpression of CYP6D1 in LPR is not further increased by PB and the non-responsiveness to PB maps to chromosome 2. It has been suggested that a single factor on chromosome 2 could be responsible for both the constitutive overexpression and lack of PB induction of CYP6D1 in LPR. We examined the PB inducibility of CYP6D1v1 promoter from LPR using dual luciferase reporter assays in Drosophila S2 cells and found the CYP6D1v1 promoter was able to mediate PB induction, similar to the CYP6D1v2 promoter from the insecticide susceptible CS strain. Therefore, variation in promoter sequences of CYP6D1v1 and v2 does not appear responsible for the lack of PB induction of CYP6D1v1 in LPR; this suggests an unidentified trans acting factor is responsible. HR96 has been implicated in having a role in PB induction in Drosophila melanogaster and M. domestica. Therefore, house fly HR96 cDNA was cloned and sequenced to examine if this trans acting factor is responsible for constitutive overexpression of CYP6D1v1 in LPR. Multiple HR96 alleles (v1-v10) were identified, but none were associated with resistance. Expression levels of HR96 were not different between LPR and CS. Thus, HR96 is not the trans acting factor responsible for the constitutive overexpression of CYP6D1 in LPR. The identity of this trans acting factor remains elusive.

Multiplicative interaction between the two major mechanisms of permethrin resistance, kdr and cytochrome P450-monooxygenase detoxification, in mosquitoes

Epistasis is the nonadditive interaction between different loci which contribute to a phenotype. Epistasis between independent loci conferring insecticide resistance is important to investigate as this phenomenon can shape the rate that resistance evolves and can dictate the level of resistance in the field. The evolution of insecticide resistance in mosquitoes is a growing and world-wide problem. The two major mechanisms that confer resistance to permethrin in Culex mosquitoes are target site insensitivity (i.e. kdr) and enhanced detoxification by cytochrome P450 monooxygenases. Using three strains of mosquitoes, and crosses between these strains, we assessed the relative contribution of the two independent loci conferring permethrin resistance, individually and when present together. We found that for all genotype combinations tested, Culex pipiens quinquefasciatus exhibited multiplicative interactions between kdr and P450 detoxification, whether the resistance alleles were homozygous or heterozygous. These results provide a basis for further analysis of the evolution and maintenance of insecticide resistance in mosquitoes.

Cytochromes P450 of insects: the tip of the iceberg

The cytochrome P450-dependent monooxygenases are an extremely important metabolic system involved in the metabolism of endogenous compounds and xenobiotics. Collectively, P450 monooxygenases can metabolize numerous substrates and carry out multiple oxidative reactions. The large number of substrates metabolized is due to the plethora of P450 isoforms and to the broad substrate specificity of some isoforms. Monooxygenases of insects have several functional roles, including growth, development, feeding and protection against xenobiotics, including resistance to pesticides and tolerance to plant toxins. This review begins with background information about P450s and their evolution, followed by a discussion of the extraordinary diversity of insect P450s. Given the enormous interest in studying individual P450s, we then provide a synopsis of the different methods that have been used in their isolation and the substrates that are known to be metabolized. We conclude by summarizing the lessons we have learned from the study of individual insect P450s, including their roles in insecticide resistance, plant-insect interactions and insect physiology. However, these studies are just the 'tip of the iceberg'. Our knowledge continues to expand at a rapid pace, suggesting that the next decade will outpace the last in terms of improving our understanding of the cytochromes P450 of insects.

Cytochromes P450 and insecticide resistance

The cytochrome P450-dependent monooxygenases (monooxygenases) are an extremely important metabolic system involved in the catabolism and anabolism of xenobiotics and endogenous compounds. Monooxygenase-mediated metabolism is a common mechanism by which insects become resistant to insecticides as evidenced by the numerous insect species and insecticides affected. This review begins by presenting background information about P450s, the role of monooxygenases in insects, and the different techniques that have been used to isolate individual insect P450s. Next, insecticide resistance is briefly described, and then historical information about monooxygenase-mediated insecticide resistance is reviewed. For any case of monooxygenase-mediated resistance, identification of the P450(s) involved, out of the dozens that are present in an insect, has proven very challenging. Therefore, the next section of the review focuses on the minimal criteria for establishing that a P450 is involved in resistance. This is followed by a comprehensive examination of the literature concerning the individual P450s that have been isolated from insecticide resistant strains. In each case, the history of the strain and the evidence for monooxygenase-mediated resistance are reviewed. The isolation and characterization of the P450(s) from the strain are then described, and the evidence of whether or not the isolated P450(s) is involved in resistance is summarized. The remainder of the review summarizes our current knowledge of the molecular basis of monooxygenase-mediated resistance and the implications for the future. The importance of these studies for development of effective insecticide resistance management strategies is discussed.

Allele-specific PCR reveals that CYP6D1 is on chromosome 1 in the house fly, Musca domestica

A cytochrome P450, termed P450lpr, is the major P450 responsible for pyrethroid resistance in the Learn-PyR (LPR) strain of house fly. Recently, the putative gene (CYP6D1) coding for P450lpr has been sequenced from the LPR and aabys strains of house fly. Allele-specific polymerase chain reaction (ASPCR) was used for linkage group analysis with backcross progeny from the wild type LPR strain and a multiple marker strain (aabys). We found that CYP6D1 is linked to chromosome 1. The possible role of regulatory or modifying genes responsible for elevated P450lpr expression is discussed in relation to the chromosomal linkage of CYP6D1.

The biochemical genetics of permethrin resistance in the Learn-PyR strain of house fly

Permethrin resistance in the Learn-PyR strain of house fly was examined in four genetically derived substrains, each being homozygous for a different "resistant" autosome of the Learn-PyR strain. The resistance of these derivative strains was characterized toxicologically and biochemically. The relative levels of resistance to permethrin conferred by each autosome were 5 greater than 3 greater than 1 greater than 2. Three factors were associated with resistance: (1) increased mixed-function oxidase (MFO) activity associated with elevated levels of cytochrome P-450, cytochrome b5, and NADPH-cytochrome c reductase (P-450 reductase) activity; (2) target-site insensitivity (kdr); and (3) decreased cuticular penetration. Permethrin resistance factors on chromosome 1 consisted of a piperonyl butoxide (PB)-suppressible mechanism correlated with increased levels of cytochromes P-450 and b5; on chromosome 2, a PB-suppressible mechanism associated with elevated amounts of cytochrome P-450; on chromosome 3, decreased cuticular penetration, kdr, and increased amounts of P-450 reductase activity; and on chromosome 5, a largely PB-suppressible mechanism correlated with elevated levels of cytochrome P-450 and P-450 reductase activity.