Larval Helicoverpa zea Transcriptional, Growth and Behavioral Responses to Nicotine and Nicotiana tabacum

Physiological and transcriptomic responses of silkworms to graphene oxide exposure

The growing use of nanomaterials has sparked significant interest in assessing the insect toxicities of nanoparticles. The silkworm, as an economically important insect, serves as a promising model for studying how insects respond to harmful substances. Here, we conducted a comprehensive investigation on the impact of graphene oxide (GO) on silkworms using a combination of physiological and transcriptome analyses. GO can enter the midguts and posterior silk glands of silkworms. High GO concentrations (> 25 mg/L) significantly (P < 0.01) inhibited larval growth. Additionally, GO (> 5 mg/L) significantly reduced the cocooning rate, and GO (> 15 mg/L) hindered oviduct development and egg laying in silkworms. GO increased the reactive oxygen species content and regulated catalase activity, suggesting that it may affect insect growth by regulating reactive oxygen detoxification. The transcriptome data analysis showed that 35 metabolism-related genes and 20 ribosome biogenesis-related genes were differentially expressed in response to GO, and their expression levels were highly correlated. Finally, we propose that a Ribosome biogenesis-Metabolic signaling network is involved in responses to GO. The research provides a new perspective on the molecular responses of insects to GO.

Fatty alcohols, a minor component of the tree tobacco surface wax, are associated with defence against caterpillar herbivory

Despite decades of research resulting in a comprehensive understanding of epicuticular wax metabolism, the function of these almost ubiquitous metabolites in plant-herbivore interactions remains unresolved. In this study, we examined the effects of CRISPR-induced knockout mutations in four Nicotiana glauca (tree tobacco) wax metabolism genes. These mutations cause a wide range of changes in epicuticular wax composition, leading to altered interactions with insects and snails. Three interaction classes were examined: chewing herbivory by seven caterpillars and one snail species, phloem feeding by Myzus persicae (green peach aphid) and oviposition by Bemisia tabaci (whitefly). Although total wax load and alkane abundance did not affect caterpillar growth, a correlation across species, showed that fatty alcohols, a minor component of N. glauca surface waxes, negatively affected the growth of both a generalist caterpillar (Spodoptera littoralis) and a tobacco-feeding specialist (Manduca sexta). This negative correlation was overshadowed by the stronger effect of anabasine, a nicotine isomer, and was apparent when fatty alcohols were added to an artificial lepidopteran diet. By contrast, snails fed more on waxy leaves. Aphid reproduction and feeding activity were unaffected by wax composition but were potentially affected by altered cutin composition. Wax crystal morphology could explain the preference of B. tabaci to lay eggs on waxy wild-type plants relative to both alkane and fatty alcohol-deficient mutants. Together, our results suggest that the varied responses among herbivore classes and species are likely to be a consequence of the co-evolution that shaped the specific effects of different surface wax components in plant-herbivore interactions.

Quantity versus quality: Effects of diet protein-carbohydrate ratios and amounts on insect herbivore gene expression

Dietary protein and digestible carbohydrates are two key macronutrients for insect herbivores, but the amounts and ratios of these two macronutrients in plant vegetative tissues can be highly variable. Typically, insect herbivores regulate their protein-carbohydrate intake by feeding selectively on nutritionally complementary plant tissues, but this may not always be possible. Interestingly, lab experiments consistently demonstrate that performance - especially growth and survival - does not vary greatly when caterpillars and nymphal grasshoppers are reared on diets that differ in their protein-carbohydrate content. This suggests insect herbivores employ post-ingestive physiological mechanisms to compensate for variation in diet protein-carbohydrate profile. However, the molecular mechanisms that underlie this compensation are not well understood. Here we explore, for the first time in an insect herbivore, the transcriptional effects of two dietary factors: protein-to-carbohydrate ratio (p:c) and total macronutrient (p + c) content. Specifically, we reared Helicoverpa zea caterpillars on three diets that varied in diet p:c ratio and one diet that varied in total p + c concentration, all within an ecologically-relevant range. We observed two key findings. Caterpillars reared on diets with elevated total p + c content showed large differences in gene expression. In contrast, only small differences in gene expression were observed when caterpillars were reared on diets with different p:c ratios (spanning from protein-biased to carbohydrate-biased). The invariable expression of many metabolic genes across these variable diets suggests that H. zea caterpillars employ a strategy of constitutive expression to deal with protein-carbohydrate imbalances rather than diet-specific changes. This is further supported by two findings. First, few genes were uniquely associated with feeding on a protein- and carbohydrate-biased diet. Second, many differentially-expressed genes were shared across protein-biased, carbohydrate-biased, and concentrated diet treatments. Our study provides insights into the post-ingestive physiological mechanisms insect herbivores employ to regulate protein-carbohydrate intake. Most notably, it suggests that H. zea, and perhaps other generalist species, use similar post-ingestive mechanisms to deal with protein-carbohydrate imbalances - regardless of the direction of the imbalance.

