RNA interference-mediated knockdown of eye coloration genes in the western tarnished plant bug (Lygus hesperus Knight)

Doublesex is essential for masculinization but not feminization in Lygus hesperus

In most holometabolous insects, sex differentiation occurs via a hierarchical cascade of transcription factors, with doublesex (dsx) regulating genes that control sex-specific traits. Although less is known in hemimetabolous insects, early evidence suggests that substantial differences exist from more evolutionarily advanced insects. Here, we identified and characterized dsx in Lygus hesperus (western tarnished plant bug), a hemipteran pest of many agricultural crops in western North America. The full-length transcript for L. hesperus dsx (Lhdsx) and several variants encode proteins with conserved DNA binding and oligomerization domains. Transcript profiling revealed that Lhdsx is ubiquitously expressed, likely undergoes alternative pre-mRNA splicing, and, unlike several model insects, is sex-biased rather than sex-specific. Embryonic RNA interference (RNAi) of Lhdsx only impacted sex development in adult males, which lacked both internal reproductive organs and external genitalia. No discernible impacts on adult female development or reproductivity were observed. RNAi knockdown of Lhdsx in nymphs likewise only affected adult males, which lacked the characteristic dimorphic coloration but had dramatically elevated vitellogenin transcripts. Gene knockout of Lhdsx by CRISPR/Cas9 editing yielded only females in G0 and strongly biased heterozygous G1 offspring to females with the few surviving males showing severely impaired genital development. These results indicate that L. hesperus male development requires Lhdsx, whereas female development proceeds via a basal pathway that functions independently of dsx. A fundamental understanding of sex differentiation in L. hesperus could be important for future gene-based management strategies of this important agricultural pest.

β-tubulin functions in spermatogenesis in Lygus hesperus Knight

Lygus hesperus Knight is an important insect pest of crops across western North America, with field management heavily reliant on the use of chemical insecticides. Because of the evolution of resistance to these insecticides, effective and environmentally benign pest management strategies are needed. Traditional sterile insect technique (SIT) has been successfully employed to manage or eradicate some insect pests but involves introducing irradiated insects with random mutations into field populations. New genetically-driven SIT techniques are a safer alternative, causing fixed mutations that manipulate individual genes in target pests to produce sterile individuals for release. Here, we identified seven β-tubulin coding genes from L. hesperus and show that Lhβtub2 is critical in male sperm production and fertility. Lhβtub2 is expressed primarily in the male testes and targeting of this gene by RNA interference or gene editing leads to male sterility.

Efficient DIPA-CRISPR-mediated knockout of an eye pigment gene in the white-backed planthopper, Sogatella furcifera

Although CRISPR/Cas9 has been widely used in insect gene editing, the need for the microinjection of preblastoderm embryos can preclude the technique being used in insect species with eggs that are small, have hard shells, and/or are difficult to collect and maintain outside of their normal environment. Such is the case with Sogatella furcifera, the white-backed planthopper (WBPH), a significant pest of Oryza sativa (rice) that oviposits inside rice stems. Egg extraction from the stem runs the risk of mechanical damage and hatching is heavily influenced by the micro-environment of the rice stem. To bypass these issues, we targeted embryos prior to oviposition via direct parental (DIPA)-CRISPR, in which Cas9 and single-guide RNAs (sgRNAs) for the WBPH eye pigment gene tryptophan 2,3-dioxygenase were injected into the hemocoel of adult females. Females at varying numbers of days posteclosion were evaluated to determine at what stage their oocyte might be most capable of taking up the gene-editing components. An evaluation of the offspring indicated that the highest G0 gene-edited efficacy (56.7%) occurred in females injected 2 d posteclosion, and that those mutations were heritably transmitted to the G1 generation. This study demonstrates the potential utility of DIPA-CRISPR for future gene-editing studies in non-model insect species and can facilitate the development of novel pest management applications.

