Molecular and functional analyses of amino acid decarboxylases involved in cuticle tanning in Tribolium castaneum

Insufficient and excessive Ca2+ intake negatively impact the life history performance and disrupt the hemolymph metabolism of Spodoptera litura

Calcium ions (Ca2+), essential as second messengers in all cells, play a pivotal role as micronutrients in insects. However, few studies have explored the effects of both insufficient and excessive Ca2+ intake on life history performance and population parameters. This study examines the impact of varying Ca2+ intake levels-insufficient (0 mg/kg), appropriate (100 mg/kg), and excessive (250 mg/kg)-on the life history performance and population parameters of Spodoptera litura using two-sex life tables. Insufficient and excessive Ca2+ intakes significantly extended the preadult development period and decreased the preadult survival rates of S. litura, compared to those on an appropriate Ca2+ intake. The population parameters (Intrinsic rate of increase (r), Finite rate of increase (λ), and Net reproductive rate (R0)) of S. litura on a 100 mg/kg diet (r = 0.1364, λ = 1.1462, R0 = 390) were significantly higher than those on a 0 mg/kg diet (r = 0.1091, λ = 1.1153, R0 = 130.52). Additionally, untargeted metabolomics analysis revealed that inappropriate Ca2+ levels (either insufficient or excessive) triggered significant up-regulation of 71.1 % and 92.8 % of the metabolites in the hemolymph, respectively, compared to the appropriate Ca2+ intake. Notably, disruptions in metabolite balance affected critical components such as melatonin and melanin within the tryptophan and tyrosine metabolism pathways. These findings underscore that both insufficient and excessive Ca2+ intakes adversely affect the life history performance and disrupt hemolymph metabolic balance in S. litura.

Comparative transcriptome analysis provides comprehensive insight into the molecular mechanisms of heat adaption in Plutella xylostella

Plutella xylostella is one of the most destructive pests for cruciferous vegetables, and is adaptability to different environmental stressors. However, we still know little about the molecular mechanisms of how P. xylostella adapt to thermal stress. Here, the comparative transcriptome analysis was conducted from the samples of control (27 °C, CK) and heat treatment (40 °C, 40 T) P. xylostella. The results showed 1253 genes were differentially expressed, with 624 and 629 genes up- and down-regulated respectively. The annotation analysis demonstrated that "Energy production and conversion", "Protein processing in endoplasmic reticulum", "Peroxisome" and "Tyrosine metabolism" pathways were significantly enriched. Additionally, we found the expression levels of heat shock protein genes (Hsps), cuticle related genes and mitochondrial genes were significantly up-regulated in 40 T insects, suggesting their vital roles in improving adaption to heat stress. Importantly, the SOD activity and MDA content of P. xylostella were both identified to be increased under high temperature stress, indicating the elevated antioxidant reactions might be involved in response to heat stress. In conclusion, the present study offered us an overview of gene expression changes after 40 °C treatments, and found some critical pathways and genes of P. xylostella might play the critical roles in resisting heat stress.

Multicopper oxidase-2 mediated cuticle formation: Its contribution to evolution and success of insects as terrestrial organisms

The insect cuticle is a non-cellular matrix composed of polysaccharide chitins and proteins. The cuticle covers most of the body surface, including the trachea, foregut, and hindgut, and it is the body structure that separates the intraluminal environment from the external environment. The cuticle is essential to sustain their lives, both as a physical barrier to maintain homeostasis and as an exoskeleton that mechanically supports body shape and movement. Previously, we proposed a theory about the possibility that the cuticle-forming system contributes to the "evolution and success of insects." The main points of our theory are that 1) insects evolved an insect-specific system of cuticle formation and 2) the presence of this system may have provided insects with a competitive advantage in the early land ecosystems. The key to this theory is that insects utilize molecular oxygen abundant in the atmosphere, which differs from closely related crustaceans that form their cuticles with calcium ions. With newly obtained knowledge, this review revisits the significance of the insect-specific system for insects to adapt to terrestrial environments and also discusses the long-standing question in entomology as to why, despite their great success in terrestrial environments, they poorly adapt to marine environments.

The BTB transcription factor, Abrupt, acts cooperatively with Chronologically inappropriate morphogenesis (Chinmo) to repress metamorphosis and promotes leg regeneration

Many insects undergo the process of metamorphosis when larval precursor cells begin to differentiate to create the adult body. The larval precursor cells retain stem cell-like properties and contribute to the regenerative ability of larval appendages. Here we demonstrate that two Broad-complex/Tramtrack/Bric-à-brac Zinc-finger (BTB) domain transcription factors, Chronologically inappropriate morphogenesis (Chinmo) and Abrupt (Ab), act cooperatively to repress metamorphosis in the flour beetle, Tribolium castaneum. Knockdown of chinmo led to precocious development of pupal legs and antennae. We show that although topical application of juvenile hormone (JH) prevents the decrease in chinmo expression in the final instar, chinmo and JH act in distinct pathways. Another gene encoding the BTB domain transcription factor, Ab, was also necessary for the suppression of broad (br) expression in T. castaneum in a chinmo RNAi background, and simultaneous knockdown of ab and chinmo led to the precocious onset of metamorphosis. Furthermore, knockdown of ab led to the loss of regenerative potential of larval legs independently of br. In contrast, chinmo knockdown larvae exhibited pupal leg regeneration when a larval leg was ablated. Taken together, our results show that both ab and chinmo are necessary for the maintenance of the larval tissue identity and, apart from its role in repressing br, ab acts as a crucial regulator of larval leg regeneration. Our findings indicate that BTB domain proteins interact in a complex manner to regulate larval and pupal tissue homeostasis.

The melanin pigment gene black mediates body pigmentation and courtship behaviour in the German cockroach Blattella germanica

Genes involved in melanin production directly impact insect pigmentation and can affect diverse physiology and behaviours. The role these genes have on sex behaviour, however, is unclear. In the present study, the crucial melanin pigment gene black was functionally characterised in an urban pest, the German cockroach, Blattella germanica. RNAi knockdown of B. germanica black (Bgblack) had no effect on survival, but did result in black pigmentation of the thoraxes, abdomens, heads, wings, legs, antennae, and cerci due to cuticular accumulation of melanin. Sex-specific variation in the pigmentation pattern was apparent, with females exhibiting darker coloration on the abdomen and thorax than males. Bgblack knockdown also resulted in wing deformation and negatively impacted the contact sex pheromone-based courtship behaviour of males. This study provides evidence for black function in multiple aspects of B. germanica biology and opens new avenues of exploration for novel pest control strategies.

Functional importance of groups I and II chitinases, CHT5 and CHT10, in turnover of chitinous cuticle during embryo hatching and post-embryonic molting in the red flour beetle, Tribolium castaneum

Chitinases (CHT) comprise a large gene family in insects and have been classified into at least eleven subgroups. Many studies involving RNA interference (RNAi) have demonstrated that depletion of group I (CHT5s) and group II (CHT10s) CHT transcripts causes lethal molting arrest in several insect species including the red flour beetle, Tribolium castaneum, presumably due to failure of degradation of chitin in their old cuticle. In this study we investigated the functions of CHT5 and CHT10 in turnover of chitinous cuticle in T. castaneum during embryonic and post-embryonic molting stages. RNAi and transmission electron microscopic (TEM) analyses indicate that CHT10 is required for cuticular chitin degradation at each molting period analyzed, while CHT5 is essential for pupal-adult molting only. We further analyzed the functions of these genes during embryogenesis in T. castaneum. Real-time qPCR analysis revealed that peak expression of CHT10 occurred prior to that of CHT5 during embryonic development as has been observed at post-embryonic molting periods in several other insect species. With immunogold-labeling TEM analysis using a fluorescein isothiocyanate-conjugated chitin-binding domain protein (FITC-CBD) probe, chitin was detected in the serosal cuticle but not in any other regions of the eggshell including the chorion and vitelline membrane layers. Injection of double-stranded RNA (dsRNA) for CHT5 (dsCHT5), CHT10 (dsCHT10) or their co-injection (dsCHT5/10) into mature adult females had no effect on their fecundity and the resulting embryos developed normally inside the egg. There were no obvious differences in the morphology of the outer chorion, inner chorion and vitelline membrane among eggs from these dsRNA-treated females. However, unlike dsCHT5 eggs, dsCHT10 and dsCHT5/10 eggs exhibited failure of turnover of the serosal cuticle in which the horizontal chitinous laminae remained intact, resulting in lethal embryo hatching defects. These results indicate that group I CHT5 is essential for pupal-adult molting, whereas group II CHT10 plays an essential role in cuticular chitin degradation in T. castaneum during both embryonic hatching and all of the post-embryonic molts. CHT10 can serve in place of CHT5 in chitin degradation, except during the pupal-adult molt when both enzymes are indispensable to complete eclosion.

