Functional characterization of the Hyles euphorbiae hawkmoth transcriptome reveals strong expression of phorbol ester detoxification and seasonal cold hardiness genes

Advances in understanding Lepidoptera cold tolerance

Ambient thermal conditions mediate insect growth, development, reproduction, survival, and distribution. With increasingly frequent and severe cold spells, it is critical to determine low-temperature performance and cold tolerances of ecologically and economically essential insect groups to predict their responses to global environmental change. This review covers the cold tolerance strategies of 49 species of Lepidoptera (moths and butterflies), focusing on species that are known as crop pests and crop storage facilities. We synthesize cold tolerance strategies of well-studied species within this order, finding that diapause is a distinctive mechanism that has independently evolved in different genera and families of Lepidoptera. However, the occurrence of diapause in each life stage is specific to the species, and in most studied lepidopteran species, the feeding stage (as larva) is the predominant overwintering stage. We also found that the onset of diapause and the improvement of cold tolerance are interdependent phenomena that typically occur together. Moreover, adopting a cold tolerance strategy is not an inherent, fixed trait and is greatly influenced by a species' geographic distribution and rearing conditions. This review further finds that freeze avoidance rather than freeze tolerance or chill susceptibility is the primary cold tolerance strategy among lepidopteran species. The cold hardiness of lepidopteran insects primarily depends on the accumulation of cryoprotectants and the depression of the supercooling point. We highlight variations in cold tolerance strategies and mechanisms among a subset of Lepidoptera, however, further work is needed to elucidate these strategies for the vast numbers of neglected species and populations to understand broad-scale responses to global change.

Comparative Transcriptome Analysis Reveals Different Responses in Three Developmental Stages of Mythimna loreyi to Cold Stress

The loreyi leafworm Mythimna loreyi (Lepidoptera: Noctuidae) is a serious pest of agriculture that causes particular damage to Gramineae crops in Asia, Europe, Australia, Africa, and the Middle East. Low temperature is one of the important environmental factors that limits the survival, distribution, colonization, and abundance of M. loreyi. However, the metabolic synthesis pathways of cold-tolerant substances in M. loreyi and the key genes involved in the regulation under cold stress remain largely unknown. In this study, we sequenced the transcriptomes of three developmental stages (larvae, pupae, and adults) of M. loreyi to discover the molecular mechanisms of their responses to cold stress. In total, sequencing generated 120.64 GB of clean data from 18 samples, of which 19,459 genes and 1740 differentially expressed genes (DEGs) were identified. The enrichment analysis of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that many DEGs were mainly enriched in pathways associated with energy metabolism and hormone metabolism. Among these, genes encoding multiple metabolic enzymes, cuticle proteins (CPs), and heat shock proteins (HSPs) were differentially expressed. These results indicate that there are significant differences among the three developmental stages of M. loreyi exposed to cold stress and provide a basis for further studying the molecular mechanisms of cold tolerance in insects.

Analysis of Genes Associated with Feeding Preference and Detoxification in Various Developmental Stages of Aglais urticae

Herbivorous insects and host plants have developed a close and complex relationship over a long period of co-evolution. Some plants provide nutrients for insects, but plants' secondary metabolites also influence their growth and development. Urtica cannabina roots and leaves are poisonous, yet Aglais urticae larvae feed on them, so we aimed to clarify the mechanism enabling this interaction. At present, studies on the detoxification mechanism of the A. urticae are rare. In our study, first, we used the A. urticae larval odor selection behavior bioassay and choice feeding preference assay to analyze the feeding preferences of A. urticae on its host plant, U. cannabina. Next, we used transcriptome sequencing to obtain the unigenes annotated and classified by various databases, such as KEGG and GO. In this study, we found that U. cannabina could attract A. urticae larvae to feed via scent, and the feeding preference assay confirmed that larvae preferred U. cannabina leaves over three other plants: Cirsium japonicum, Cannabis sativa, and Arctium lappa. The activity of detoxifying enzymes GST and CarE changed in larvae that had consumed U. cannabina. Furthermore, through transcriptomic sequencing analysis, 77,624 unigenes were assembled from raw reads. The numbers of differentially expressed genes were calculated using pairwise comparisons of all life stages; the expression of detoxification enzyme genes was substantially higher in larvae than in the pupal and adult stages. Finally, we identified and summarized 34 genes associated with detoxification enzymes, such as UDP-glucose 4-epimerase gene, 5 Glutathione S-transferase genes, 4 Carboxylesterase genes, 4 Cytochrome P450 genes, 10 ATP-binding cassette genes, 4 Superoxide dismutase, and Peroxidase. Moreover, we identified 28 genes associated with the development of A. urticae. The qRT-PCR results were nearly consistent with the transcriptomic data, showing an increased expression level of four genes in larvae. Taken together, this study examines the correlation between A. urticae and host plants U. cannabina, uncovering a pronounced preference for A. urticae larvae toward host plants. Consistent with RNA-seq, we investigated the mechanism of A. urticae's interaction with host plants and identified detoxification-related genes. The present study provides theoretical support for studying insect adaptation mechanisms and biological control.