Genome editing of the vermilion locus generates a visible eye color marker for Oncopeltus fasciatus

Insects display a vast array of eye and body colors. Genes encoding products involved in biosynthesis and deposition of pigments are ideal genetic markers, contributing, for example, to the power of Drosophila genetics. Oncopeltus fasciatus is an emerging model for hemimetabolous insects, a member of the piercing-sucking feeding order Hemiptera, that includes pests and disease vectors. To identify candidate visible markers for O. fasciatus, we used parental and nymphal RNAi to identify genes that altered eye or body color while having no deleterious effects on viability. We selected Of-vermilion for CRISPR/Cas9 genome editing, generating three independent loss-of-function mutant lines. These studies mapped Of-vermilion to the X-chromosome, the first assignment of a gene to a chromosome in this species. Of-vermilion homozygotes have bright red, rather than black, eyes and are fully viable and fertile. We used these mutants to verify a role for Of-xdh1, ortholog of Drosophila rosy, in contributing to red pigmentation using RNAi. Rather than wild-type-like red bodies, bugs lacking both vermilion and xdh1 have bright yellow bodies, suggesting that ommochromes and pteridines contribute to O. fasciatus body color. Our studies generated the first gene-based visible marker for O. fasciatus and expanded the genetic toolkit for this model system.

Genetic modification of the ant Lasius niger using CRISPR-Cas9 technology

CRISPR-Cas9 has become one of the most prominent gene editing tools available and it has been utilized in various organisms from bacteria to fungi, plants, and animals. In this study, we developed a CRISPR-Cas9 protocol for the black garden ant Lasius niger, a common and easily available study species for lab and field experiments. To create indel mutations using CRISPR-Cas9 in L. niger, we targeted three different locations in a well-studied eye pigmentation gene cinnabar, generating several mutations that disrupt the ommochrome biosynthesis pathway and result in the lack of the pigment and therefore, abnormal eye coloration in adult workers. We also developed a protocol to collect L. niger eggs, inject them with CRISPR-Cas9 construct, and rear the eggs into mature adult workers with the assistance of nursing workers. We demonstrated for the first time in L. niger that CRISPR-Cas9 is an excellent tool to create targeted mutations for this species. Our protocol can be referred to when developing similar studies for other species of ants and eusocial insects.

Disruption of tetrahydrobiopterin (BH4) biosynthesis pathway affects cuticle pigmentation in Henosepilachna vigintioctopunctata

Tetrahydrobiopterin (BH4) is produced from guanosine triphosphate (GTP) under catalyzation of GTP cyclohydrolase I (GTPCH), 6-pyruvoyltetrahydropterin synthase (PTPS) and sepiapterin reductase (SR), among others. In Drosophila melanogaster, BH4 and other pteridines are required for cuticle tanning and eye pigmentation. In this study, two Hvgtpch (Hvgtpch-a and Hvgtpch-b), an Hvptps and an Hvsr transcripts were identified in a serious defoliator Henosepilachna vigintioctopunctata. Hvgtpch-a and Hvgtpch-b were highly expressed just before and/or right after the molt, in contrast to Hvptps and Hvsr. RNA interference (RNAi) by injection of a dsgtpch targeting the common fragment of Hvgtpch-a and Hvgtpch-b into the third instar larvae caused albino fourth-instar larvae and pupae. Around 80% of the Hvgtpch RNAi larvae failed to pupate. The remaining 20% of Hvgtpch RNAi pupated beetles did not completely remove the larval/pupal exuviae after emerged as adults and eventually died. Depletion of Hvgtpch at the fourth instar stage resulted in under-pigmented pupae and adults, with significantly low pupation and emergence rates. The Hvgtpch RNAi adults rarely moved and fed on plant leaves; they died within a week after emergence. Silence of Hvptps or Hvsr at the third- and fourth-instar stages led to similar but less serious phenotypes, with lowest influence in the Hvsr RNAi ladybirds. Moreover, RNAi of Hvgtpch, Hvptps or Hvsr did not affect coloration of the larval ocelli and pupal/adult compound eyes. Therefore, our results demonstrated that pteridines are involved in melanin formation but not in eye pigmentation in H. vigintioctopunctata. Moreover, our findings will enable the development of a dsgtpch-based pesticide to control H. vigintioctopunctata larvae.