Discovery and Engineering of a Novel Bacterial L-Aspartate α-Decarboxylase for Efficient Bioconversion

L-aspartate α-decarboxylase (ADC) is a pyruvoyl-dependent decarboxylase that catalyzes the conversion of L-aspartate to β-alanine in the pantothenate pathway. The enzyme has been extensively used in the biosynthesis of β-alanine and D-pantothenic acid. However, the broad application of ADCs is hindered by low specific activity. To address this issue, we explored 412 sequences and discovered a novel ADC from Corynebacterium jeikeium (CjADC). CjADC exhibited specific activity of 10.7 U/mg and Km of 3.6 mM, which were better than the commonly used ADC from Bacillus subtilis. CjADC was then engineered leveraging structure-guided evolution and generated a mutant, C26V/I88M/Y90F/R3V. The specific activity of the mutant is 28.8 U/mg, which is the highest among the unknown ADCs. Furthermore, the mutant displayed lower Km than the wild-type enzyme. Moreover, we revealed that the introduced mutations increased the structural stability of the mutant by promoting the frequency of hydrogen-bond formation and creating a more hydrophobic region around the active center, thereby facilitating the binding of L-aspartate to the active center and stabilizing the substrate orientation. Finally, the whole-cell bioconversion showed that C26V/I88M/Y90F/R3V completely transformed 1-molar L-aspartate in 12 h and produced 88.6 g/L β-alanine. Our study not only identified a high-performance ADC but also established a research framework for rapidly screening novel enzymes using a protein database.

MpADC, an L-aspartate-α-decarboxylase, from Myzus persicae, that enables production of β-alanine with high yield by whole-cell enzymatic catalysis

BACKGROUND

β-Alanine is a precursor of many important pharmaceutical products and food additives, its market demand is continuously increasing nowadays. Whole-cell catalysis relying on the recombinant expression of key β-alanine synthesizing enzymes is an important method to produce β-alanine. Nevertheless, β-alanine synthesizing enzymes found so far have problems including easy inactivation, low expression or poor catalytic activity, and it remains necessary to develop new enzymes.

RESULTS

Herein, we characterized an L-aspartate-α-decarboxylase, MpADC, from an aphid, Myzus persicae. It showed excellent catalytic activity at pH 6.0-7.5 and 37 °C. With the help of chaperone co-expression and N-terminal engineering guided by AlphaFold2 structure prediction, the expression and catalytic ability of MpADC in Escherichia coli were significantly improved. Using 50 g/L of E. coli cells expressing the MpADC-∆39 variant cultured in a 15-L fermenter, 232.36 g/L of β-alanine was synthesized in 13.5 h, with the average β-alanine yield of 17.22 g/L/h, which is best known so far.

CONCLUSIONS

Our research should facilitate the production of β-alanine in an environment-friendly manner.

ebony underpins Batesian mimicry in melanic stoneflies

The evolution of Batesian mimicry - whereby harmless species avoid predation through their resemblance to harmful species - has long intrigued biologists. In rare cases, Batesian mimicry is linked to intraspecific colour variation, in which only some individuals within a population resemble a noxious 'model'. Here, we assess intraspecific colour variation within a widespread New Zealand stonefly, wherein highly melanized individuals of Zelandoperla closely resemble a chemically defended aposematic stonefly, Austroperla cyrene. We assess convergence in the colour pattern of these two species, compare their relative palatability to predators, and use genome-wide association mapping to assess the genetic basis of this resemblance. Our analysis reveals that melanized Zelandoperla overlap significantly with Austroperla in colour space but are significantly more palatable to predators, implying that they are indeed Batesian mimics. Analysis of 194,773 genome-wide SNPs reveals an outlier locus (ebony) strongly differentiating melanic versus non-melanic Zelandoperla. Genotyping of 338 specimens from a single Zelandoperla population indicates that ebony explains nearly 70% of the observed variance in melanism. As ebony has a well-documented role in insect melanin biosynthesis, our findings indicate this locus has a conserved function across deeply divergent hexapod lineages. Distributional records suggest a link between the occurrence of melanic Zelandoperla and the forested ecosystems where the model Austroperla is abundant, suggesting the potential for adaptive shifts in this system underpinned by environmental change.

Ovariole-specific Yellow-g and Yellow-g2 proteins are required for fecundity and egg chorion rigidity in the red flour beetle, Tribolium castaneum

Most insects reproduce by laying eggs that have an eggshell/chorion secreted by follicle cells, which serves as a protective barrier for developing embryos. Thus, eggshell formation is vital for reproduction. Insect yellow family genes encode for secreted extracellular proteins that perform different, context-dependent functions in different tissues at various stages of development involving, for example, cuticle/eggshell coloration and morphology, molting, courtship behavior and embryo hatching. In this study we investigated the function of two of this family's genes, yellow-g (TcY-g) and yellow-g2 (TcY-g2), on the formation and morphology of the eggshell of the red flour beetle, Tribolium castaneum. Real-time PCR analysis revealed that both TcY-g and TcY-g2 were specifically expressed in the ovarioles of adult females. Loss of function produced by injection of double-stranded RNA (dsRNA) for either TcY-g or TcY-g2 gene resulted in failure of oviposition. There was no effect on maternal survival. Ovaries dissected from those dsRNA-treated females exhibited ovarioles containing not only developing oocytes but also mature eggs in their egg chambers. However, the ovulated eggs were collapsed and ruptured, resulting in swollen lateral oviducts and calyxes. TEM analysis showed that lateral oviducts were filled with electron-dense material, presumably from some cellular content leakage out of the collapsed eggs. In addition, morphological abnormalities in lateral oviduct epithelial cells and the tubular muscle sheath were evident. These results support the hypothesis that both TcY-g and TcY-g2 proteins are required for maintaining the rigidity and integrity of the chorion, which is critical for resistance to mechanical stress and/or rehydration during ovulation and egg activation in the oviducts of T. castaneum. Because Yellow-g and Yellow-g2 are highly conserved among insect species, both genes are potential targets for development of gene-based insect pest population control methods.

Cuticle supplementation and nitrogen recycling by a dual bacterial symbiosis in a family of xylophagous beetles

Many insects engage in stable nutritional symbioses with bacteria that supplement limiting essential nutrients to their host. While several plant sap-feeding Hemipteran lineages are known to be simultaneously associated with two or more endosymbionts with complementary biosynthetic pathways to synthesize amino acids or vitamins, such co-obligate symbioses have not been functionally characterized in other insect orders. Here, we report on the characterization of a dual co-obligate, bacteriome-localized symbiosis in a family of xylophagous beetles using comparative genomics, fluorescence microscopy, and phylogenetic analyses. Across the beetle family Bostrichidae, most investigated species harbored the Bacteroidota symbiont Shikimatogenerans bostrichidophilus that encodes the shikimate pathway to produce tyrosine precursors in its severely reduced genome, likely supplementing the beetles' cuticle biosynthesis, sclerotisation, and melanisation. One clade of Bostrichid beetles additionally housed the co-obligate symbiont Bostrichicola ureolyticus that is inferred to complement the function of Shikimatogenerans by recycling urea and provisioning the essential amino acid lysine, thereby providing additional benefits on nitrogen-poor diets. Both symbionts represent ancient associations within the Bostrichidae that have subsequently experienced genome erosion and co-speciation with their hosts. While Bostrichicola was repeatedly lost, Shikimatogenerans has been retained throughout the family and exhibits a perfect pattern of co-speciation. Our results reveal that co-obligate symbioses with complementary metabolic capabilities occur beyond the well-known sap-feeding Hemiptera and highlight the importance of symbiont-mediated cuticle supplementation and nitrogen recycling for herbivorous beetles.

Functional characterization of tyrosine melanin genes in the white-backed planthopper and utilization of a spray-based nanoparticle-wrapped dsRNA technique for pest control

As a significant pest of rice the white-backed planthopper (WBPH) Sogatella furcifera is a focus of pest management. However, traditional chemical-based control methods risk the development of pesticide resistance as well as severe ecological repercussions. Although nanoparticle-encapsulated dsRNAs provide a promising alternative method for sustainable pest management, gene targets specific to WBPH have yet to be optimized. Genes in the tyrosine-melanin pathway impact epidermal melanization and sclerotization, two processes essential for insect development and metabolism, have been proposed as good candidate targets for pest management. Seven genes (aaNAT, black, DDC, ebony, tan, TH, and yellow-y) in this group were identified from WBPH genome and functionally characterized by using RNAi for their impact on WBPH body color, development, and mortality. Knockdown of SfDDC, Sfblack, SfaaNAT, and Sftan caused cuticles to turn black, whereas Sfyellow-y and Sfebony knockdown resulted in yellow coloration. SfTH knockdown resulted in pale-colored bodies and high mortality. Additionally, an Escherichia coli expression system for large-scale dsRNA production was coupled with star polycation nanoparticles to develop a sprayable RNAi method targeting SfTH that induced high WBPH mortality rates on rice seedlings. These findings lay the groundwork for the development of large-scale dsRNA nanoparticle sprays as a WBPH control method.