Can natural selection and druggable targets synergize? Of nutrient scarcity, cancer, and the evolution of cooperation

Since the dawn of molecular biology, cancer therapy has focused on druggable targets. Despite some remarkable successes, cell-level evolution remains a potent antagonist to this approach. We suggest that a deeper understanding of the breakdown of cooperation can synergize the evolutionary and druggable-targets approaches. Complexity requires cooperation, whether between cells of different species (symbiosis) or between cells of the same organism (multicellularity). Both forms of cooperation may be associated with nutrient scarcity, which in turn may be associated with a chemiosmotic metabolism. A variety of examples from modern organisms supports these generalities. Indeed, mammalian cancers-unicellular, glycolytic, and fast-replicating-parallel these examples. Nutrient scarcity, chemiosmosis, and associated signaling may favor cooperation, while under conditions of nutrient abundance a fermentative metabolism may signal the breakdown of cooperation. Manipulating this metabolic milieu may potentiate the effects of targeted therapeutics. Specific opportunities are discussed in this regard, including avicins, a novel plant product.

Predator-induced stress responses in insects: A review

Predators can induce extreme stress and profound physiological responses in prey. Insects are the most dominant animal group on Earth and serve as prey for many different predators. Although insects have an extraordinary diversity of anti-predator behavioral and physiological responses, predator-induced stress has not been studied extensively in insects, especially at the molecular level. Here, we review the existing literature on physiological predator-induced stress responses in insects and compare what is known about insect stress to vertebrate stress systems. We conclude that many unrelated insects share a baseline pathway of predator-induced stress responses that we refer to as the octopamine-adipokinetic hormone (OAH) axis. We also present best practices for studying predator-induced stress responses in prey insects. We encourage investigators to compare neurophysiological responses to predator-related stress at the organismal, neurohormonal, tissue, and cellular levels within and across taxonomic groups. Studying stress-response variation between ecological contexts and across taxonomic levels will enable the field to build a holistic understanding of, and distinction between, taxon- and stimulus-specific responses relative to universal stress responses.

Larval transcriptomic response to host plants in two related phytophagous lepidopteran species: implications for host specialization and species divergence

Background

Most phytophagous insects have morphological, behavioral and physiological adaptations allowing them to specialize on one or a few plant species. Identifying the mechanisms involved in host plant specialization is crucial to understand the role of divergent selection between different environments in species diversification, and to identify sustainable targets for the management of insect pest species. In the present study, we measured larval phenotypic and transcriptomic responses to host plants in two related phytophagous lepidopteran species: the European corn borer (ECB), a worldwide pest of maize, and the adzuki bean borer (ABB), which feeds of various dicotyledons. Our aim was to identify the genes and functions underlying host specialization and/or divergence between ECB and ABB.

Results

At the phenotypic level, we observed contrasted patterns of survival, weight gain and developmental time between ECB and ABB, and within ECB and ABB reared on two different host plants. At the transcriptomic level, around 8% of the genes were differentially expressed (DE) between species and/or host plant. 70% of these DE genes displayed a divergent pattern of expression between ECB and ABB, regardless of the host, while the remaining 30% were involved in the plastic response between hosts. We further categorized plastic DE genes according to their parallel or opposite pattern between ECB and ABB to specifically identify candidate genes involved in the species divergence by host specialization. These candidates highlighted a comprehensive response, involving functions related to plant recognition, digestion, detoxification, immunity and development. Last, we detected viral, bacterial, and yeast genes whose incidence contrasted ECB and ABB samples, and maize and mugwort conditions. We suggest that these microorganism communities might influence the survival, metabolism and defense patterns observed in ECB and ABB larvae.