Mining Lygus hesperus (western tarnished plant bug) transcriptomic data for transient receptor potential channels: Expression profiling and functional characterization of a Painless homolog

The transient receptor potential (TRP) family of cation channels are evolutionarily conserved proteins with critical roles in sensory physiology. Despite extensive studies in model species, knowledge of TRP channel functional diversity and physiological impact remains limited in many non-model insect species. To assess the TRP channel repertoire in a non-model agriculture pest species (Lygus hesperus), publicly available transcriptomic datasets were mined for potential homologs. Among the transcripts identified, 30 are predicted to encompass complete open reading frames that encode proteins representing each of the seven TRP channel subfamilies. Although no homologs were identified for the Pyrexia and Brivido channels, the TRP complement in L. hesperus exceeded the 13-16 channels reported in most insects. This diversity appears to be driven by a combination of alternative splicing, which impacted members of six subfamilies, and gene expansion of the TRPP subfamily. To validate the in silico data and provide more detailed analyses of L. hesperus TRP functionality, the putative Painless homolog was selected for more in depth analysis and its functional role in thermosensation examined in vitro. RT-PCR expression profiling revealed near ubiquitous expression of the Painless transcript throughout nymphal and adult development. Electrophysiological data generated using a Xenopus oocyte recombinant expression system indicated activation parameters for L. hesperus Painless homolog that are consistent with a role in noxious heat (40°-45 °C) thermosensation.

RNAi-Mediated Manipulation of Cuticle Coloration Genes in Lygus hesperus Knight (Hemiptera: Miridae)

Cuticle coloration in insects is a consequence of the accumulation of pigments in a species-specific pattern. Numerous genes are involved in regulating the underlying processes of melanization and sclerotization, and their manipulation can be used to create externally visible markers of successful gene editing. To clarify the roles for many of these genes and examine their suitability as phenotypic markers in Lygus hesperus Knight (western tarnished plant bug), transcriptomic data were screened for sequences exhibiting homology with the Drosophila melanogaster proteins. Complete open reading frames encoding putative homologs for six genes (aaNAT, black, ebony, pale, tan, and yellow) were identified, with two variants for black. Sequence and phylogenetic analyses supported preliminary annotations as cuticle pigmentation genes. In accord with observable difference in color patterning, expression varied for each gene by developmental stage, adult age, body part, and sex. Knockdown by injection of dsRNA for each gene produced varied effects in adults, ranging from the non-detectable (black 1, yellow), to moderate decreases (pale, tan) and increases (black 2, ebony) in darkness, to extreme melanization (aaNAT). Based solely on its expression profile and highly visible phenotype, aaNAT appears to be the best marker for tracking transgenic L. hesperus.

RNAi-induced knockdown of white gene in the southern green stink bug (Nezara viridula L.)

The southern green stink bug (SGSB) Nezara viridula L. is one of the most common stink bug species in the United States and can cause significant yield loss in a variety of crops. A suitable marker for the assessment of gene-editing tools in SGSB has yet to be characterized. The white gene, first documented in Drosophila, has been a useful target to assess the efficiency of introduced mutations in many species as it controls pigmentation processes and mutants display readily identifiable phenotypes. In this study we used the RNAi technique to investigate functions and phenotypes associated with the white ortholog in the SGSB and to validate white as a marker for genetic transformation in this species. This study revealed that white may be a suitable marker for germline transformation in the SGSB as white transcript knockdown was not lethal, did not impair embryo development and provided a distinguishable phenotype. Our results demonstrated that the white ortholog in SGSB is involved in the pathway for ommochrome synthesis and suggested additional functions of this gene such as in the integument composition, management of hemolymph compounds and riboflavin mobilization.

CRISPR-mediated knockout of cardinal and cinnabar eye pigmentation genes in the western tarnished plant bug

