Identification and expression analysis of diapause hormone and pheromone biosynthesis activating neuropeptide (DH-PBAN) in the legume pod borer, Maruca vitrata Fabricius

Molecular and Functional Characterization of Pyrokinin-Like Peptides in the Western Tarnished Plant Bug Lygus hesperus (Hemiptera: Miridae)

Simple Summary

Neuropeptides regulate most insect biological functions. One such group of peptides, the pyrokinins (PKs), are distinguished by a C-terminal FXPRLamide. While widely distributed in most insects, they are poorly characterized in plant bugs. To address this limitation, we identified the PK transcript in the western tarnished plant bug (Lygus hesperus) and examined its expression. The Lygus PK transcript is predicted to yield three PK-like peptides but only two (LyghePKa and LyghePKb) have the characteristic C-terminal amide. The transcript is expressed throughout development and is most abundant in heads. A custom FXPRLamide antibody revealed immunoreactive cells throughout the Lygus central nervous system consistent with typical neuropeptide expression. To assess potential functional roles of the peptides, a fluorescence-based Ca²⁺ influx assay using cultured insect cells stably expressing a moth PK receptor was performed. LyghePKa was unable to stimulate receptor activation, whereas LyghePKb triggered a robust response. The in vivo pheromonotropic activity of the two peptides was likewise assessed using three different moth species. Like the cell culture system, only the LyghePKb peptide was active. The study suggests evolutionary divergence of the PK gene in plant bugs and provides critical insights into likely biological functions in the western tarnished plant bug.

Abstract

The pyrokinin (PK) family of insect neuropeptides, characterized by C termini consisting of either WFGPRLamide (i.e., PK1) or FXPRLamide (i.e., PK2), are encoded on the capa and pk genes. Although implicated in diverse biological functions, characterization of PKs in hemipteran pests has been largely limited to genomic, transcriptomic, and/or peptidomic datasets. The Lygus hesperus (western tarnished plant bug) PK transcript encodes a prepropeptide predicted to yield three PK2 FXPRLamide-like peptides with C-terminal sequences characterized by FQPRSamide (LyghePKa), FAPRLamide (LyghePKb), and a non-amidated YSPRF. The transcript is expressed throughout L. hesperus development with greatest abundance in adult heads. PRXamide-like immunoreactivity, which recognizes both pk- and capa-derived peptides, is localized to cells in the cerebral ganglia, gnathal ganglia/suboesophageal ganglion, thoracic ganglia, and abdominal ganglia. Immunoreactivity in the abdominal ganglia is largely consistent with capa-derived peptide expression, whereas the atypical fourth pair of immunoreactive cells may reflect pk-based expression. In vitro activation of a PK receptor heterologously expressed in cultured insect cells was only observed in response to LyghePKb, while no effects were observed with LyghePKa. Similarly, in vivo pheromonotropic effects were only observed following LyghePKb injections. Comparison of PK2 prepropeptides from multiple hemipterans suggests mirid-specific diversification of the pk gene.

Physiological Alterations in Deletion Mutants of Two Insulin-Like Peptides Encoded in Maruca vitrata Using CRISPR/Cas9

Most insect species encode multiple insulin-like peptides (ILPs) that exhibit functional overlaps in mediating physiological processes such as development and reproduction. Why do they need multiple ILPs? To address this question, we tested a hypothesis of the requirement of multiple ILPs by generating mutants lacking individual ILP genes using the CRISPR/Cas9 technology. Two ILPs (ILP1 and ILP2) in the legume pod borer, Maruca vitrata, mediate similar physiological processes such as hemolymph sugar level, larval development, and adult reproduction. Individual knock-out mutants (ΔILP1 and ΔILP2) were generated. They showed successful development from larvae to adults. However, they suffered from high hemolymph sugar levels by enhancing trehalose titers in the hemolymph. The hyperglycemic effect was more evident in ΔILP2 mutants than in ΔILP1 mutants. Both mutants showed increased expression of trehalose-6-phosphate synthase but suppressed expression of trehalase. These mutants also showed altered expression patterns of insulin signaling components. Expression levels of insulin receptor and Akt genes were upregulated, while those of FOXO and Target of rapamycin genes were downregulated in these mutants. These alterations of signal components resulted in significant retardation of immature development and reduced body sizes. ΔILP1 or ΔILP2 females exhibited poor oocyte development. Bromo-uridine incorporation was much reduced at the germarium of ovarioles of these mutants compared with wild females. Expression of the vitellogenin gene was also reduced in these mutants. Furthermore, males of these deletion mutants showed impaired reproductive activities when they mated with wild-type females. These results suggest that both ILPs are required for mediating larval development and adult reproduction in M. vitrata.