Combinatorial expression of ebony and tan generates body color variation from nymph through adult stages in the cricket, Gryllus bimaculatus

Insect body colors and patterns change markedly during development in some species as they adapt to their surroundings. The contribution of melanin and sclerotin pigments, both of which are synthesized from dopamine, to cuticle tanning has been well studied. Nevertheless, little is known about how insects alter their body color patterns. To investigate this mechanism, the cricket Gryllus bimaculatus, whose body color patterns change during postembryonic development, was used as a model in this study. We focused on the ebony and tan genes, which encode enzymes that catalyze the synthesis and degradation, respectively, of the precursor of yellow sclerotin N-β-alanyl dopamine (NBAD). Expression of the G. bimaculatus (Gb) ebony and tan transcripts tended to be elevated just after hatching and the molting period. We found that dynamic alterations in the combined expression levels of Gb'ebony and Gb'tan correlated with the body color transition from the nymphal stages to the adult. The body color of Gb'ebony knockout mutants generated by CRISPR/Cas9 systemically darkened. Meanwhile, Gb'tan knockout mutants displayed a yellow color in certain areas and stages. The phenotypes of the Gb'ebony and Gb'tan mutants probably result from an over-production of melanin and yellow sclerotin NBAD, respectively. Overall, stage-specific body color patterns in the postembryonic stages of the cricket are governed by the combinatorial expression of Gb'ebony and Gb'tan. Our findings provide insights into the mechanism by which insects evolve adaptive body coloration at each developmental stage.

Research progress of L-aspartate-α-decarboxylase and its isoenzyme in the β-alanine synthesis

Driven by the massive demand in recent years, the production of β-alanine has significantly progressed in chemical and biological ways. Although the chemical method is relatively mature compared to biological synthesis, its high cost of waste disposal and environmental pollution does not meet the environmental protection standard. Hence, the biological method has become more prevalent as a potential alternative to the chemical synthesis of β-alanine in recent years. As a result, the aspartate pathway from L-aspartate to β-alanine (the most significant rate-limiting step in the β-alanine synthesis) catalyzed by L-aspartate-α-decarboxylase (ADC) has become a research hotspot in recent years. Therefore, it is vital to comprehensively understand the different enzymes that possess a similar catalytic ability to ADC. This review will investigate the exploratory process of unique synthesis features and catalytic properties of ADC/ADC-like enzymes in particular creatures with similar catalytic capacity or high sequence homology. At the same time, we will discuss the different β-alanine production methods which can apply to future industrialization.

Melanin synthesis genes BgTH and BgDdc affect body color and cuticle permeability in Blattella germanica

Melanin is involved in cuticle pigmentation and sclerotization of insects, which is critical for maintaining structural integrity and functional completeness of insect cuticle. The 2 key enzymes of tyrosine hydroxylase (TH) and dopa decarboxylase (DDC) predicted in melanin biosynthesis are usually conserved in insects. However, it is unclear whether their function is related to epidermal permeability. In this study, we identified and cloned the gene sequences of BgTH and BgDdc from Blattella germanica, and revealed that they both showed a high expression at the molting, and BgTH was abundant in the head and integument while BgDdc was expressed highest in the fat body. Using RNA interference (RNAi), we found that knockdown of BgTH caused molting obstacles in some cockroaches, with the survivors showing pale color and softer integuments, while knockdown of BgDdc was viable and generated an abnormal light brown body color. Desiccation assay showed that the dsBgTH-injected adults died earlier than control groups under a dry atmosphere, but dsBgDdc-injected cockroaches did not. In contrast, when dsRNA-treated cockroaches were reared under a high humidity condition, almost no cockroaches died in all treatments. Furthermore, with eosin Y staining assay, we found that BgTH-RNAi resulted in a higher cuticular permeability, and BgDdc-RNAi also caused slight dye penetration. These results demonstrate that BgTH and BgDdc function in body pigmentation and affect the waterproofing ability of the cuticle, and the reduction of cuticular permeability may be achieved through cuticle melanization.

RR-1 cuticular protein TcCPR69 is required for growth and metamorphosis in Tribolium castaneum

Cuticle is not only critical for protecting insects from noxious stimuli but is also involved in a variety of metabolic activities. Cuticular proteins (CPs) affect cuticle structure and mechanical properties during insect growth, reproduction, and environmental adaptation. Here, we describe the identification and characterization of a member of the RR-1 subfamily of CPs with an R&R consensus (CPR) in Tribolium castaneum (TcCPR69). Although it was previously reported to be highly expressed in the wings, we found that knocking down TcCPR69 by RNA interference (RNAi) did not cause obvious wing abnormalities but markedly disrupted the growth and metamorphosis of beetles with 100% cumulative mortality; additionally, the chitin content of the pharate adult was decreased and the new abdominal cuticle was significantly thinner before molting. TcCPR69 showed chitin-binding ability and the expression levels of key genes involved in chitin metabolism (trehalase [TcTRE], chitin synthase [TcCHSA and TcCHSB], and chitinase [TcCHT5 and TcCHT10]) were also decreased by TcCPR69 knockdown. TcCPR69 gene expression peaked shortly after molting and was increased 2.61 fold at 12 h after 20-hydroxyecdysone (20E) injection. This was reversed by RNAi of the ecdysone-related genes ecdysone receptor (TcECR) and fushi tarazu transcription factor 1 (TcFTZ-F1). These results indicate that TcCPR69 is positively regulated by 20E signaling to contribute to cuticle formation and maintain chitin accumulation during the growth and metamorphosis of beetles.

Transcriptome Analysis to Identify Responsive Genes under Sublethal Concentration of Bifenazate in the Diamondback Moth, Plutella xylostella (Linnaeus, 1758) (Lepidoptera: Plutellidae)

Bifenazate is a novel acaricide that has been widely used to control spider mites. Interestingly, we found bifenazate had a biological activity against the diamondback moth (Plutella xylostella), one of the most economically important pests on crucifer crops around the world. However, the molecular mechanisms underlying the response of P. xylostella to bifenazate treatment are not clear. In this study, we first estimated the LC₃₀ dose of bifenazate for third-instar P. xylostella larvae. Then, in order to identify genes that respond to the treatment of this insecticide, the comparative transcriptome profiles were used to analyze the gene expression changes in P. xylostella larvae after exposure to LC₃₀ of bifenazate. In total, 757 differentially expressed genes (DEGs) between bifenazate-treated and control P. xylostella larvae were identified, in which 526 and 231 genes were up-regulated and down-regulated, respectively. The further Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the xenobiotics metabolisms pathway was significantly enriched, with ten detoxifying enzyme genes (four P450s, five glutathione S-transferases (GSTs), and one UDP-Glucuronosyltransferase (UGT)) were up-regulated, and their expression patterns were validated by qRT-PCR as well. Interestingly, the present results showed that 17 cuticular protein (CP) genes were also remarkably up-regulated, including 15 CPR family genes. Additionally, the oxidative phosphorylation pathway was found to be activated with eight mitochondrial genes up-regulated in bifenazate-treated larvae. In contrast, we found some genes that were involved in tyrosine metabolism and purine pathways were down-regulated, indicating these two pathways of bifenazate-exposed larvae were significantly inhibited. In conclusion, the present study would help us to better understand the molecular mechanisms of sublethal doses of bifenazate detoxification and action in P. xylostella.

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.

Abnormal overexpression of SoxD enhances melanin synthesis in the Ursa mutant of Bombyx mori

The pigment and structural color of insects play crucial roles in body protection, ecological adaptation, and signal communication. Epidermal melanization is a common and main coloring pattern, which results in broad phenotypic diversity. Melanin is one of the compounds contributing to dark brown-black pigmentation, which is synthesized from dopamine and tyrosine by the melanin metabolism pathway. The Ursa mutant of the silkworm Bombyx mori is a body-color mutant characterized by excessive melanin pigmentation in the larval epidermis. However, the exact gene responsible for this phenotype remains unclear. Here, we performed positional cloning of the gene responsible for Ursa, which was mapped to an 83-kb region on chromosome 14. The genomic region contains a protein-coding gene encoding a transcription factor, which was designated BmSoxD. The mutation site was determined by analysis of nucleotide sequences of the genomic region corresponding to BmSoxD, which identified a 449-bp transposable sequence similar to that of the B. mori transposon Helitron inserted into the sixth intron. BmSoxD was dramatically overexpressed in the epidermis of Ursa at the end of the molting stage compared with that of wild-type B. mori. Overexpression of BmSoxD led to upregulation of genes involved in the melanin metabolism pathway, whereas knocking down BmSoxD via small interfering RNAs blocked melanin pigment production in the larval epidermis. These data indicate that the mutation in BmSoxD is responsible for the Ursa mutant phenotype. We propose that the transposable sequence insertion causes abnormal overexpression of BmSoxD at the molting stage in the Ursa mutant, resulting in excessive melanin synthesis and its accumulation in epidermal cells.