Conclusions

The comprehensive approach developed in the present study allowed to identify phenotypic specialization patterns and underlying candidate molecular mechanisms, and highlighted the putative role of microorganisms in the insect-host plant interaction. These findings offer the opportunity to pinpoint specific and sustainable molecular or physiological targets for the regulation of ECB pest populations.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4589-x) contains supplementary material, which is available to authorized users.

Genomics of Lepidoptera saliva reveals function in herbivory

Lepidoptera herbivores deposit copious amounts of saliva when feeding. Their saliva is produced by the paired mandibular and labial glands and evidence indicates that it may play an important role in allowing an herbivore to establish on its host plant. Genomic studies of Lepidoptera saliva are beginning to reveal the role of saliva in herbivory. Molecules involved in digestion, detoxification, immunity, defense against plant secondary chemicals, chemoreception and so on have been identified using high throughput genomic tools. These genomic tools have also revealed changes that occur in Lepidoptera saliva when caterpillars feed on different host plants. However, there are other factors either biotic or abiotic (e.g., larval stage, larval health, temperature, water stress, etc.) that might also affect its composition. Though further functional and ecological studies are still necessary to fully understand the role of Lepidoptera saliva on herbivory, here we review current trends.

Diet dependent metabolic responses in three generalist insect herbivores Spodoptera spp

Adaption to dietary changes is critical in the evolution of host plant ranges in polyphagous insects. We compared three taxa of lepidopteran herbivores from the predominantly generalist genus Spodoptera showing different degrees of polyphagy: Spodoptera littoralis, with a broad host range including both mono- and dicotyledonous plants, and two Spodoptera frugiperda strains [Corn (i.e. maize) (C) and Rice (R)] adapted primarily to different grass species. When feeding on maize we show a lower performance in the broad generalist taxon compared to the grass adapted taxa. Among these taxa, the maize adapted S. frugiperda C-strain generally performed better than the R-strain on maize leaves. On artificial pinto diet, all taxa performed well. Our RNA-Seq analysis of midgut transcriptomes from 3rd instar larvae feeding on maize showed broader transcriptional readjustments in the generalist S. littoralis compared to grass adapted S. frugiperda strains. Substantial alteration in the expression levels of midgut physiological function related transcripts, such as digestive and detoxifying enzymes, transporters, immunity, and peritrophic membrane associated transcripts, existed in all taxa. We found high background expression of UDP-glucosyl transferases, which are known to neutralize maize leaf toxins, in the maize adapted S. frugiperda C-strain, contributing to its fitness on maize compared to the R-strain. Our findings provide evidence for divergent diet specific response of digestive physiology within these Spodoptera taxa. Unexpectedly, the C- and R-strains of S. frugiperda fed on the same diet showed large differences in expression patterns between these two closely related taxa.

Baculovirus-Induced Climbing Behavior Favors Intraspecific Necrophagy and Efficient Disease Transmission in Spodoptera exigua

Shortly prior to death, many species of Lepidoptera infected with nucleopolyhedrovirus climb upwards on the host plant. This results in improved dissemination of viral occlusion bodies over plant foliage and an increased probability of transmission to healthy conspecific larvae. Following applications of Spodoptera exigua multiple nucleopolyhedrovirus for control of Spodoptera exigua on greenhouse-grown sweet pepper crops, necrophagy was observed by healthy S. exigua larvae that fed on virus-killed conspecifics. We examined whether this risky behavior was induced by olfactory or phagostimulant compounds associated with infected cadavers. Laboratory choice tests and olfactometer studies, involving infected and non-infected cadavers placed on spinach leaf discs, revealed no evidence for greater attraction of healthy larvae to virus-killed over non-infected cadavers. Physical contact or feeding on infected cadavers resulted in a very high incidence of transmission (82-93% lethal disease). Observations on the behavior of S. exigua larvae on pepper plants revealed that infected insects died on the uppermost 10% of foliage and closer to the plant stem than healthy conspecifics of the same stage, which we considered clear evidence of baculovirus-induced climbing behavior. Healthy larvae that subsequently foraged on the plant were more frequently observed closer to the infected than the non-infected cadaver. Healthy larvae also encountered and fed on infected cadavers significantly more frequently and more rapidly than larvae that fed on non-infected cadavers. Intraspecific necrophagy on infected cadavers invariably resulted in virus transmission and death of the necrophagous insect. We conclude that, in addition to improving the dissemination of virus particles over plant foliage, baculovirus-induced climbing behavior increases the incidence of intraspecific necrophagy in S. exigua, which is the most efficient mechanism of transmission of this lethal pathogen.