A high-quality, long-read genome assembly of the whitelined sphinx moth (Lepidoptera: Sphingidae: Hyles lineata) shows highly conserved melanin synthesis pathway genes

The sphinx moth genus Hyles comprises 29 described species inhabiting all continents except Antarctica. The genus diverged relatively recently (40-25 MYA), arising in the Americas and rapidly establishing a cosmopolitan distribution. The whitelined sphinx moth, Hyles lineata, represents the oldest extant lineage of this group and is one of the most widespread and abundant sphinx moths in North America. Hyles lineata exhibits the large body size and adept flight control characteristic of the sphinx moth family (Sphingidae), but it is unique in displaying extreme larval color variation and broad host plant use. These traits, in combination with its broad distribution and high relative abundance within its range, have made H. lineata a model organism for studying phenotypic plasticity, plant-herbivore interactions, physiological ecology, and flight control. Despite being one of the most well-studied sphinx moths, little data exist on genetic variation or regulation of gene expression. Here, we report a high-quality genome showing high contiguity (N50 of 14.2 Mb) and completeness (98.2% of Lepidoptera BUSCO genes), an important first characterization to facilitate such studies. We also annotate the core melanin synthesis pathway genes and confirm that they have high sequence conservation with other moths and are most similar to those of another, well-characterized sphinx moth, the tobacco hornworm (Manduca sexta).

The complete mitogenome sequence of the hawk moth, Theretra latreillii subsp. lucasii (Lepidoptera: Sphingidae) from Zhejiang Province, China

The sphingid, Theretra latreillii subsp. lucasii is a common hawk moth distributed in southeast Asia and Australian regions. Although barcode analyses have been published, its complete mitogenome sequence has not been deciphered. In this study, the complete mitogenome of T. latreillii lucasii (GeneBank accession no. MW539688) was sequenced using Illumina HiSeq X Ten system for mitogenome-based phylogenetic analysis. The mitogenome was 15,354 bp in length and comprises 13 protein-coding genes (PCGs), two ribosomal RNA (rRNA) genes, and 22 transfer RNAs (tRNAs) with the typical gene order and orientation of Sphingidae mitogenomes. The nucleotide composition of majority strand is 41.2% for A, 7.4% for G, 12.0% for C, and 39.4% for T, with an A + T content of 80.6%. Phylogenetic analysis using the 13 PCGs fully resolved T. latreillii lucasii in a clade with T. japonica, Macroglossum stellatarum, and Ampelophaga rubiginosa, with high nodal support both by Bayesian inference and maximum-likelihood methods, forming the Macroglossini monophyletic group.

Expression analysis of genes related to cold tolerance in Dendroctonus valens

Pine beetles are well known in North America for their widespread devastation of pine forests. However, Dendroctonus valens LeConte is an important invasive forest pest in China also. Adults and larvae of this bark beetle mainly winter at the trunks and roots of Pinus tabuliformis and Pinus sylvestris; larvae, in particular, result in pine weakness or even death. Since the species was introduced from the United States to Shanxi in 1998, its distribution has spread northward. In 2017, it invaded a large area at the junction of Liaoning, Inner Mongolia and Hebei provinces, showing strong cold tolerance. To identify genes relevant to cold tolerance and the process of overwintering, we sequenced the transcriptomes of wintering and non-wintering adult and larval D. valens using the Illumina HiSeq platform. Differential expression analysis methods for other non-model organisms were used to compare transcript abundances in adults and larvae at two time periods, followed by the identification of functions and metabolic pathways related to genes associated with cold tolerance. We detected 4,387 and 6,091 differentially expressed genes (DEGs) between sampling dates in larvae and adults, respectively, and 1,140 common DEGs, including genes encoding protein phosphatase, very long-chain fatty acids protein, cytochrome P450, and putative leucine-rich repeat-containing proteins. In a Gene Ontology (GO) enrichment analysis, 1,140 genes were assigned to 44 terms, with significant enrichment for cellulase activity, hydrolase activity, and carbohydrate metabolism. Kyoto Encyclopedia of Genes and Genomes (KEGG) classification and enrichment analyses showed that the lysosomal and purine metabolism pathways involved the most DEGs, the highly enriched terms included autophagy-animal, pentose and glucuronate interconversions and lysosomal processes. We identified 140 candidate genes associated with cold tolerance, including genes with established roles in this trait (e.g., genes encoding trehalose transporter, fructose-1,6-bisphosphatase, and trehalase). Our comparative transcriptome analysis of adult and larval D. valens in different conditions provides basic data for the discovery of key genes and molecular mechanisms underlying cold tolerance.