Molecular and functional analysis of eclosion hormone-like gene involved in post-eclosion behavior in a beetle

Eclosion hormone (EH) is a neurohormone that plays a key role in the regulation of insect pre-ecdysis behavior at the end of each molt. Previous research has reported more than one EH gene was found in certain insects, with their functions and mechanisms still unclear. Here, aside from the classical EH gene orthologous group, we characterized another novel orthologous cluster of eclosion hormone-like (EHL) genes in Arthropoda and investigated the roles of EHL during development in Tribolium castaneum. T. castaneum EHL (TcEHL) shows high expression levels during pupal - adult development, which also positively responded to 20-hydroxyecdysone (20E) treatment as well as RNA interference (RNAi) of ECR (20E nuclear receptor). Knockdown of TcEHL prevented the tanning of the adult cuticle and caused lethal phenotypes. Further analysis indicated that knockdown of TcEHL could upregulate expression levels of the classical TcEH, and downregulate the ecdysis behavior cascade genes, as well as tanning pathway enzymes. This suggests a critical role for TcEHL in adult eclosion and cuticle tanning. In addition, our data indicated that TcEHL is responsible for the female reproduction process. Taken together, these results suggest that TcEHL has specific roles in adult cuticle tanning during the post-eclosion process and female reproduction. They also suggest that EHL gene is the ancestral copy for the EH family and it is functionally shuffled by synfunctionalization.

Nuclear receptor HR3 mediates transcriptional regulation of chitin metabolic genes during molting in Tribolium castaneum

BACKGROUND

Chitin, a major component of insect cuticles, plays a critical role in insect molting and morphogenesis. Thus, coordination of chitin remodeling during insect development requires tight transcriptional control of the chitin metabolism genes involved in chitin synthesis, assembly and degradation. However, the molecular mechanism underlying transcriptional coordination of chitin metabolism genes during beetle development is not yet completely understood.

RESULTS

We cloned the full-length cDNA encoding hormone receptor 3 (TcHR3) from Tribolium castaneum and showed a critical role of TcHR3 in modulating chitin metabolism gene expression during molting. Genome-wide transcriptome analysis of HR3-deficient old larvae using RNA sequencing analysis revealed a positive correlation between TcHR3 and transcription of chitin metabolism genes involved in chitin synthesis and degradation. In addition, HR3 overexpression significantly induced the gene promoter activity of N-acetylglucosaminidase 1 (NAG1) involved in chitin degradation and UDP-N-acetylglucosamine pyrophosphorylase 1 (UAP1) involved in chitin synthesis. Chromatin immunoprecipitation analysis revealed that HR3 could directly bind to HR3-response element of NAG1 and UAP1 promoters. Finally, HR3-deficient late instar larvae and prepupae exhibited defects in larval-larval and larval-pupal molting, respectively, leading to eventual larval death because developing larvae were trapped inside the old cuticle as a result of abnormal chitin metabolism.

CONCLUSION

TcHR3 is a transcriptional regulator of chitin metabolic genes for molting of T. castaneum. Controlling the molting system by TcHR3 might be a new management strategy for selective control of red flour beetle infestation. © 2022 Society of Chemical Industry.

Temporal changes in the physical and mechanical properties of beetle elytra during maturation

Changes in physical properties of Tenebrio molitor and Tribolium castaneum elytra (hardened forewings) were studied to understand how the development of microstructure and chemical interactions determine cuticle mechanical properties. Analysis of these properties supports a model in which cuticular material is continuously secreted from epidermal cells to produce an extracellular matrix so that the outermost layers mature first. It is hypothesized that enzymatic crosslinking and pigmentation reactions along with dehydration help to stabilize the protein-chitin network within the initial layers of cuticle shortly after eclosion. Mature layers are proposed to bear most of the mechanical loads. The frequency dependence of the storage modulus and the tan δ values decreased during the beginning of maturation, reaching constant values after 48 h post-eclosion. A decrease of tan δ indicates an increase in crosslinking of the material. The water content declined from 75% to 31%, with a significant portion lost from within the open spaces between the dorsal and ventral cuticular layers. Dehydration had a less significant influence than protein crosslinking on the mechanical properties of the elytron during maturation. When Tribolium cuticular protein TcCP30 expression was decreased by RNAi, the tan δ and frequency dependence of E' of the elytron did not change during maturation. This indicates that TcCP30 plays a role in the crosslinking process of the beetle's exoskeleton. This study was inspired by previous work on biomimetic multicomponent materials and helps inform future work on creating robust lightweight materials derived from natural sources. STATEMENT OF SIGNIFICANCE: Examination of changes in the physical properties of the elytra (hardened forewings) of two beetle species advanced understanding of how the molecular interactions influence the mechanical properties of the elytra. Physical characterization, including dynamic mechanical analysis, determined that the outer portion of the elytra matured first, while epidermal cells continued to secrete reactive components until the entire structure reached maturation. RNA interference was used to identify the role of a key protein in the elytra. Suppression of its expression reduced the formation of crosslinked polymeric components in the elytra. Identifying the molecular interactions in the matrix of proteins and polysaccharides in the elytra together with their hierarchical architecture provides important design concepts in the development of biomimetic materials.

Silencing tyrosine hydroxylase or dopa decarboxylase gene disrupts cuticle tanning during larva-pupa-adult transformation in Henosepilachna vigintioctopunctata

BACKGROUND

The 28-spotted potato ladybird, Henosepilachna vigintioctopunctata, is a notorious defoliator of many solanaceous and cucurbitaceous plants. Tyrosine hydroxylase (TH) and dopa decarboxylase (DDC) are responsible for cuticle tanning pathway in insects.

RESULTS

We identified HvTH and HvDDC in H. vigintioctopunctata, and found that high levels of them were accumulated just before or right after molting. Injection of dsHvTH or feeding 3-iodo-tyrosine (3-IT) at the third instar larval stage repressed tanning of the larval cuticle, reduced larval feeding, inhibited larval growth, and consequently caused 100% of larval mortality. Knockdown of HvDDC at the third instar larval stage hardly affected the coloration of larval head, and partially inhibited pigmentation of larval bodies and around 80% of the HvDDC RNAi larvae developed into albino pupae and adults. Moreover, depletion of HvTH or HvDDC at the fourth instar larval stage resulted in albino pupae and adults. The HvTH or HvDDC hypomorph adults fully or partially failed to remove the larval/pupal exuviae, possessed pale and abnormal wings, and poorly tanned heads and bodies, and eventually, struggled for several days without feeding on leaves before death.

CONCLUSION

These results show that TH and DDC play key roles in larval and adult cuticle tanning and development in H. vigintioctopunctata. Also, these findings suggest that dopa- and dopamine-originated pigments are essential for larval and adult feeding behavior and the molting process during emergence. © 2022 Society of Chemical Industry.

Nuclear receptor estrogen-related receptor modulates antimicrobial peptide expression for host innate immunity in Tribolium castaneum

Antimicrobial peptides (AMPs) are core components of innate immunity to protect insects against microbial infections. Nuclear receptors (NRs) are ligand-dependent transcription factors that can regulate the expression of genes critical for insect development including molting and metamorphosis. However, the role of NRs in host innate immune response to microbial infection remains poorly understood in Tribolium castaneum (T. castaneum). Here, we show that estrogen-related receptor (ERR), an insect ortholog of the mammalian ERR family of NRs, is a novel transcriptional regulator of AMP genes for innate immune response of T. castaneum. Tribolium ERR (TcERR) expression was induced by immune deficiency (IMD)-Relish signaling in response to infection by Escherichia coli (E. coli), a Gram-negative bacterium, as demonstrated in TcIMD-deficient beetles. Interestingly, genome-wide transcriptome analysis of TcERR-deficient old larvae using RNA-sequencing analysis showed that TcERR expression was positively correlated with gene transcription levels of AMPs including attacins, defensins, and coleoptericin. Moreover, chromatin immunoprecipitation analysis revealed that TcERR could directly bind to ERR-response elements on promoters of genes encoding defensin3 and coleoptericin, critical for innate immune response of T. castaneum. Finally, TcERR-deficient old larvae infected with E. coli displayed enhanced bacterial load and significantly less host survival. These findings suggest that TcERR can coordinate transcriptional regulation of AMPs and host innate immune response to bacterial infection.

MicroRNAs miR-14 and miR-2766 regulate tyrosine hydroxylase to control larval-pupal metamorphosis in Helicoverpa armigera

BACKGROUND

The cotton bollworm, Helicoverpa armigera, is a worldwide polyphagous pest, causing huge economic losses in vegetable, cotton and corn crops, among others. Owing to long-term exposure to Bacillus thuringiensis (Bt) toxins, evolution of resistance has been detected in this pest. As a conservative and effective neurotransmitter, dopamine (DA) has an important role in insect growth and development. In this study, we investigated the regulatory functions of DA and its associated non-coding RNA in metamorphosis in H. armigera.

