Cloning, expression, and functional analysis of two acetylcholinesterase genes in Spodoptera litura (Lepidoptera: Noctuidae)

Assessment of Sex-Specific Toxicity and Physiological Responses to Thymol in a Common Bean Pest Acanthoscelides obtectus Say

Acanthoscelides obtectus Say (Coleoptera: Chrysomelidae: Bruchinae), is one of the most important pests of the common bean Phaseolus vulgaris L. Without appropriate management it may cause significant seed loss in storages. In search for means of environmentally safe and effective protection of beans we assessed biological activity of thymol, an oxygenated monoterpene present in essential oils of many aromatic plants. We studied contact toxicity of thymol on bean seeds and its effects on adult longevity and emergence in F1 generation. Furthermore, we determined acetylcholinesterase (AChE), superoxide dismutase (SOD), catalase (CAT), mixed-function oxidase (MFO), carboxylesterases (CarE) and glutathione S-transferase (GST) activities in response to 24 h exposure of beetles to sublethal and lethal thymol concentrations. Our results showed that thymol decreased adult survival, longevity and percentage of adult emergence. Higher median lethal concentration (LC50) was recorded in females indicating their higher tolerance comparing to males. Overall, activities of SOD, CAT and CarE increased at sublethal and MFO increased at both sublethal and lethal thymol concentrations. On the other hand, GST and AChE activities decreased along with the increase in thymol concentrations from sublethal (1/5 of LC50, 1/2 of LC50) to lethal (LC50). Enzyme responses to the presence of thymol on bean seed were sex-specific. In the control group females had lower CarE and higher SOD, CAT and GST activity than males. In treatment groups, females had much higher CAT activity and much lower CarE activity than males. Our results contribute to deeper understanding of physiological mechanisms underlying thymol toxicity and tolerance which should be taken into account in future formulation of a thymol-based insecticide.

Characterization and functional analysis of two acetylcholinesterase genes in Bradysia odoriphaga Yang et Zhang (Diptera: Sciaridae)

Two acetylcholinesterase genes (Boace1 and Boace2) were cloned from Bradysia odoriphaga, a devastating soil pest that mainly damages Chinese chives. The Boace1 encodes BoAChE1 protein consisting of 696 amino acid residues, while Boace2 encodes BoAChE2 containing 638 amino acids. Phylogenetic analysis showed that Boace1 and Boace2 are appeared to be distinct clusters. The gene expression patterns at different development stages and various body parts tissues were examined, and their biological functions were characterized by RNA interference and analog docking prediction. The results showed that both Boace genes were expressed in all developmental stages and examined tissues. The transcript level of Boace2 was significantly higher than Boace1 in all tested samples, and Boace1 was found most abundant in the head while Boace2 was highly expressed in the fat body of B. odoriphaga. The silencing of Boace1 and Boace2 significantly decreased the AChE activity of 36.6% and 14.8% separately, and increased the susceptibility of B. odoriphaga to phoxim, with 60.8% and 44.7% mortality. Besides, overexpression and gene duplication of Boace1 were found in two field resistant populations, and two major mutations, A319S and G400V, were detected in Boace1. Moreover, the docking results revealed that BoAChE1 had a higher affinity towards organophosphorus than BoAChE2. It is concluded that Boace2 is the most abundant ace type in B. odoriphaga, while both Boace play vital roles. Boace1 might play a major neurological function and more likely be the prime target for insecticides, while Boace2 might play some important unidentified roles.

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.

Exposure to herbicides reduces larval sensitivity to insecticides in Spodoptera litura (Lepidoptera: Noctuidae)

Herbicides and insecticides are widely used in modern agriculture. It has been reported in various studies that application of insecticides can increase tolerance of herbivorous insects to insecticides. However, limited information exists on susceptibility to insecticides when insects are exposed to herbicides. This study was conducted to investigate the potential impact of the herbicides trifluralin and 2-methyl-4-chlorophenoxyacetic acid sodium salt (MCPA-Na) on the susceptibility of the nocturnal moth Spodoptera litura to the insecticides λ-cyhalothrin, phoxim and bifenthrin. We found that larvae exposed to trifluralin or MCPA-Na became significantly less susceptible to both insecticides than non-exposed control larvae. Herbicide-treated larvae did not show altered growth under the used test conditions. However, heads of herbicide-treated larvae showed increased expression of the acetylcholinesterase genes SlAce1 and SlAce2. Moreover, the fat body and midgut of herbicide-treated larvae displayed elevated expression of detoxification genes (the carboxylesterase gene SlCarE; the glutathione S-transferase genes SlGSTe2 and SlGSTe3; the cytochrome P450 monooxygenase genes CYP6B48, CYP9A40 and CYP321B1). The CYP6B48 gene exhibited highest inducibility. In conclusion, the data of this study suggest that exposure of S. litura larvae to herbicides may stimulate detoxification mechanisms that compromise the efficacy of insecticides.

