Transcriptomic Analysis Reveals Insect Hormone Biosynthesis Pathway Involved in Desynchronized Development Phenomenon in Hybridized Sibling Species of Tea Geometrids (Ectropis grisescens and Ectropis obliqua)

Recent Advances in the Specialized Metabolites Mediating Resistance to Insect Pests and Pathogens in Tea Plants (Camellia sinensis)

Tea is the second most popular nonalcoholic beverage consumed in the world, made from the buds and young leaves of the tea plants (Camellia sinensis). Tea trees, perennial evergreen plants, contain abundant specialized metabolites and suffer from severe herbivore and pathogen attacks in nature. Thus, there has been considerable attention focusing on investigating the precise function of specialized metabolites in plant resistance against pests and diseases. In this review, firstly, the responses of specialized metabolites (including phytohormones, volatile compounds, flavonoids, caffeine, and L-theanine) to different attacks by pests and pathogens were compared. Secondly, research progress on the defensive functions and action modes of specialized metabolites, along with the intrinsic molecular mechanisms in tea plants, was summarized. Finally, the critical questions about specialized metabolites were proposed for better future research on phytohormone-dependent biosynthesis, the characteristics of defense responses to different stresses, and molecular mechanisms. This review provides an update on the biological functions of specialized metabolites of tea plants in defense against two pests and two pathogens.

Comparative Analyses of Reproductive Caste Types Reveal Vitellogenin Genes Involved in Queen Fertility in Solenopsis invicta

The red imported fire ant (Solenopsis invicta Buren) is a social pest species with a robust reproductive ability that causes extensive damage. Identification of the genes involved in queen fertility is critical in order to better understand the reproductive biology and screening for the potential molecular targets in S. invicta. Here, we used the mRNA deep sequencing (RNA-seq) approach to identify differentially expressed genes (DEGs) in the transcriptomes of three reproductive caste types of S. invicta, including queen (QA) and winged female (FA) and male (MA) ants. The genes that were specific to and highly expressed in the queens were then screened, and the Vg2 and Vg3 genes were chosen as targets to explore their functions in oogenesis and fertility. A minimum of 6.08 giga bases (Gb) of clean reads was obtained from all samples, with a mapping rate > 89.78%. There were 7524, 7133, and 977 DEGs identified in the MA vs. QA, MA vs. FA, and FA vs. QA comparisons, respectively. qRT-PCR was used to validate 10 randomly selected DEGs, including vitellogenin 2 (Vg2) and 3 (Vg3), and their expression patterns were mostly consistent with the RNA-seq data. The S. invicta Vgs included conserved domains and motifs that are commonly found in most insect Vgs. SiVg2 and SiVg3 were highly expressed in queens and winged females and were most highly expressed in the thorax, followed by the fat body, head, and epidermis. Evaluation based on a loss-of-function-based knockdown analysis showed that the downregulation of either or both of these genes resulted in smaller ovaries, less oogenesis, and less egg production. The results of transcriptional sequencing provide a foundation for clarifying the regulators of queen fertility in S. invicta. The functions of SiVg2 and SiVg3 as regulators of oogenesis highlight their importance in queen fecundity and their potential as targets of reproductive disruption in S. invicta control.

Genome-wide identification and immune response analysis of mitogen-activated protein kinase cascades in tea geometrid, Ectropis grisescens Warren (Geometridae, Lepidoptera)

BACKGROUND

Tea geometrid Ectropis grisescens (Geometridae: Lepidoptera), is one of the most destructive defoliators in tea plantations in China. The MAPK cascade is known to be an evolutionarily conserved signaling module, acting as pivotal cores of host-pathogen interactions. Although the chromosome-level reference genome of E. grisescens was published, the whole MAPK cascade gene family has not been fully identified yet, especially the expression patterns of MAPK cascade gene family members upon an ecological biopesticide, Metarhizium anisopliae, remains to be understood.

RESULTS

In this study, we have identified 19 MAPK cascade gene family members in E. grisescens, including 5 MAPKs, 4 MAP2Ks, 8 MAP3Ks, and 2 MAP4Ks. The molecular evolution characteristics of the whole Eg-MAPK cascade gene family, including gene structures, protein structural organization, chromosomal localization, orthologs construction and gene duplication, were systematically investigated. Our results showed that the members of Eg-MAPK cascade gene family were unevenly distributed in 13 chromosomes, and the clustered members in each group shared similar structures of the genes and proteins. Gene expression data revealed that MAPK cascade genes were expressed in all four developmental stages of E. grisescens and were fairly and evenly distributed in four different larva tissues. Importantly, most of the MAPK cascade genes were induced or constitutively expressed upon M. anisopliae infection.

CONCLUSIONS

In summary, the present study was one of few studies on MAPK cascade gene in E. grisescens. The characterization and expression profiles of Eg-MAPK cascades genes might help develop new ecofriendly biological insecticides to protect tea trees.