RESULTS

Expression profiles indicated that DA and DA pathway genes were highly expressed during larval-pupal metamorphosis in H. armigera. RNA interference and pharmacological experiments confirmed that tyrosine hydroxylase (TH), dopa decarboxylase, vesicular amine transporter and DA receptor 2 are critical genes related to the development of H. armigera from larvae to pupae. We also found that miR-14 and miR-2766 targeted the 3' untranslated region to post-transcriptionally regulate HaTH function. Application of miR-2766 and miR-14 antagomirs significantly increased levels of HaTH transcripts and proteins, while injection of miR-2766 and miR-14 agomirs not only suppressed messenger RNA and protein levels of HaTH, but also resulted in defective pupation in H. armigera.

CONCLUSION

These results suggest that DA deficiency inhibits larval-pupal metamorphosis in H. armigera. Potentially, DA pathway genes and their microRNAs could be used as a novel target for H. armigera management. © 2022 Society of Chemical Industry.

A fat body transcriptome analysis of the immune responses of Rhodnius prolixus to artificial infections with bacteria

BACKGROUND

Rhodnius prolixus is an important vector of Trypanosoma cruzi, the causal agent of Chagas disease in humans. Despite the medical importance of this and other triatomine vectors, the study of their immune responses has been limited to a few molecular pathways and processes. Insect immunity studies were first described for holometabolous insects such as Drosophila melanogaster, and it was assumed that their immune responses were conserved in all insects. However, study of the immune responses of triatomines and other hemimetabolous insects has revealed discrepancies between these and the Drosophila model.

METHODS

To expand our understanding of innate immune responses of triatomines to pathogens, we injected fifth instar nymphs of R. prolixus with the Gram-negative (Gr-) bacterium Enterobacter cloacae, the Gram-positive (Gr+) bacterium Staphylococcus aureus, or phosphate-buffered saline (PBS), and evaluated transcript expression in the fat body 8 and 24 h post-injection (hpi). We analyzed the differential expression of transcripts at each time point, and across time, for each treatment.

RESULTS

At 8 hpi, the Gr- bacteria-injected group had a large number of differentially expressed (DE) transcripts, and most of the changes in transcript expression were maintained at 24 hpi. In the Gr+ bacteria treatment, few DE transcripts were detected at 8 hpi, but a large number of transcripts were DE at 24 hpi. Unexpectedly, the PBS control also had a large number of DE transcripts at 24 hpi. Very few DE transcripts were common to the different treatments and time points, indicating a high specificity of the immune responses of R. prolixus to different pathogens. Antimicrobial peptides known to be induced by the immune deficiency pathway were induced upon Gr- bacterial infection. Many transcripts of genes from the Toll pathway that are thought to participate in responses to Gr+ bacteria and fungi were induced by both bacteria and PBS treatment. Pathogen recognition receptors and serine protease cascade transcripts were also overexpressed after Gr- bacteria and PBS injections. Gr- injection also upregulated transcripts involved in the metabolism of tyrosine, a major substrate involved in the melanotic encapsulation response to pathogens.

CONCLUSIONS

These results reveal time-dependent pathogen-specific regulation of immune responses in triatomines, and hint at strong interactions between the immune deficiency and Toll pathways.

Comparison of gene expression profiles among caste differentiations in the termite Reticulitermes speratus

Termite castes express specialized phenotypes for their own tasks and are a good example of insect polyphenism. To understand the comprehensive gene expression profiles during caste differentiation, RNA-seq analysis based on the genome data was performed during the worker, presoldier, and nymphoid molts in Reticulitermes speratus. In this species, artificial induction methods for each molt have already been established, and the time scale has been clarified. Three different periods (before the gut purge (GP), during the GP, and after the molt) were discriminated in each molt, and two body parts (head and other body regions) were separately sampled. The results revealed that many differentially expressed genes (head: 2884, body: 2579) were identified in each molt. Based on the independent real-time quantitative PCR analysis, we confirmed the different expression patterns of seven out of eight genes in the presoldier molt. Based on the GO and KEGG enrichment analyses, the expressions of genes related to juvenile hormone titer changes (e.g., JH acid methyltransferase), nutrition status (e.g., Acyl-CoA Delta desaturase), and cell proliferation (e.g., insulin receptor), were shown to specifically fluctuate in each molt. These differences may have a crucial impact on caste differentiation. These data are important resources for future termite sociogenomics.

AA15 lytic polysaccharide monooxygenase is required for efficient chitinous cuticle turnover during insect molting

Microbial lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of crystalline polysaccharides including chitin and cellulose. The discovery of a large assortment of LPMO-like proteins widely distributed in insect genomes suggests that they could be involved in assisting chitin degradation in the exoskeleton, tracheae and peritrophic matrix during development. However, the physiological functions of insect LPMO-like proteins are still undetermined. To investigate the functions of insect LPMO15 subgroup I-like proteins (LPMO15-1s), two evolutionarily distant species, Tribolium castaneum and Locusta migratoria, were chosen. Depletion by RNAi of T. castaneum TcLPMO15-1 caused molting arrest at all developmental stages, whereas depletion of the L. migratoria LmLPMO15-1, prevented only adult eclosion. In both species, LPMO15-1-deficient animals were unable to shed their exuviae and died. TEM analysis revealed failure of turnover of the chitinous cuticle, which is critical for completion of molting. Purified recombinant LPMO15-1-like protein from Ostrinia furnacalis (rOfLPMO15-1) exhibited oxidative cleavage activity and substrate preference for chitin. These results reveal the physiological importance of catalytically active LPMO15-1-like proteins from distant insect species and provide new insight into the enzymatic mechanism of cuticular chitin turnover during molting.

Superoxide dismutase 6 is required during metamorphosis for the development of properly movable legs in Tribolium castaneum

The body form of holometabolous insects dramatically transforms from larval to adult stages during metamorphosis that occurs in the pupal stage. The larval disorganization and then new adult tissues are built up at this time. In motoneuron, larval neuronal cells degenerate, and new adult neurons are remodeled. Finally, adult neurons reconnect to new adult muscles. However, the factors that control metamorphosis have not yet been fully elucidated. Here, we show that an antioxidant enzyme, Tribolium castaneum superoxide dismutase 6 (TcSOD6), is secreted into the haemolymph and is required for proper movable legs during metamorphosis. TcSOD6 has a unique domain architecture and is mainly expressed in the pupal stage. The depletion of TcSOD6 expression in the pupa inhibits normal axon development and results in adults that display dysfunctional leg motions, suggesting that SOD6 expression is required for the development of properly movable legs. Therefore, we speculate that TcSOD6 might participate in some of the processes for larval neurons to be remodelled to new adult functions in the legs during metamorphosis, providing new insight into the evolution of SOD functions.

Atypical strategies for cuticle pigmentation in the blood-feeding hemipteran Rhodnius prolixus

Pigmentation in insects has been linked to mate selection and predator evasion, thus representing an important aspect for natural selection. Insect body color is classically associated to the activity of tyrosine pathway enzymes, and eye color to pigment synthesis through the tryptophan and guanine pathways, and their transport by ABC proteins. Among the hemiptera, the genetic basis for pigmentation in kissing bugs such as Rhodnius prolixus, that transmit Chagas disease to humans, has not been addressed. Here we report the functional analysis of R. prolixus eye and cuticle pigmentation genes. Consistent with data for most insect clades, we show that knockdown for yellow results in a yellow cuticle, while scarlet and cinnabar knockdowns display red eyes as well as cuticle phenotypes. In addition, tyrosine pathway aaNATpreto knockdown resulted in a striking dark cuticle that displays no color pattern or UV reflectance. In contrast, knockdown of ebony and tan, that encode NBAD branch tyrosine pathway enzymes, did not generate the expected dark and light brown phenotypes, respectively, as reported for other insects. We hypothesize that R. prolixus, which requires tyrosine pathway enzymes for detoxification from the blood diet, evolved an unusual strategy for cuticle pigmentation based on the preferential use of a color erasing function of the aaNATpreto tyrosine pathway branch. We also show that genes classically involved in the generation and transport of eye pigments regulate red body color in R. prolixus. This is the first systematic approach to identify the genes responsible for the generation of color in a blood-feeding hemiptera, providing potential visible markers for future transgenesis.