Short term transcriptional responses of P450s to phytochemicals in insects and mites

Cytochrome P450 monooxygenases (P450s) play a key role in the detoxification of phytochemicals in arthropod herbivores. We present here an overview of recent progress in understanding the breadth and specificity of gene expression plasticity of P450s in response to phytochemicals. We discuss experimental setups and new findings in mechanisms of P450 regulation. Whole genome transcriptomic analysis of arthropod herbivores, either after direct administration of phytochemicals or after host plant shifts, allowed to integrate various levels of chemical complexity and lead to the unbiased identification of responsive P450 genes. However, despite progress in identification of inducible P450s, the link between induction and metabolism is still largely unexplored, and to what extent the overall response is biologically functional should be further investigated. In the near future, such studies will be more straightforward as forward and reverse genetic tools become more readily available.

miR-31-5p regulates cold acclimation of the wood-boring beetle Monochamus alternatus via ascaroside signaling

BACKGROUND

Survival to cold stress in insects living in temperate environments requires the deployment of strategies that lead to physiological changes involved in freeze tolerance or freeze avoidance. These strategies may consist of, for instance, the induction of metabolic depression, accumulation of cryoprotectants, or the production of antifreeze proteins, however, little is known about the way such mechanisms are regulated and the signals involved in their activation. Ascarosides are signaling molecules usually known to regulate nematode behavior and development, whose expression was recently found to relate to thermal plasticity in the Japanese pine sawyer beetle Monochamus alternatus. Accumulating evidence also points to miRNAs as another class of regulators differentially expressed in response to cold stress, which are predicted to target genes involved in cold adaptation of insects. Here, we demonstrate a novel pathway involved in insect cold acclimation, through miRNA-mediated regulation of ascaroside function.

RESULTS

We initially discovered that experimental cold acclimation can enhance the beetle's cold hardiness. Through screening and functional verification, we found miR-31-5p, upregulated under cold stress, significantly contributes to this enhancement. Mechanistically, miR-31-5p promotes production of an ascaroside (asc-C9) in the beetle by negatively targeting the rate-limiting enzyme, acyl-CoA oxidase in peroxisomal β-oxidation cycles. Feeding experiments with synthetic asc-C9 suggests it may serve as a signal to promote cold acclimation through metabolic depression and accumulation of cryoprotectants with specific gene expression patterns.

CONCLUSIONS

Our results point to important roles of miRNA-mediated regulation of ascaroside function in insect cold adaptation. This enhanced cold tolerance may allow higher survival of M. alternatus in winter and be pivotal in shaping its wide distribution range, greatly expanding the threat of pine wilt disease, and thus can also inspire the development of ascaroside-based pest management strategies.

Mechanisms of Novel Host Use by Bactrocera tau (Tephritid: Diptera) Revealed by RNA Transcriptomes

Use of novel plant hosts can facilitate the establishment and range expansion of herbivorous invasive species. However, the inherent mechanisms of novel host use are still unclear in many herbivorous species. Here, we examine mechanisms of novel host use in the invasive tephritid fruit fly Bactrocera tau (Walker)(Diptera: Tephritidae) by documenting changes in the RNA transcriptomes associated with a novel host. RNA transcripts of B. tau were obtained with high-throughput sequencing from samples continuously reared on two traditional Cucurbitaceae hosts and a novel host (banana). We found transcriptome variation was strongly associated with feeding on banana. Moreover, B. tau feeding on banana contained more differentially expressed genes (DEGs) and more annotated categories of DEGs in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database with 1,595 DEGs and 21 major annotated pathways. The annotated categories of DEGs in individuals reared on banana differed with from those individuals feeding on other hosts and were enriched in oxidative phosphorylation, citrate cycle pathway, and four other carbohydrate pathways. For B. tau feeding on banana, the predominant numbers of upregulated genes in the mitochondrial NADH (56 on average) and a relatively higher numbers of upregulated genes (13 on average) were found in oxidative phosphorylation and the TCA pathway, respectively. Changes in RNA transcriptomes associated with novel host use, especially for genes related to energy and carbohydrate metabolism, help to explain how B. tau can be successful in use of novel hosts and may be useful in developing novel strategies for control of tephritid flies.

