A link between host plant adaptation and pesticide resistance in the polyphagous spider mite Tetranychus urticae

Abstract

Plants produce a wide range of allelochemicals to defend against herbivore attack, and generalist herbivores have evolved mechanisms to avoid, sequester, or detoxify a broad spectrum of natural defense compounds. Successful arthropod pests have also developed resistance to diverse classes of pesticides and this adaptation is of critical importance to agriculture. To test whether mechanisms to overcome plant defenses predispose the development of pesticide resistance, we examined adaptation of the generalist two-spotted spider mite, Tetranychus urticae, to host plant transfer and pesticides. T. urticae is an extreme polyphagous pest with more than 1,100 documented hosts and has an extraordinary ability to develop pesticide resistance. When mites from a pesticide-susceptible strain propagated on bean were adapted to a challenging host (tomato), transcriptional responses increased over time with ~7.5% of genes differentially expressed after five generations. Whereas many genes with altered expression belonged to known detoxification families (like P450 monooxygenases), new gene families not previously associated with detoxification in other herbivores showed a striking response, including ring-splitting dioxygenase genes acquired by horizontal gene transfer. Strikingly, transcriptional profiles of tomato-adapted mites resembled those of multipesticide-resistant strains, and adaptation to tomato decreased the susceptibility to unrelated pesticide classes. Our findings suggest key roles for both an expanded environmental response gene repertoire and transcriptional regulation in the life history of generalist herbivores. They also support a model whereby selection for the ability to mount a broad response to the diverse defense chemistry of plants predisposes the evolution of pesticide resistance in generalists.

Fig. 1.

Venn diagrams depicting overlap among differentially expressed genes [log₂(FC) ≥ 1, FDR < 0.05] from pairwise comparisons of mites shifted from bean to tomato and of resistant mites. Blue, up-regulated genes; orange, down-regulated genes. (A) Comparisons for shift to tomato for 2 h, 12 h, and five generations. (B) Comparisons for strains MAR-AB, MR-VP, and Tomato-5G.

Fig. 2.

Global changes in gene expression of two multiresistant T. urticae strains (MR-VP and MAR-AB) relative to the London susceptible strain, compared with gene expression changes upon host plant change (Tomato-5G). (A) Commonly differentially expressed genes [log₂(FC) ≥ 1, FDR < 0.05] in two multiresistant strains (MR-VP and/or MAR-AB: “Resistance”) and after host plant change for five generations (Tomato-5G): black, differentially expressed genes in Tomato-5G and MAR-AB; red, differentially expressed genes in Tomato-5G and MR-VP; yellow, differentially expressed genes in Tomato-5G, MR-VP, and MAR-AB (the Log₂ of the average of fold changes of commonly differentially expressed genes of MR-VP and MAR-AB is plotted). (B) Fold changes of differentially expressed genes [log₂(FC) ≥ 1, FDR < 0.05], known to be implicated in detoxification and transport, in two multiresistant strains (MR-VP and/or MAR-AB: Resistance) and after host plant change for five generations (Tomato-5G): red, P450 monooxygenases (P450s); black, ATP-binding cassette transporters, classes B and C (ABC-B/Cs); green, lipocalins; pink, carboxyl-cholinesterases (CCEs); yellow, glutathione S-transferases (GSTs); light blue, intradiol ring-cleavage dioxygenases (ID-RCDs); gray, MFS transporters (OrthoMCL clusters 10032, 10082, and 10236).

Fig. 3.

Intradiol ring-cleavage dioxygenases (ID-RCDs) in T. urticae. (A) Phylogenetic relationship of T. urticae ID-RCDs linked to expression levels (log₂(FC)) in acaricide-multiresistant strains (MR-VP and MAR-AB) and after host plant shift to tomato (S. lycopersicum) for five generations. Genes with detected orthologs in T. evansi are depicted with an “e”. (B) Alignment of conserved residues in “classical” and secreted ID-RCDs. CTD, catechol ID-RCD; PCD, protocatechuate ID-RCD; HQD, hydroxyquinol ID-RCD; IDL, intradiol dioxygenase-like (cd03457); Aci, Acinetobacter sp.; R. opa, Rhodococcus opacus; N. sim, Nocardia simplex; A. fum, A. fumigatus. Tetur07g02040, tetur13g04550, and tetur20g01790 are ID-RCD representatives of T. urticae. The two His-2 Tyr nonheme iron (III) binding sites are indicated by shading. Residues defined by crystallographic (–48) studies to have an influence on substrate interaction in classical ID-RCDs (CTD, PCD, and HQD) are indicated by solid circles. The predicted binding residues of epicatechin in the protein sequence of A. fumigatus are indicated in boldface type (69). (C) Maximun-likelihood unrooted tree of 17 ID-RCDs of T. urticae (and five T. evansi orthologs) with 232 bacterial and fungal sequences. Color codes indicate the percentage of secretion within the clade: yellow, not secreted; blue, <55% secreted; green, 55–85% secreted; and orange, >85% secreted. All members of the T. urticae clade are secreted and form a sister clade to fungal secreted dioxygenases, sharing a most recent common ancestor with plant and entomopathogenic Proteobacteria. The classical biochemically characterized ID-RCDs (CTD, PCD, and HQD) are not secreted and cluster together as an outgroup. The phylogenetic positions of the ID-RCD protein sequences of Naegleria gruberi (Protozoa), Shistosoma mansoni (Metazoa), P. infestans (oomycete), and Haloferax volcanii (Archaea) are indicated by *, **, ***, and ****, respectively.

Fig. 4.

Effect of host plant on acaricide toxicity. Percentage mortality of spider mites from the London strain to pesticide treatment. Shown are LC₉₀ values for exposure to various acaricides (pyridaben, milbemectin, tebufenpyrad, fenbutatin oxide, and bifenthrin) before (London) and after adaptation on tomato (London on tomato). An asterisk indicates significant differences determined by a t test for paired samples.

Fig. P1.

(A) Transcriptional response by members of four gene families implicated in host response (bean vs. tomato, horizontal) and pesticide response (susceptible vs. resistant strains, vertical). Abbreviations: ID-RCDs, intradiol ring-cleavage dioxygenases; MFS, major facilitator superfamily; P450s, P450 monooxygenases. (B) Model for rapid evolution of pesticide resistance in generalist herbivores compared with specialists (adapted from ref. 5). Preadaptation to multiple host plants is postulated to increase polymorphism in environmental responses, leading to several subsets of alleles. The initial stages of selection by a pesticide mimic those of a host plant shift, rapidly selecting the best-adapted subset of environmental response alleles and providing a larger population from which a rare (high-) resistance allele can be selected, thus accelerating the development of agriculturally significant resistance. A similar transcriptome signature occurs after both types of selection (i.e., host plant shift and pesticide) because it is drawn from a similar subset of genotypes.