Abstract
The polyphagous feeding habits of the corn earworm, Helicoverpa zea (Boddie), underscore its status as a major agricultural pest with a wide geographic distribution and host plant repertoire. To study the transcriptomic response to toxins in diet, we conducted a microarray analysis of H. zea caterpillars feeding on artificial diet, diet laced with nicotine and Nicotiana tabacum (L.) plants. We supplemented our analysis with growth and aversion bioassays. The transcriptome reflects an abundant expression of proteases, chitin, cytochrome P450 and immune-related genes, many of which are shared between the two experimental treatments. However, the tobacco treatment tended to elicit stronger transcriptional responses than nicotine-laced diet. The salivary factor glucose oxidase, known to suppress nicotine induction in the plant, was upregulated by H. zea in response to tobacco but not to nicotine-laced diet. Reduced caterpillar growth rates accompanied the broad regulation of genes associated with growth, such as juvenile hormone epoxide hydrolase. The differential expression of chemosensory proteins, such as odorant binding-protein-2 precursor, as well as the neurotransmitter nicotinic-acetylcholine-receptor subunit 9, highlights candidate genes regulating aversive behavior towards nicotine. We suggest that an observed coincidental rise in cannibalistic behavior and regulation of proteases and protease inhibitors in H. zea larvae signify a compensatory response to induced plant defenses.
Highlights
Helicoverpa zea is an agriculturally important generalist pest on a large number of crop plants
The low level of nicotine was the estimated concentration found in tobacco leaves and was enough to have a significant effect on H. zea larval growth rates
We observed no cases of cannibalism among caterpillar pairs sharing artificial diet and 13 cases of cannibalism among caterpillar pairs sharing artificial diet laced with nicotine (Figure 3) (N = 60, Wilcoxon, p = 0.0044)
Summary
Helicoverpa zea is an agriculturally important generalist pest on a large number of crop plants. Because the toxicity of nicotine presents a formidable obstacle to growth, understanding how H. zea caterpillars cope with tobacco chemical defense may yield further insight on how the corn earworm has gained such a wide geographic distribution and extensive menu of host plants Because both behavioral and physiological adjustments are essential features of the generalist feeding strategy, we supplemented a transcriptomic profile of sixth-instar H. zea caterpillars feeding on nicotine-laced diet and tobacco plants with a series of aversion bioassays. In these bioassays, we presented H. zea caterpillars with a choice between two feeding constraints, posed by the toxicity of nicotine against the risk of predation in the form of cannibalism. We expected differences in diet to be accompanied by differences in both the behavior and transcriptome of H. zea
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