The Importance of ATM and ATR in Physcomitrella patens DNA Damage Repair, Development, and Gene Targeting

Coordinated by ataxia-telangiectasia-mutated (ATM) and ATM and Rad3-related (ATR), two highly conserved kinases, DNA damage repair ensures genome integrity and survival in all organisms. The Arabidopsis thaliana (A. thaliana) orthologues are well characterized and exhibit typical mammalian characteristics. We mutated the Physcomitrellapatens (P. patens) PpATM and PpATR genes by deleting functionally important domains using gene targeting. Both mutants showed growth abnormalities, indicating that these genes, particularly PpATR, are important for normal vegetative development. ATR was also required for repair of both direct and replication-coupled double-strand breaks (DSBs) and dominated the transcriptional response to direct DSBs, whereas ATM was far less important, as shown by assays assessing resistance to DSB induction and SuperSAGE-based transcriptomics focused on DNA damage repair genes. These characteristics differed significantly from the A. thaliana genes but resembled those in yeast (Saccharomyces cerevisiae). PpATR was not important for gene targeting, pointing to differences in the regulation of gene targeting and direct DSB repair. Our analysis suggests that ATM and ATR functions can be substantially diverged between plants. The differences in ATM and ATR reflect the differences in DSB repair pathway choices between A. thaliana and P. patens, suggesting that they represent adaptations to different demands for the maintenance of genome stability.

Comparative transcriptome analysis of differentially expressed genes in Bradysia odoriphaga Yang et Zhang (Diptera: Sciaridae) at different acute stress temperatures

The gnat, Bradysia odoriphaga Yang et Zhang, is an important underground pest in Asia. B. odoriphaga differ in heat and cold tolerance and exhibit quite different developmental strategies. To understand the underlying mechanisms, we sequenced and compared the transcriptome of B. odoriphaga under 40 °C (a stressful high temperature), 25 °C, and 4 °C (a stressful low temperature) for 1 h. We found that metabolism- and ribosome-related genes were modulated. In high temperature (40 °C), heat shock protein (HSP) genes, detoxication genes, metabolism genes, protein turnover genes, and stress signal transduction genes were differentially expressed. In low temperature (4 °C), genes related with heat shock protein (HSP) and detoxication were differentially expressed. Our study increases our understanding of the complex molecular mechanisms involved in the responses of B. odoriphaga to acute temperature stress and provides a potential strategy for pest management.

Transcriptomic analysis of differentially expressed genes in the oriental armyworm Mythimna separata Walker at different temperatures

The oriental armyworm Mythimna separata Walker is a serious migratory and polyphagous pest that damages major crops and some pastures from the family Gramineae. Temperature is a crucial abiotic factor that affects its survival, development and reproduction, but the thermal responses of this moth at the molecular level are largely unknown. In this research, we sequenced the transcriptomes of oriental armyworms that were reared at three temperatures (20 °C, 25 °C and 30 °C) using an Illumina high-throughput RNA-sequencing (RNA-seq) method. We obtained 54.0 Gb of clean reads and 113,396 transcripts. From a total of 46,681 unigenes identified, 22,911 were annotated to the non-redundant (NR) database. We identified 333 downregulated and 1588 upregulated genes in 20 °C versus 25 °C, and 1096 downregulated and 875 upregulated genes at 30 °C versus 25 °C by differential expression of genes (DEGs). GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analyses revealed several functional terms related to carbohydrate metabolism, energy metabolism, and xenobiotics metabolism. DEGs involved in glycolysis, the citrate cycle, oxidative phosphorylation, and the composition of myofilaments were significantly downregulated, while most heat shock protein genes (HSPs) and genes in the ubiquitin-mediated proteasome pathway were upregulated at 30 °C. Many cytochrome P450 monooxygenase genes (CYPs) in clan 3 were upregulated at 20 °C, while two genes involved in ecdysteroid biosynthesis, CYP302A1 and CYP315A1, were upregulated at 30 °C. These data may improve the understanding of the complex molecular mechanisms involved in the thermal responses of M. separata.