The western tarnished plant bug, Lygus hesperus, is a key hemipteran pest of numerous agricultural, horticultural, and industrial crops in the western United States and Mexico. A lack of genetic tools in L. hesperus hinders progress in functional genomics and in developing innovative pest control methods such as gene drive. Here, using RNA interference (RNAi) against cardinal (LhCd), cinnabar (LhCn), and white (LhW), we showed that knockdown of LhW was lethal to developing embryos, while knockdown of LhCd or LhCn produced bright red eye phenotypes, in contrast to wild-type brown eyes. We further used CRISPR/Cas9 (clustered regularly interspaced palindromic repeats/CRISPR-associated) genome editing to generate germline knockouts of both LhCd (Card) and LhCn (Cinn), producing separate strains of L. hesperus characterized by mutant eye phenotypes. Although the cardinal knockout strain Card exhibited a gradual darkening of the eyes to brown typical of the wild-type line later in nymphal development, we observed bright red eyes throughout all life stages in the cinnabar knockout strain Cinn, making it a viable marker for tracking gene editing in L. hesperus. These results provide evidence that CRISPR/Cas9 gene editing functions in L. hesperus and that eye pigmentation genes are useful for tracking the successful genetic manipulation of this insect.

Biology, Ecology, and Pest Management of the Tarnished Plant Bug, Lygus lineolaris (Palisot de Beauvois) in Southern Row Crops

Simple Summary

The tarnished plant bug, Lygus lineolaris, is a polyphagous, sap-feeder that causes significant economic damage in several field crops, especially cotton (Gossypium hirsutum L.) in the mid-southern United States. In 2020, it was reported that 4.8 million acres of cotton were infested by Lygus spp. in the United States. A broad host range, polyphagous feeding behavior and high mobility of this pest along with resistance development to conventional pesticides helped them establish as a significant pest of concern for cotton growers in the mid-south. Since the publication of a review by Layton (2000) on damage caused by Lygus lineolaris, many new research studies have been published on the Lygus biology, ecology, and integrated pest management strategies. A comprehensive review paper that summarizes these latest research developments and Lygus management strategies will be useful for researchers and cotton growers. In this review, we report and discuss the latest developments in Lygus research and the new control strategies that have been developed in the last two decades.

Abstract

The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), (Hemiptera: Miridae) is considered the most damaging pest of cotton (Gossypium hirsutum L.) in the mid-southern United States, although it is established throughout the United States, southern Canada, and northern Mexico. The introduction of transgenic crops for the control of moths in the Heliothine complex and eradication of the boll weevil, Anthonomus grandis, from much of the United States led to greatly reduced pesticide use in cotton fields, which allowed L. lineolaris to emerge as a new primary pest of cotton in the mid-southern United States. Since the publication of a review by Layton (2000) on damage caused by Lygus lineolaris, many new studies have been published on the changes in host range, population dynamics, sampling methods and thresholds, cultural practices, sex pheromones and attractant blends, novel pesticides and insecticide resistance mechanisms, olfactory and feeding behaviors, introduction of biological control agents, host-plant resistance mechanisms, and new molecular and genetic tools for integrated pest management of Lygus species in cotton and other important crops. Here, we review and discuss the latest developments in L. lineolaris research in the last two decades.

Genome-wide identification and gene-editing of pigment transporter genes in the swallowtail butterfly Papilio xuthus

Background

Insect body coloration often functions as camouflage to survive from predators or mate selection. Transportation of pigment precursors or related metabolites from cytoplasm to subcellular pigment granules is one of the key steps in insect pigmentation and usually executed via such transporter proteins as the ATP-binding cassette (ABC) transmembrane transporters and small G-proteins (e.g. Rab protein). However, little is known about the copy numbers of pigment transporter genes in the butterfly genomes and about the roles of pigment transporters in the development of swallowtail butterflies.

Results

Here, we have identified 56 ABC transporters and 58 Rab members in the genome of swallowtail butterfly Papilio xuthus. This is the first case of genome-wide gene copy number identification of ABC transporters in swallowtail butterflies and Rab family in lepidopteran insects. Aiming to investigate the contribution of the five genes which are orthologous to well-studied pigment transporters (ABCG: white, scarlet, brown and ok; Rab: lightoid) of fruit fly or silkworm during the development of swallowtail butterflies, we performed CRISPR/Cas9 gene-editing of these genes using P. xuthus as a model and sequenced the transcriptomes of their morphological mutants. Our results indicate that the disruption of each gene produced mutated phenotypes in the colors of larvae (cuticle, testis) and/or adult eyes in G0 individuals but have no effect on wing color. The transcriptomic data demonstrated that mutations induced by CRISPR/Cas9 can lead to the accumulation of abnormal transcripts and the decrease or dosage compensation of normal transcripts at gene expression level. Comparative transcriptomes revealed 606 ~ 772 differentially expressed genes (DEGs) in the mutants of four ABCG transporters and 1443 DEGs in the mutants of lightoid. GO and KEGG enrichment analysis showed that DEGs in ABCG transporter mutants enriched to the oxidoreductase activity, heme binding, iron ion binding process possibly related to the color display, and DEGs in lightoid mutants are enriched in glycoprotein binding and protein kinases.