Molecular and functional properties of two Spodoptera exigua acetylcholinesterase genes

Acetylcholinesterase (AChE) is a vital enzyme that hydrolyzes acetylcholine. Here, full-length complementary DNAs (cDNAs) of two acetylcholinesterase genes (SeAce1 and SeAce2) were obtained from Spodoptera exigua, a widespread phytophagous pest in agriculture. The complete SeAce1 cDNA comprised 5447 nucleotides including an open reading frame (ORF) encoding 694 amino acids, while SeAce2 cDNA encompassed a 1917-bp ORF which would likely yield 638 amino acids. Both SeAce1 and SeAce2 contained specific characteristics of functional AChE. A phylogenetic tree of all lepidopteran insect Aces showed S. exigua clustered with S. litura, Helicoverpa assulta, and H. armigera, all of which are Noctuidae. In S. exigua, SeAce1 gene expression levels (reverse transcription polymerase chain reaction [RT-PCR] and quantitative RT-PCR) were markedly increased compared with SeAce2 in all developmental phases and tissue types. Both genes were down regulated by inserting the corresponding dsRNAs in 5th instar larvae, which resulted in 56.7% (SeAce1) and 24.6% (SeAce2) death. Downregulation of both SeAce1 and SeAce2 significantly reduced fecundity and vitellogenin gene expression in S. exigua. These results revealed the biological functions of the two Ace genes (SeAce1 and SeAce2), providing novel insights into the development of strategies for controlling insect pests.

Plant allelochemicals affect tolerance of polyphagous lepidopteran pest Helicoverpa armigera (Hübner) against insecticides

Cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), is a polyphagous lepidopteran pest distributed worldwide with a broad spectrum of host plants. However, the mechanism of H. armigera adaptation to various insecticides and defensive allelochemicals in its host plants is not fully understood. Therefore, this study examined the influence of consumption of plant allelochemicals on larval tolerance to methomyl and chlorpyrifos insecticides in H. armigera and its possible mechanism. Twelve plant allelochemicals were screened to evaluate their effects on larval sensitivity to methomyl. Of which flavone, coumarin, DIMBOA (2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and visnagin significantly reduced larval sensitivity to methomyl. Application of cytochrome P450 inhibitor piperonyl butoxide (PBO) significantly increased the mortality of methomyl-treated larvae. In contrast, PBO addition significantly decreased the mortality of chlorpyrifos-treated larvae. Moreover, allelochemical consumption enhanced the activities of glutathione S-transferase, carboxylesterase, cytochrome P450 and acetylcholinesterase in the midgut and fat body. The qRT-PCR analysis confirms that P450 genes, CYP6B2, CYP6B6 and CYP6B7 were induced by the four allelochemicals in the midguts and the fat bodies. In conclusion, the generalist H. armigera can take benefit of plant allelochemicals from its host plants to elaborate its defense against insecticides.

Molecular Characterization and Expression Analysis of Two Acetylcholinesterase Genes From the Small White Butterfly Pieris rapae (Lepidoptera: Pieridae)

Acetylcholinesterases (AChEs) are essential for the hydrolysis of the neurotransmitter acetylcholine and play crucial roles in the termination of neurotransmission. AChEs are encoded by the ace genes. However, the ace genes from the small white butterfly, Pieris rapae (L.) (Lepidoptera: Pieridae), remained uncharacterized. In this study, two aces (Prace1 and Prace2) were identified from P. rapae. Prace1 encoded a PrAChE1 protein consisting of 694 amino acid residues, and Prace2 encoded the 638-amino-acid PrAChE2. The two identified PrAChEs both had features typical of AChEs, including the catalytic triad, choline-binding sites, an oxyanion hole, an acyl pocket, a peripheral anionic subsite, an FGESAG motif and 14 conserved aromatic amino acids. Phylogenetic analysis showed that Prace1 and Prace2 were clustered into two distinct groups: ace1 and ace2, respectively. The two Praces were distributed on different genomic scaffolds: Prace1 on scaffold 156 and Prace2 on scaffold 430. Additionally, Prace1 consisted of three exons and two introns, whereas Prace2 consisted of six exons and five introns. One amino acid mutation (Gly324Ala) in PrAChE1 and two (Ser291Gly and Ser431Phe) in PrAChE2 were consistent with mutations in other insect AChEs that are associated with insecticide insensitivity. Both Prace1 and Prace2 were highly expressed at the fifth-instar larval stage and in the larval head, and the transcriptional levels of Prace1 were significantly higher than those of Prace2 in all of the tested life stages and tissues. This is the first report characterizing two ace genes in P. rapae. The results pave the way for functional study of these genes.