Host Plant Induction and Pesticides

The adaptive plasticity conferred by the inducibility of P450 enzymes on different diets can have important consequences for insect control and the bionomics of pest insects. It is far from being just an ecological oddity or an interesting set of tales of insect natural history. It is well recognized that the same insect species fed different (host) plants will show differences in their response to pesticides

(Ahmad, 1986; Yu, 1986; Lindroth, 1991), and that these differences often reflect the induction of P450 enzymes, as well as of other enzymes, glutathione S-transferases, epoxide hydrolases, etc. The complexity of plant chemistry makes it difficult to account for the contribution of each individual chemical to this response and key components are often analyzed first (e.g., Moldenke et al., 1992). Similarly, the multiplicity of P450 genes and the range of P450 enzyme specificity makes it difficult to predict the outcome of exposure to a plant chemical. The toxi-cological importance of the plant diet on the herbivore's P450 status (induction, inhibition) is well recognized in pharmacology where the joint use of chemical therapy and traditional herbs can have unpredicted outcomes (Zhou et al., 2003).

Larvae of the European corn borer, Ostrinia nubi-lalis, fed leaves from corn varieties with increasing DIMBOA content and thus increasing levels of resistance to leaf damage had correspondingly increased levels of total midgut P450 and p-nitroa-nisole O-demethylation activity (Feng et al., 1992). These studies suggest that constitutive host plant resistance may affect the insect response to xeno-biotics. In addition, the induction of host plant defense by insect damage may itself be a signal for induction of herbivore P450 enzymes, as shown in H. zea. The plant defense signal molecules jasmo-nate and salicylate induce CYP6B8, B9, B27, and B28 (Li et al., 2002d) in both fat body and midgut. The response to salicylate is relatively specific as p-hydroxybenzoate, but not methylparaben, also acts as an inducer.

Treatment with 2-tridecanone, a toxic allelo-chemical from trichomes of wild tomato, protects H. zea larvae against carbaryl toxicity (Kennedy, 1984) and H. virescens larvae against diazinon toxicity (Riskallah etal., 1986b). In H. virescens larvae, the compound caused both qualitative and quantitative changes in P450 spectral properties (Riskallah et al., 1986a), an induction confirmed by its effect on specific P450 genes in the gut of M. sexta larvae (Snyder et al., 1995; Stevens et al., 2000). Larvae of H. virescens with a genetic resistance to 2-trideca-none have increased P450 levels and P450 marker activities (benzphetamine demethylation, benzo[a]-pyrene hydroxylation, phorate sulfoxidation), and these can be further increased by feeding 2-trideca-none (Rose et al., 1991). A laboratory population of H. zea can rapidly display increased tolerance to a-cypermethrin by selection of an increased P450 detoxification ability with a high dose of dietary xanthotoxin (Li et al., 2000b).

Beyond the host plant, it is the whole biotic and chemical environment that determines the response of an insect to pesticide exposure. Herbicides and insecticide solvents can serve as inducers (Brattsten and Wilkinson, 1977; Kao et al., 1995; Miota et al., 2000). Aquatic larvae are exposed to natural or anthropogenic compounds that alter their P450 detoxification profile (David et al., 2000, 2002; Suwanchaichinda and Brattsten, 2002). Virus infection affects P450 levels (Brattsten, 1987), and the expression of several P450 genes is affected during the immune response (see Section 4.1.5.1.2).

4.1.4.5. Insecticide Resistance

4.1.4.5.1. Phenotype, genotype, and causal relationships Insecticide resistance is achieved in a selected strain or population by: (1) an alteration of the target site; (2) an alteration of the effective dose of insecticide that reaches the target; or (3) a combination of the two. The resistance phenotypes have long been analyzed according to these useful biochemical and physiological criteria. At the molecular genetic level, several classes of mutations can account for these phenotypes (Taylor and Feyereisen, 1996) and a causal relationship between a discrete mutation and resistance has been clearly established for several cases of target site resistance (ffrench-Constant et al., 1999). The molecular mutations responsible for P450-mediated insecticide resistance are only beginning to be explored. In contrast to CYP51, which is a target for a major class of fungicides, no insect P450 has been recognized as a primary target for a commercial insecticide. Thus, biochemical changes in P450 structure or activity can lead to changes in insecticide sequestration, activation, or inactivation, so that all the classes of molecular mutations (structural, up- or downregu-lation, see Taylor and Feyereisen, 1996) can be theoretically involved in P450-mediated resistance. When the number of P450 genes is taken into account, it is little wonder that P450 enzymes are so often involved in insecticide resistance, and that it has been so difficult finding, and establishing, the role of resistance mutations for P450 genes.

Traditionally, the first line of evidence for a role of a P450 enzyme in resistance has been the use of an insecticide synergist (e.g., piperonyl butoxide), a suppression or decrease in the level of resistance by treatment with the synergist being diagnostic. In cases too many to list here, this initial and indirect evidence is probably correct, however there are cases where piperonyl butoxide synergism has not been explained by increased detoxification (Kennaugh et al., 1993). Piperonyl butoxide may also be a poor inhibitor of the P450(s) responsible for resistance, so that the use of a second, unrelated synergist may be warranted (Brown et al., 1996; Zhang et al.,

1997). In addition, the synergist as P450 inhibitor can decrease the activation of a proinsecticide, so that lack of resistance suppression can be misleading. Chlorpyriphos resistance in D. melanogaster from vineyards in Israel maps to the right arm of chromosome 2 (see Section 4.1.4.5.5) and is enhanced by piperonyl butoxide rather than suppressed (Ringo etal., 1995).