Silencing of Adc and Ebony Causes Abnormal Darkening of Cuticle in Henosepilachna vigintioctopunctata

N-β-alanyldopamine (NBAD) is a precursor of N-acylquinone sclerotin utilized for cross-linking between cuticular proteins for cuticle during insect molting. The importance of NBAD in cuticle tanning has not been well compared among different developing stages of insects. Henosepilachna vigintioctopunctata, a typical polyphagous pest feeding on a large number of Solanaceae and Cucurbitaceae plants in Asian countries, displays diverse cuticle pigmentation patterns among developing stages and body regions. Here, we found that the expression of three genes (Hvadc, Hvebony, and Hvtan) involved in NBAD biosynthesis peaked in the 4-day-old pupae or 0-day-old adults of H. vigintioctopunctata. At the first, second, third, and fourth larval instar and pupal stage, their transcript levels were high just before and/or right after the molting. Moreover, they were more abundantly transcribed at the larval heads than in the bodies. RNA interference (RNAi) of either Hvadc or Hvebony at the third instar larvae selectively deepened the color of the larval head capsules, antennae, mouthpart, scoli, strumae, and legs; and depletion of the two genes blackened the pupal head capsules, antennae, mouthpart, and legs. However, the knockdown of either Hvadc or Hvebony darkened the whole bodies of the adults. Conversely, RNAi of Hvtan at the third instar stage had little influence on the pigmentation in the larvae, pupae, and adults. These findings demonstrated that Adc and Ebony are important in cuticle pigmentation of H. vigintioctopunctata and suggested that larger quantities of NBAD were present in adults and play more important roles in pigmentation than larvae/pupae.

Drosophila yellow-h encodes dopaminechrome tautomerase: A new enzyme in the eumelanin biosynthetic pathway

Melanin is a widely distributed phenolic pigment that is biosynthesized from tyrosine and its hydroxylated product, dopa, in all animals. However, recent studies reveal a significant deviation from this paradigm, as insects appear to use dopamine rather than dopa as the major precursor of melanin. This observation calls for a reconsideration of the insect melanogenic pathway. While phenoloxidases and laccases can oxidize dopamine for dopaminechrome production, the fate of dopaminechrome remains undetermined. Dopachrome decarboxylase/tautomerase, encoded by yellow-f/f2 of Drosophila melanogaster, can convert dopaminechrome into 5,6-dihydroxyindole, but the same enzyme from other organisms does not act on dopaminechrome, suggesting the existence of a specific dopaminechrome tautomerase (DPT). We now report the identification of this novel enzyme that biosynthesizes 5,6-dihydroxyindole from dopaminechrome in Drosophila. Dopaminechrome tautomerase acted on both dopaminechrome and N-methyl dopaminechrome but not on dopachrome or other aminochromes tested. Our biochemical and molecular studies reveal that this enzyme is encoded by the yellow-h gene, a member of the yellow gene family, and advance our understanding of the physiological functions of this gene family. Identification and characterization of DPT clarifies the precursor for melanin biosynthetic pathways and proves the existence of an independent melanogenic pathway in insects that utilizes dopamine as the primary precursor.

Nuclear receptor HR96 up-regulates cytochrome P450 for insecticide detoxification in Tribolium castaneum

BACKGROUND

Red flour beetle, Tribolium castaneum (T. castaneum), is a major agricultural pest that causes significant damage to stored grains and products. Although hormone receptor 96 (HR96) is known to be the single ortholog corresponding to mammalian constitutive androstane receptor and pregnane X receptor, the structural features of Tribolium HR96 (TcHR96) and its role in insecticide-mediated transcription control of cytochrome P450 enzyme genes in T. castaneum have not been elucidated yet.

RESULTS

We cloned full-length complementary DNA encoding TcHR96 and revealed the role of TcHR96 in transcriptional control of cytochrome P450 enzyme genes. Interestingly, genome-wide transcriptome analysis of HR96-deficient beetles using RNA sequencing showed a positive correlation between TcHR96 and gene transcription of metabolizing enzymes involved in phase I detoxification processes. Moreover, TcHR96 overexpression significantly increased the promoter activity of genes encoding phase I P450 enzymes such as CYP4Q4, CYP4G7, CYP4BR3, and CYP345A1. Chromatin immunoprecipitation analysis showed that TcHR96 could directly bind to the promoter of gene encoding CYP345A1, an enzyme for metabolizing insecticides in T. castaneum. Furthermore, imidacloprid, a neonicotinoid insecticide, significantly increased gene expression of phase I P450 enzymes in old larvae of T. castaneum, which were reversed by TcHR96 knockdown. Finally, TcHR96 knockdown significantly decreased the resistance of old larvae to imidacloprid concomitant with reduction of imidacloprid-mediated phase I P450 enzyme gene expression.

CONCLUSION

TcHR96 plays a major role in transcriptional control of P450 enzyme for imidacloprid detoxification. Controlling TcHR96 might facilitate the regulation of insecticide tolerance in T. castaneum, thus providing a promising new strategy to manage pest beetle populations. © 2021 Society of Chemical Industry.

Expression profiles of tyrosine metabolic pathway genes and functional analysis of DOPA decarboxylase in puparium tanning of Bactrocera dorsalis (Hendel)

BACKGROUND

Tanning is an important physiological process with critical roles in cuticle pigmentation and sclerotization. Previous studies have shown that insect cuticle tanning is closely associated with the tyrosine metabolism pathway, which consists of a series of enzymes.

RESULTS

In this study, 24 tyrosine metabolism pathway genes were identified in the oriental fruit fly Bactrocera dorsalis (Hendel) genome. Gene expression profiles throughout 15 developmental stages of B. dorsalis were established based on our previous RNA sequencing data, and we found that 13 enzyme genes could be involved in the process of pupariation. Accordingly, a tyrosine-mediated tanning pathway during the pupariation of B. dorsalis was predicted and a critical enzyme, 3,4-dihydroxyphenylalanine (DOPA) decarboxylase (DDC), was used to explore its possible roles in formation of the puparium. First, a real-time quantitative polymerase chain reaction confirmed that BdDDC had an epidermis-specific expression pattern, and was highly expressed during larval metamorphosis in B. dorsalis. Subsequent disruption of BdDDC by feeding 5-day-old larvae with DDC inhibitor (l-α-methyl-DOPA) could lead to: (i) a significant decrease in BdDDC enzyme activity and dopamine concentration; (ii) defects in puparium pigmentation; (iii) impairment of the morphology and less thickness of the puparium; and (iv) lower pupal weight and obstacles to eclosion.

CONCLUSION

This study provided a potential tyrosine metabolic pathway that was responsible for insect tanning during pupariation, and the BdDDC enzyme has been shown to have crucial roles in larval-pupal tanning of B. dorsalis. © 2021 Society of Chemical Industry.

Aspartate-β-alanine-NBAD pathway regulates pupal melanin pigmentation plasticity of ladybird Harmonia axyridis

Phenotypic plasticity is observed in many animal species and it is effective for them to cope with many types of environmental threats. The multicolored Asian ladybird Harmonia axyridis shows a cuticular pigmentation plasticity that can be rapidly induced by temperature changes, and in the form of changeable melanin spot patterns to adjust heat-absorbing. Here, H. axyridis with thermal stimulation were selected for determining the molecular regulations behind it. First, we confirmed the melanin level changes of H. axyridis pupa could be induced by temperature, and then screened the efficient time window for thermal sensing of H. axyridis pre-pupa; it is suggested that the late stage of pre-pupa (late stage of 4th instar larva) is the critical period to sense thermal signals and adjust its pupal melanin spot area size to adapt to upcoming thermal conditions. The Ha-ADC (aspartate decarboxylase) and Ha-ebony (NBAD synthase) of aspartate-β-alanine-NBAD pathway were then proved in regulation of cuticular melanization for pupa through RNA interference experiments; knockdown of these two genes enlarged the melanin spot size. Finally, we designed a random injection of Ha-ADC at different pre-pupal stages, to further study the regulation window during this process. Combined with all evidence observed, we suggested the spot size determination can be regulated very close to the time point of pupation, and genes of the aspartate-β-alanine-NBAD pathway play an important role at the molecular level. In brief, H. axyridis exhibits a flexible active physiological regulation through transcriptional modification to thermal changes.

The Identification of Boll Weevil, Anthonomus grandis grandis (Coleoptera: Curculionidae), Genes Involved in Pheromone Production and Pheromone Biosynthesis

Simple Summary

The boll weevil is a destructive pest of commercial cotton throughout the Americas. An eradication program in the United States has removed the boll weevil from most of its range. However, weevil populations in South Texas remain a threat to eradicated areas. Pheromone traps are used to monitor boll weevil activity, and when a weevil is captured, eradication programs rely on malathion for control. However, the effectiveness of pheromone traps in detecting incipient boll weevil populations is reduced during certain times of the year. Additionally, human safety and environmental concerns, as well as the potential development of malathion-resistant populations, have prompted program managers to seek alternative control methods. We sequenced and compared pheromone-producing and non-pheromone-producing weevils to identify genes involved in pheromone production, which, in turn, could be an environmentally friendly way to target gene-level pest control that is specific to the boll weevil. Our results revealed genes involved in pheromone production, as well as insect development and immunity, which may be targeted for boll weevil suppression.