Conclusions

Our data indicated these transporter proteins play an important role in body color of P. xuthus. Our study provides new insights into the function of ABC transporters and small G-proteins in the morphological development of butterflies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07400-z.

Functional divergence of white genes in Henosepilachna vigintioctopunctata revealed by RNA interference

Henosepilachna vigintioctopunctata is a serious pest of Solanaceae and Cucurbitaceae in many Asian countries. RNA interference (RNAi) can effectively reduce transcript abundance in this beetle, offering opportunities to explore the biological function of specific genes. The white gene encodes a half-type ATP-binding cassette transporter that plays an essential role in tryptophan, guanine and uric acid transport across membranes. Mutations that disrupt the function of white are known to cause eye pigmentation phenotypes in many insect species. Here, we found evidence for five white gene paralogues present in H. vigintioctopunctata transcriptome datasets sequenced from a range of developmental stages. We individually knocked down each of the five white genes through the injection of corresponding double-stranded RNAs (dsRNAs) to the fourth-instar larvae to determine whether functional divergence has occurred. We found that injecting 1 μg dswhite3 caused compound eye colour of pupae and adults to develop as red/brown and brown, respectively, compared with black eyes in control beetles. Injection of 2 μg dswhite3 increased RNAi efficacy and produced a clearer eye colour phenotype. At both doses, the ocular diaphragm (a ring of black pigment surrounding each eye) did not change in the white3 RNAi hypomorphs. Moreover, our data revealed that injection of dswhite2 at the fourth-instar larval stage impaired the climbing ability of both male and female adults. Our results confirmed, for the first time, functional divergence of duplicated white genes in an insect species.

RNAi-Based Functional Genomics in Hemiptera

RNA interference (RNAi) is a powerful approach for sequence-specific gene silencing, displaying tremendous potential for functional genomics studies in hemipteran insects. Exploiting RNAi allows the biological roles of critical genes to be defined and aids the development of RNAi-based biopesticides. In this review, we provide context to the rapidly expanding field of RNAi-based functional genomics studies in hemipteran insects. We highlight the most widely used RNAi delivery strategies, including microinjection, oral ingestion and topical application. Additionally, we discuss the key variables affecting RNAi efficacy in hemipteran insects, including insect life-stage, gene selection, the presence of nucleases, and the role of core RNAi machinery. In conclusion, we summarise the application of RNAi in functional genomics studies in Hemiptera, focusing on genes involved in reproduction, behaviour, metabolism, immunity and chemical resistance across 33 species belonging to 14 families.

TRPA1 modulates noxious odor responses in Lygus hesperus

Lygus hesperus isa key pest of many economically important crops across western North America. Central to many aspects of the lives of these insects is chemical signalling, with identified roles in host plant selection, aggregation and passive mate guarding. The development of novel monitoring and control approaches for this insect will rely on a sound understanding of how these cues are perceived and processed, and their impact on behavior. Towards this end, we investigated allyl isothiocyanate, cinnamaldehyde and citronellal, compounds that are noxious repellents to other insects. We found that L. hesperus avoided areas containing the three compounds and that exposure induced increases in movement velocity and duration in both nymphs and adults. This suggests these compounds may work as repellents. To better understand the underlying physiology of this response, RNA interference by dsRNA injection was used to inhibit the expression of two chemosensory-associated proteins, the odorant receptor co-receptor (Orco) and the transient receptor potential A (TRPA1) channel. While knockdown of Orco did not change the reaction of adult females to citronellal, TRPA1 silencing effectively eliminated the induced increase to movement, suggesting a chemoperceptory role in citronellal detection.