An independent and additional line of evidence is the measurement of total P450 levels or metabolism of selected model substrates. An increase in either or both being viewed as diagnostic. Again, such evidence is tantalizing but indirect, and the absence of change uninformative. The validation of a model substrate for resistance studies requires substantial knowledge about the P450(s) involved, and is therefore best assessed a posteriori.

An increase in the metabolism of the insecticide itself in the resistant strain is more conclusive. For instance, permethrin metabolism to 4'-hydroxyper-methrin was higher in microsomes from Culex quin-quefasciatus larvae that are highly resistant to permethrin (Kasai et al., 1998b) than in their susceptible counterparts. Total P450 and cytochrome b5 levels were 2.5 times higher in the resistant strain. Both permethrin toxicity and metabolism were inhibited by two unrelated synergists, TCPPE and piperonyl butoxide. A similarly convincing approach was taken to show P450 involvement in the resistance of housefly larvae of the YPPF strain to pyriproxifen. Gut and fat body microsomes were shown to metabolize the IGR to 4'-OH-pyriproxy-fen and 5"-OH-pyriproxyfen at higher rates than microsomes of the susceptible strains and this metabolism was synergist-suppressible (Zhang et al.,

1998). The major, dominant resistance factor was linked to chromosome 2 in that strain (Zhang et al., 1997).

Increased levels of transcripts for one or more P450 genes in insecticide-resistant strains has now been reported in many cases (see Table 5). This suggests that overexpression of one or more P450 genes is a common phenomenon of metabolic resistance but does not by itself establish a causal relationship with resistance. In some cases, the increased mRNA levels have been related to increased transcription (Liu and Scott, 1998), or increased protein levels (Liu and Scott, 1998; Sabourault etal., 2001). Genetic linkage between increased mRNA or protein levels for a particular P450 and resistance has been obtained to the chromosome level (CYP6A1, Cyp6a2, Cyp6a8, CYP6D1, CYP9A1, CYP12A1: Carino et al., 1994; Liu and Scott, 1996; Rose et al., 1997; Guzov et al., 1998; Maitra et al., 2000), and closer to marker genes (Cyp6g1, CYP6A1: Daborn et al., 2001; Sabourault et al., 2001). Linkage is just the first step in establishing a causal link between a P450 gene and resistance.

The following is a discussion of specific cases of P450 genes associated with insecticide resistance that have been studied in greater detail. Evidence for mutations causing constitutive overexpression in cis and trans, as well as an example of point mutations in a P450 coding sequence are currently available. The variety of mechanisms, even in a single species in response to the same insecticide, is striking. The paucity of available data on the molecular definition of the resistant genotype and on its causal relationship to resistance is also striking when compared to the wealth of data on target site resistance (ffrench-Constant et al., 1999).

4.1.4.5.2. CYP6A1 and diazinon resistance in the housefly Rutgers strain CYP6A1 was the first insect P450 cDNA to be cloned, and the gene was shown to be phenobarbital-inducible and constitu-tively overexpressed in the multiresistant Rutgers strain (Feyereisen et al., 1989). A survey of 15 housefly strains (Carino et al., 1992) showed that CYP6A1 is constitutively overexpressed at various degrees in eight resistant strains, but not in all resistant strains, notably R-Fc known to possess a P450-based resistance mechanism. Thus, the first survey with a P450 molecular probe confirmed the results of the first survey of housefly strains with marker P450 activities (aldrin epoxidation and naphthalene hydroxylation; Schonbrod et al., 1968): there is no simple relationship between resistance and a molecular marker, here the level of expression of a single P450 gene. That different P450 genes would be involved in different cases of insecticide resistance was a sobering observation (Carino et al., 1992), even before the total number of P450 genes in an insect genome was known. The constitutive overexpression of CYP6A1 was observed in larvae and in adults of both sexes. Overexpression was shown in both developmental stages to be linked to a semidominant factor on chromosome 2 (Carino et al., 1994), but the CYP6A1 gene was mapped to chromosome 5 (Cohen et al., 1994). The gene copy number being identical between Rutgers and a standard susceptible strain (sbo), gene amplification could not be invoked to explain overexpression (Carino et al., 1994), and the existence of a chromosome 2 trans-acting factor(s) differentially regulating CYP6A1 expression in the Rutgers and sbo strains was implied. Competitive ELISA using purified recombinant CYP6A1 protein as standard showed that the elevated mRNA levels were indeed translated into elevated protein levels (Sabourault

Table 5 P450 overexpression in insecticide-resistant strains

P450 overexpressed

Strain

Resistance pattern

Reference

Musca domestica CYP6A1

CYP6A1a CYP6D1

P450Lpr/CYP6D1a CYP6D1/CYP6D1a CYP6D1 CYP6D3 CYP12A1 Drosophila melanogaster Cyp4e2 Cyp6a2

CYP6A2a Cyp6a8

Cyp6g1

Cyp12d1/2

Drosophila simulans CYP6G1b

Heliothis virescens

CYP9A1 Helicoverpa armigera CYP4G8 CYP6B7 Lygus lineolaris

CYP6X1 Anopheles gambiae

CYP6Z1 Culex quinquefasciatus

CYP6F1 Culex pipiens pallens

CYP4H21, H22, H23 CYP4J4, CYP4J6 Diabrotica virgifera

CYP4 Blattella germanica P450MAa

Rutgers and other strains Rutgers LPR LPR

NG98, Georgia

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