Larval-Transcriptome Dynamics of Ectropis grisescens Reveals Differences in Virulence Mechanism between Two EcobNPV Strains

The biological insecticide, Ectropis obliqua nucleopolyhedrovirus (EcobNPV), has been applied to control the major tea-pest Ectropis grisescens. Previously, the virus strain EcobNPV-QF4 showed higher a mortality rate (58.2% vs. 88.2%) and shorter median lethal-time (13.9 d vs. 15.4 d) on E. grisescens than the strain EcobNPV-QV. However, the mechanism of the difference in virulence between the two strains remains unclear. Using the leaf-disc method, we detected the virulence of the two strains on 3^rd-instar larvae, and found that median lethal-dose (LD₅₀) of EcobNPV-QF4 is 55-fold higher than that of EcobNPV-QV (4.35 × 10⁸ vs. 7.89 × 10⁶). Furthermore, fourteen larva transcriptomes of E. grisescens were subsequently sequenced at seven time-points after ingestion of the two virus strains, yielding 410.72 Gb of raw reads. Differential gene-expression analysis shows that 595, 87, 27, 108, 0, 12, and 290 genes were up-regulated in EcobNPV-QF4 at 0, 2, 6, 12, 24, 36 h and 48 h post ingestion (hpi), while 744, 68, 152, 8, 1, 0, 225 were down-regulated. KEGG enrichment showed that when the virus first invades (eats the leaf-discs), EcobNPV-QF4 mainly affects pathways such as ribosome (p-value = 2.47 × 10^-29), and at 48 hpi EcobNPV-QF4, causes dramatic changes in the amino-acid-synthesis pathway and ribosome pathway (p-value = 6.94 × 10^-13) in E. grisescens. Among these, thirteen key genes related to immunity were screened. The present study provides the first ever comprehensive analysis of transcriptional changes in E. grisescens after ingestion of the two strains of EcobNPV.

Transcriptome analysis reveals gene expression differences in Liriomyza trifolii exposed to combined heat and abamectin exposure

Liriomyza trifolii is an invasive pest of horticultural and vegetable crops that possesses robust competitive advantages that enable it to replace closely-related species. High temperatures often occur concomitantly with insecticide usage during L. trifolii outbreaks. In this study, we compared the transcriptomes of L. trifolii exposed to high temperature (40 °C T40), insecticide (LC₅₀ of technical grade abamectin, I50) and combined high temperature and abamectin exposure (IT5040, I50 followed by T40; and TI4050, T40 followed by I50). RNA-seq generated and revealed 44,633 unigenes with annotation data; these were compared with COG and KEGG databases for functional classification and enrichment analysis. Compared with the I50 treatment, COG classification indicated that ‘post-translational modification, protein turnover, chaperones’ was enriched in the IT5040 treatment. In the TI4050 treatment, ‘carbohydrate transport and metabolism’ was the most abundant group. The most enriched KEGG pathways in the TI4050 and IT5040 treatments were ‘longevity regulating pathway - multiple species’ and ‘protein processing in endoplasmic reticulum’, respectively. Subsequent annotation and enrichment analyses indicated that stress-related genes such as CYP450s and HSPs were differentially expressed in the I50 vs. TI4050 or I50 vs. IT5040 treatment groups. Three commercial insecticide formulations were also used to further verify the expression of selected differentially-expressed genes. This study will be conductive to consider the temperature effect on insecticide tolerance in L. trifolii, and provides a framework for improving the application efficiency of insecticides in hot weather, which will ultimately reduce the overuse of pesticides.

Wolbachia Strain wGri From the Tea Geometrid Moth Ectropis grisescens Contributes to Its Host's Fecundity

Members of the Wolbachia genus manipulate insect-host reproduction and are the most abundant bacterial endosymbionts of insects. The tea Geometrid moth Ectropis grisescens (Warren) (Lepidoptera: Geometridae) is the most devastating insect pest of tea plants [Camellia sinensis (L.) O. Kuntze] in China. However, limited data on the diversity, typing, or phenotypes of Wolbachia in E. grisescens are available. Here, we used a culture-independent method to compare the gut bacteria of E. grisescens and other tea Geometridae moths. The results showed that the composition of core gut bacteria in larvae of the three Geometridae moth species was similar, except for the presence of Wolbachia. Moreover, Wolbachia was also present in adult female E. grisescens samples. A Wolbachia strain was isolated from E. grisescens and designated as wGri. Comparative analyses showed that this strain shared multilocus sequence types and Wolbachia surface protein hypervariable region profiles with cytoplasmic incompatibility (CI)-inducing strains in supergroup B; however, the wGri-associated phenotypes were undetermined. A reciprocal cross analysis showed that Wolbachia-uninfected females mated with infected males resulted in 100% embryo mortality (0% eggs hatched per female). Eggs produced by mating between uninfected males and infected females hatched normally. These findings indicated that wGri induces strong unidirectional CI in E. grisescens. Additionally, compared with uninfected females, Wolbachia-infected females produced approximately 30-40% more eggs. Together, these results show that this Wolbachia strain induces reproductive CI in E. grisescens and enhances the fecundity of its female host. We also demonstrated that wGri potential influences reproductive communication between E. grisescens and Ectropis obliqua through CI.