Abstract

Eradication programs for the boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), rely almost exclusively on pheromone traps to indicate the need for insecticide applications. However, the effectiveness of traps in detecting weevil populations is reduced during certain times of the year, particularly when cotton is actively fruiting. Consequently, this could result in fields becoming heavily infested with weevils. It is widely speculated that the lack of weevil captures in traps during this period is largely due to the overwhelming amount of pheromone released by weevils in the field, which outcompete the pheromone released from traps. Thus, this work sought to identify genes involved in pheromone production so that new control methods that target these genes can be explored. We conducted an RNA-seq experiment that revealed 2479 differentially expressed genes between pheromone-producing and non-pheromone-producing boll weevils. Of those genes, 1234 were up-regulated, and 1515 were down-regulated, and most had gene annotations associated with pheromone production, development, or immunity. This work advances our understanding of boll weevil pheromone production and brings us one step closer to developing gene-level control strategies for this cotton pest.

RNA-seq analysis of gene expression changes in cuticles during the larval-pupal metamorphosis of Plutella xylostella

The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) is a holometabolous insect that its cuticles must undergo the significant changes during the larval-pupal metamorphosis development. To elucidate these changes at molecular levels, RNA-seq analysis of cuticles from LLS (later fourth instar larval stage), PPS (prepupal stage) and PS (pupal stage) were performed in P. xylostella. In this paper, a total of 17,710 transcripts were obtained in the larval-pupal transition of P. xylostella, and out of which 2293 (881 up-regulated and 1412 down-regulated) and 2989 transcripts (2062 up-regulated and 927 down-regulated) were identified to be differentially expressed between LLS and PPS, as well as PPS and PS, respectively. The further GO and KEGG analysis of differentially expressed genes (DEGs) revealed that the 'structural constituent of cuticle', 'chitin metabolic process', 'chitin binding', 'tyrosine metabolism' and 'insect hormone biosynthesis' pathways were significantly enriched, indicating these pathways might be involved in the process of larval pupation in P. xylostella. Then, we found some genes that encoded cuticular proteins, chitinolytic enzymes, chitin synthesis enzymes, and cuticle tanning proteins changed their expression levels remarkably, indicating these genes might play important roles in the restruction (degradation and biosynthesis) of insect cuticles during the larval metamorphosis. Additionally, the significant changes in the mRNA levels of 20-hydroxyecdysone (20E) and juvenile hormone (JH) related genes suggested their crucial roles in regulating cuticle remodeling during the larval metamorphosis of P. xylostella. In conclusion, the present study provide us the comprehensive gene expression profiles to explore the molecular mechanisms of cuticle metamorphosis in P. xylostella, which laid a molecular basis to study roles of specific pathways and genes in insect development.

Identification of Oxygen-Independent Pathways for Pyridine Nucleotide and Coenzyme A Synthesis in Anaerobic Fungi by Expression of Candidate Genes in Yeast

Neocallimastigomycetes are unique examples of strictly anaerobic eukaryotes. This study investigates how these anaerobic fungi bypass reactions involved in synthesis of pyridine nucleotide cofactors and coenzyme A that, in canonical fungal pathways, require molecular oxygen. Analysis of Neocallimastigomycetes proteomes identified a candidate l-aspartate-decarboxylase (AdcA) and l-aspartate oxidase (NadB) and quinolinate synthase (NadA), constituting putative oxygen-independent bypasses for coenzyme A synthesis and pyridine nucleotide cofactor synthesis. The corresponding gene sequences indicated acquisition by ancient horizontal gene transfer (HGT) events involving bacterial donors. To test whether these enzymes suffice to bypass corresponding oxygen-requiring reactions, they were introduced into fms1Δ and bna2Δ Saccharomyces cerevisiae strains. Expression of nadA and nadB from Piromyces finnis and adcA from Neocallimastix californiae conferred cofactor prototrophy under aerobic and anaerobic conditions. This study simulates how HGT can drive eukaryotic adaptation to anaerobiosis and provides a basis for elimination of auxotrophic requirements in anaerobic industrial applications of yeasts and fungi. IMPORTANCE NAD (NAD⁺) and coenzyme A (CoA) are central metabolic cofactors whose canonical biosynthesis pathways in fungi require oxygen. Anaerobic gut fungi of the Neocallimastigomycota phylum are unique eukaryotic organisms that adapted to anoxic environments. Analysis of Neocallimastigomycota genomes revealed that these fungi might have developed oxygen-independent biosynthetic pathways for NAD⁺ and CoA biosynthesis, likely acquired through horizontal gene transfer (HGT) from prokaryotic donors. We confirmed functionality of these putative pathways under anaerobic conditions by heterologous expression in the yeast Saccharomyces cerevisiae. This approach, combined with sequence comparison, offers experimental insight on whether HGT events were required and/or sufficient for acquiring new traits. Moreover, our results demonstrate an engineering strategy for enabling S. cerevisiae to grow anaerobically in the absence of the precursor molecules pantothenate and nicotinate, thereby contributing to alleviate oxygen requirements and to move closer to prototrophic anaerobic growth of this industrially relevant yeast.

Regional patterning and regulation of melanin pigmentation in insects

Insects display an immense diversity in melanin pigmentation, which is generated by the interplay between the regulatory genes (that provide general patterning information) and effector genes (that provide coloration of the pattern). However, recent studies encompassing several different orders (Hemiptera, Blattodea, Coleoptera, and Lepidoptera) have shown that knockdowns of melanin producing genes alone can generate distinct region-specific patterns. This review surveys the most recent studies to further document the regional patterning of effector genes, and highlights the new advances and their implications for future research.

Immune function differences between two color morphs of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) at different life stages

Several studies demonstrated that in insects cuticle melanism is interrelated with pathogen resistance, as melanin-based coloration and innate immunity possess similar physiological pathways. For some insects, higher pathogen resistance was observed in darker individuals than in individuals with lighter cuticular coloration. Here, we investigated the difference in immune response between two color morphs (black and red) and between the life stages (pupa and adult) of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae). Here in this study, cuticle thickness, microbial test (antimicrobial activity, phenoloxidase activity, and hemocyte density), and immune-related gene expression were evaluated at different stages of RPW. Study results revealed that cuticle thickness of black phenotype was thicker than red phenotype at old-pupa stage, while no significant difference found at adult stage. These results may relate to the development processes of epidermis in different stages of RPW. The results of antimicrobial activity, phenoloxidase (PO) activity, and hemocyte density analyses showed that adults with a red phenotype had stronger pathogen resistance than those with a black phenotype. In addition to antimicrobial activity and PO activity, we tested relative gene expression in the fat body of old pupae. The results of hemolymph antimicrobial analysis showed that old pupae with a red phenotype were significantly different from those with a black phenotype at 12 hr after Staphylococcus aureus injection, suggesting that red phenotype pupae were more sensitive to S. aureus. Examination of gene expression in the fat body also revealed that the red phenotype had a higher immune response than the black phenotype. Our results were inconsistent with the previous conclusion that dark insects had increased immune function, suggesting that the relationship between cuticle pigmentation and immune function in insects was not a direct link. Additional possible factors that are associated with the immune response, such as life-history, developmental, physiological factors also need to be considered.

Lepidopteran wing scales contain abundant cross-linked film-forming histidine-rich cuticular proteins

Scales are symbolic characteristic of Lepidoptera; however, nothing is known about the contribution of cuticular proteins (CPs) to the complex patterning of lepidopteran scales. This is because scales are resistant to solubilization, thus hindering molecular studies. Here we succeeded in dissolving developing wing scales from Bombyx mori, allowing analysis of their protein composition. We identified a distinctive class of histidine rich (His-rich) CPs (6%-45%) from developing lepidopteran scales by LC-MS/MS. Functional studies using RNAi revealed CPs with different histidine content play distinct and critical roles in constructing the microstructure of the scale surface. Moreover, we successfully synthesized films in vitro by crosslinking a 45% His-rich CP (BmorCPR152) with laccase2 using N-acetyl- dopamine or N-β-alanyl-dopamine as the substrate. This molecular study of scales provides fundamental information about how such a fine microstructure is constructed and insights into the potential application of CPs as new biomaterials.

Advances in laccase-triggered anabolism for biotechnology applications

This is the first comprehensive overview of laccase-triggered anabolism from fundamental theory to biotechnology applications. Laccase is a typical biological oxidordeuctase that induces the one-electronic transfer of diverse substrates for engendering four phenoxy radicals with concomitant reduction of O₂ into 2H₂O. In vivo, laccase can participate in anabolic processes to create multifarious functional biopolymers such as fungal pigments, plant lignins, and insect cuticles, using mono/polyphenols and their derivatives as enzymatic substrates, and is thus conducive to biological tissue morphogenesis and global carbon storage. Exhilaratingly, fungal laccase has high redox potential (E° = 500-800 mV) and thermodynamic efficiency, making it a remarkable candidate for utilization as a versatile catalyst in the green and circular economy. This review elaborates the anabolic mechanisms of laccase in initiating the polymerization of natural phenolic compounds and their derivatives in vivo via radical-based self/cross-coupling. Information is also presented on laccase immobilization engineering that expands the practical application ranges of laccase in biotechnology by improving the enzymatic catalytic activity, stability, and reuse rate. Particularly, advances in biotechnology applications in vitro through fungal laccase-triggered macromolecular biosynthesis may provide a key research direction beneficial to the rational design of green chemistry.