Chromosome-level genome reference and genome editing of the tea geometrid

The tea geometrid is a destructive insect pest on tea plants, which seriously affects tea production in terms of both yield and quality and causes severe economic losses. The tea geometrid also provides an important study system to address the ecological adaptive mechanisms underlying its unique host plant adaptation and protective resemblance. In this study, we fully sequenced and de novo assembled the reference genome of the tea geometrid, Ectropis grisescens, using long sequencing reads. We presented a highly continuous, near-complete genome reference (787.4 Mb; scaffold N50: 26.9 Mb), along with the annotation of 18,746 protein-coding genes and 53.3% repeat contents. Importantly, we successfully placed 97.8% of the assembly in 31 chromosomes based on Hi-C interactions and characterized the sex chromosome based on sex-biased sequencing coverage. Multiple quality-control assays and chromosome-scale synteny with the model species all supported the high quality of the presented genome reference. We focused biological annotations on gene families related to the host plant adaptation and camouflage in the tea geometrid and performed comparisons with other representative lepidopteran species. Important findings include the E. grisescens-specific expansion of CYP6 P450 genes that might be involved in metabolism of tea defensive chemicals and unexpected massive expansion of gustatory receptor gene families that suggests potential polyphagy for this tea pest. Furthermore, we developed an efficient genome editing system based on CRISPR/Cas9 technology and successfully implement mutagenesis of a Hox gene in the tea geometrid. Our study provides key genomic resources both for exploring unique mechanisms underlying the ecological adaptation of tea geometrids and for developing environment-friendly strategies for tea pest management.

Comparative characterization of microbiota between the sibling species of tea geometrid moth Ectropis obliqua Prout and E. grisescens Warren

For a wide range of insect species, the microbiota has potential roles in determining host developmental programme, immunity and reproductive biology. The tea geometrid moths Ectropis obliqua and E. grisescens are two closely related species that mainly feed on tea leaves. Although they can mate, infertile hybrids are produced. Therefore, these species provide a pair of model species for studying the molecular mechanisms of microbiotal involvement in host reproductive biology. In this study, we first identified and compared the compositions of microbiota between these sibling species, revealing higher microbiotal diversity for E. grisescens. The microbiota of E. obliqua mainly comprised the phyla Firmicutes, Proteobacteria and Cyanobacteria, whereas that of E. grisescens was dominated by Proteobacteria, Actinobacteria and Firmicutes. At the genus level, the dominant microbiota of E. grisescens included Wolbachia, Enterobacter and Pseudomonas and that of E. obliqua included Melissococcus, Staphylococcus and Enterobacter. Furthermore, we verified the rate of Wolbachia to infect 80 samples from eight different geographical populations, and the results supported that only E. grisescens harboured Wolbachia. Taken together, our findings indicate significantly different microbiotal compositions for E. obliqua and E. grisescens, with Wolbachia possibly being a curial factor influencing the reproductive isolation of these species. This study provides new insight into the mechanisms by which endosymbiotic bacteria, particularly Wolbachia, interact with sibling species.

Characterization of Dendrolimus houi Lajonquiere (Lepidoptera: Lasiocampidae) Transcriptome across All Life Stages

Dendrolimus houi Lajonquiere is a phytophagous caterpillar infesting many economically important coniferous tree species in China, causing serious economic and ecological environment losses. Based on previous research, it has one generation per year in South China and East China in contrast to two generations per year in Yunnan province in southwestern China. The species is potentially resilient to climatic extremes in these regions with the eggs and 1st instar larvae surviving in the winter (5 °C), older instar larvae and pupae surviving high temperatures in the summer (35 °C), suggesting some temperature stress tolerance during different developmental stages. However, little is known in this species at the genetic and genomic level. In this study, we used high throughput sequencing to obtain transcriptome data from different developmental stages (eggs, 1st-3rd instar larvae, 4th-5th instar larvae, 6th-7th instar larvae, pupae, male and female adults), which were collected from Fujian province. In total, we obtained approximately 90 Gb of data, from which 33,720 unigenes were assembled and 17,797 unigenes were annotated. We furtherly analyzed the differentially expressed genes (DGEs) across all stages, the largest number between the eggs and 1st instar larvae stage and gene expression varied significantly in different developmental stages. Furthermore, 4138 SSR genes and 114,977 SNP loci were screened from transcriptome data. This paper will be a foundation for further study towards improved integrated pest management strategies for this species.