RNA interference-mediated functional characterization of Group I chitin deacetylases in Holotrichia parallela Motschulsky

Chitin deacetylases (CDAs, EC 3.5.1.41) catalyze the N-deacetylation of chitin to produce chitosan, which is essential for insect survival. Hence, CDAs are promising targets for the development of novel insecticidal drugs. In this study, the putative Group I chitin deacetylase genes HpCDA1, HpCDA2-1 and HpCDA2-2 were identified from Holotrichia parallela. Conserved domain database search identified a chitin-binding peritrophin-A domain (ChBD), a low-density lipoprotein receptor class A domain (LDLa), and a putative CDA-like catalytic domain. RT-qPCR analysis showed that the Group I HpCDAs were expressed in various tissues and predominant in the integument. The developmental expression patterns from the first-instar to third-instar larvae showed that HpCDAs were highly expressed on the first day and gradually declined after molting. The functional characteristics of the Group I CDAs in cuticle organization were examined using RNA interference (RNAi) and transmission electron microscopy (TEM) methods. Administration of double-stranded HpCDA (dsHpCDA) through larval injection could suppress the expression levels of HpCDA1 and HpCDA2, thus resulting in abnormal or lethal phenotypes. TEM analysis revealed that RNAi of either HpCDA1 or HpCDA2 remarkably affected the cuticle integrity, as evidenced by cuticle disorganization and chitin laminae disruption, suggesting the crucial role of CDAs in chitin modification. These experimental results demonstrate the important contribution of putative key genes involved in chitin metabolism, and provide a foundation for developing new strategies to control H. parallela.

Proteomic and transcriptomic analyses of saliva and salivary glands from the Asian citrus psyllid, Diaphorina citri

Salivary secretions play critical roles in interactions among insects, insect-vectored pathogens, and host plants. The Asian citrus psyllid Diaphorina citri is a sap-sucking Hemipteran that serves as a vector for Candidatus Liberibacter asiaticus, the causal agent of citrus greening disease ("Huanglongbing" or HLB). D. citri continuously injects saliva into host plants using specialized stylets so as to feed and transmit the HLB pathogen. Knowledge on the composition and function of salivary proteins of this pest is very limited. In this study, proteomic and transcriptomic approaches were adopted to characterize the protein composition of the saliva and salivary glands in D. citri. A total of 246 and 483 proteins were identified in saliva and dissected salivary glands, respectively, via LC-MS/MS analyses. Comparative analyses of the identified proteins were performed between D. citri and other reported Hemipteran insect species. Transcription levels of the genes coding for the identified proteins were determined via RNA-sequencing among different tissues including salivary glands and other digestive tissues. Identification of putative effectors that are expressed exclusively or abundantly in salivary glands provides the foundation for future functional studies towards the understanding of their roles in interactions among D. citri, HLB pathogen, and their citrus host. BIOLOGICAL SIGNIFICANCE: This is a systematic analysis on proteins in saliva and dissected salivary glands. A high percentage of novel proteins have been identified due to the large amounts of samples collected. This report gives a more comprehensive repertoire of potential effector proteins that may be possibly involved in modulating host defense, altering nutrient metabolism, and facilitating Ca. L. asiaticus transmission.

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.

Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-Associated Protein 9 Mediated Knockout Reveals Functions of the yellow-y Gene in Spodoptera litura

Yellow genes are thought to be involved in the melanin biosynthetic pathway and play a crucial role in pigmentation reactions in insects. However, little research has been done on yellow genes in lepidopteran pests. To clarify the function of one of the yellow genes (yellow-y) in Spodoptera litura, we cloned the full-length of yellow-y, and investigated its spatial and temporal expression profiles by quantitative real-time PCR (qPCR). It revealed that yellow-y was highly expressed in larva of fourth, fifth, and sixth instars, as well as in epidermis (Ep), fat bodies (FB), Malpighian tubes (MT), and midguts (MG) of the larvae; whereas it was expressed in very low levels in different tissues of adults, and was almost undetected in pupa. This expression profile suggests an important role of yellow-y in larvae, minor role in adults, and no role in pupae. To confirm this, we disrupted yellow-y using the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system, and obtained G0 insects with mutation in yellow-y. The mutation in yellow-y clearly rendered the larvae body, a color yellower than that of wide type insects, and in addition, the mutation resulted in abnormal segmentation and molting for older larvae. The mutation of yellow-y also made various adult tissues (antennae, proboscis, legs, and wings) yellowish. However, the mutation had no effect on pigmentation of the pupal cuticle. Taken together, our study clearly demonstrated the role of yellow-y not only in the body pigmentation of larvae and adults, and but also in segmentation and molting of larvae, providing new insights into the physiology of larval development, as well as a useful marker gene for genome editing based studies.

Micromorphological details and identification of chitinous wall structures in Rapana venosa (Gastropoda, Mollusca) egg capsules

The present study evaluated the structural and ultrastructural characteristics of Rapana venosa egg capsules, starting from observations of their antifouling activity and mechanical resistance to water currents in mid-shore habitats. Optical microscopy, epifluorescence, and electron microscopy were used to evaluate the surface and structure of the R. venosa egg capsules. These measurements revealed an internal multilamellar structure of the capsule wall with in-plane distributions of layers with various orientations. It was found that the walls contained vacuolar structures in the median layer, which provided the particular characteristics. Mechanical, viscoelastic and swelling measurements were also carried out. This study revealed the presence and distribution of chitosan in the capsule of R. venosa. Chitosan identification in the egg capsule wall structure was carried out through SEM-EDX measurements, colorimetric assays, FT-IR spectra and physical-chemical tests. The biopolymer presence in the capsule walls may explain the properties of their surfaces as well as the mechanical resistance of the capsule and its resistance to chemical variations in the living environment.

MicroRNA-277 regulates dopa decarboxylase to control larval-pupal and pupal-adult metamorphosis of Helicoverpa armigera

Insect metamorphosis is a complex process involving many metabolic pathways, such as juvenile hormones and molting hormones, bioamines, microRNAs (miRNAs), etc. However, relatively little is known about the biogenic amines and their miRNAs to regulate cotton bollworm metamorphosis. Here we show that one miRNA, miR-277 regulates larval-pupal and pupal-adult metamorphosis of cotton bollworm by targeting the 3'UTR of Dopa decarboxylase (DDC), a synthetic catalytic enzyme of dopamine. Injection of miR-277 agomir inhibited the expression of DDC at the mRNA and protein levels, leading to defects in the pupation and emergence of H. armigera that was consistent with the phenotype obtained by injection of DDC double-stranded RNA (dsRNA). Injection of miR-277 antagomir induced the mRNA and protein expression of DDC and rescued the phenotype of pupation failure caused by DDC gene silencing. Unexpectedly, miR-277 antagomir can also cause failure of emergence of H. armigera and both agomir and antagomir of miR-277 injection could cause abnormal phenotypes in wing veins. This study reveals that elaborate regulation of miRNA and its target gene expression is prerequisite for insect development, which provides a new insight to study the developmental mechanisms of insect wing veins.

Yellow-g and Yellow-g2 proteins are required for egg desiccation resistance and temporal pigmentation in the Asian tiger mosquito, Aedes albopictus

Eggs from Aedes mosquitoes exhibit desiccation resistance that helps them to survive and spread as human disease vectors throughout the world. Previous studies have suggested that eggshell/chorion melanization and/or serosal cuticle formation are important for desiccation resistance. In this study, using dsRNAs for target genes, we analyzed the functional importance of two ovary-specific yellow genes, AalY-g and AalY-g2, in the resistance to egg desiccation of the Asian tiger mosquito, Aedes albopictus, a species in which neither the timing of the melanization nor temporal development of the serosal cuticle is correlated with desiccation resistance. Injections of dsAalY-g, dsAalY-g2 or dsAalY-g/g2 (co-injection) into adult females have no effect on their fecundity. However, initial melanization is delayed by 1-2 h with the eggshells eventually becoming black similar to that observed in eggs from dsEGFP-injected control females. In addition, the shape of the eggs from dsAalY-g, -g2 and -g/g2-treated females is abnormally crescent-shaped and the outermost exochorion is more fragile and partially peeled off. dsEGFP control eggs, like those from the wild-type strain, acquire resistance to desiccation between 18 and 24 h after oviposition (HAO). In contrast, ~80% of the 24 HAO dsAalY-g and dsAalY-g2 eggs collapse when they are transferred to a low humidity environment. In addition, there is no electron-dense outer endochorion evident in either dsAalY-g or dsAalY-g2 eggs. These results support the hypothesis that AalY-g and AalY-g2 regulate the timing of eggshell darkening and are required for integrity of the exochorion as well as for rigidity, normal morphology and formation of the outer endochorion, a structure that apparently is critical for desiccation resistance of the Ae. albopictus egg.