Diapause hormone directly stimulates the prothoracic glands of diapause larvae under juvenile hormone regulation in the bamboo borer, Omphisa fuscidentalis Hampson

Larval diapause in many lepidopteran insects is induced and maintained by high juvenile hormone (JH). In the case of the bamboo borer, Omphisa fuscidentalis, the effect of JH is the opposite: The application of juvenile hormone analog (JHA: S-methoprene) terminates larval diapause, unlike in other insect species. Here, we analyzed the expression of JH-receptor Met, DH-PBAN, and Kr-h1 in the subesophageal ganglion (SG) from October to April using semi-quantitative polymerase chain reaction (PCR). The results show that OfMet and OfDH-PBAN messenger RNA in the SG are mainly expressed during the larval diapause stage, while OfKr-h1 increases during the pupal stage. Using tissue culture techniques and an enzyme-linked immunosorbent assay (ELISA), diapause hormone (DH) was found to induce ecdysteroidogenesis in the culture medium of the prothoracic gland (PG) after incubation for 30 min with 25 ng and 50 ng of DH. Thus, DH is a novel stimulator for the PG. We identified a DHR homolog in the bamboo borer and confirmed that it is expressed in the PG. In addition, for in vitro experiments, DH increased the expression levels of OfDHR, OfEcR-A, and ecdysone-inducible genes in the PG. These results demonstrate that DH can function as a prothoracicotropic factor, and this function of DH might be through of DHR expressed on PG cells. Consequently, DH is one of the key factors in larval diapause break which is triggered by JH in the bamboo borer, O. fuscidentalis.

Pheromone-binding proteins based phylogenetics and phylogeography of Maruca spp. from Asia, Africa, Oceania, and South America

Variations in the functional response of legume pod borer (Maruca vitrata) populations to sex pheromone blends were observed in Asia and Africa. Hence, this study was carried out to understand the differences in pheromone-binding proteins (PBPs) among Maruca populations in Asia, Africa, Oceania, and South America. A de novo transcriptome assembly was adopted to sequence the entire transcribed mRNAs in M. vitrata from Taiwan. The raw-sequence data were assembled using homologous genes from related organisms in GenBank to detect M. vitrata PBPs (MvitPBPs). Sections of the cDNA of MvitPBP of different length were used to design primers to amplify the full-length cDNA of PBPs. All three PBP sequences comprised three exons interspersed by two introns. In total, 92 MvitPBP1 haplotypes, 77 MvitPBP2 haplotypes, and 64 MvitPBP3 haplotypes were identified in 105, 98, and 68 Maruca individuals, respectively. High pairwise F ST values (0.41-0.73) and phylogenetic analyses distinguished the putative Maruca species in South America from those occurring in rest of the world, and possibly two putative subspecies in Asia and Africa. The haplotype networks and Automatic Barcode Gap Discovery analyses also confirmed these results. The negative Tajima's D and Fu's F S values showed the recent demographic expansion of Maruca populations. Thus, this study confirmed the presence of different Maruca species and/or subspecies in different continents based on the diversity within PBP genes. Additional sampling and studies are suggested for Oceania and South America. The genetic differences among Maruca populations should be carefully considered while using sex pheromone lures and bio-control agents.

Insulin signaling mediates previtellogenic development and enhances juvenile hormone-mediated vitellogenesis in a lepidopteran insect, Maruca vitrata

BACKGROUND

Insulin/insulin-like growth peptide signaling (IIS) down-regulates hemolymph sugar level and facilitates larval growth in the soybean pod borer, Maruca vitrata. The objective of this study is to determine whether IIS of M. vitrata can mediate ovarian development of adult females.

RESULTS

A pair of ovaries consists of 8 ovarioles, each of which is separated into distal germarium and proximal vitellarium in M. vitrata. In the germarium, oocyte development occurred with active mitotic activity which was visible by incorporating bromodeoxyribose uridine. Previtellogenic development and subsequent vitellogenesis began soon after adult emergence. They continued with increase of female age. Oocyte development was facilitated by up-regulation of vitellogenin (Vg) and Vg receptor (VgR) gene expression. Larval diets significantly influenced on ovarian development of M. vitrata because oocyte development varied with pupal size derived from larvae treated with different nutritional diets. Its ovarian development was dependent on endocrine signal(s) from the head because decapitation soon after adult emergence prevented oogenesis and subsequent vitellogenesis along with marked reduction of Vg and VgR expression. Topical application of juvenile hormone (JH) significantly recovered its ovarian development whereas farnesoic acid (a precursor of JH biosynthesis) or 20-hydroxyecdysone treatment did not. JH stimulated vitellogenesis and choriogenesis, but not previtellogenic development. In contrast, insulin injection to decapitated females stimulated oocyte differentiation and vitellogenesis along with increase of Vg and VgR expression. To further analyze the effect of insulin on ovarian development, expression of four IIS components (InR, FOXO, Akt, and TOR) genes was manipulated by RNA interference. Hemocoelic injection of gene-specific double stranded RNAs significantly reduced their target gene mRNA levels and interfered with ovarian development. An addition of insulin to JH treatment against decapitated females enhanced the gonadotropic effect of JH by stimulating oogenesis.

CONCLUSIONS

IIS plays crucial role in mediating previtellogenic development of M. vitrata in response to nutrient signal. It also enhances the gonadotropic effect of JH II on vitellogenesis.

Sex-specific spatial and temporal gene expressions of Pheromone biosynthesis activating neuropeptide (PBAN) and binding proteins (PBP/OBP) in Spoladea recurvalis

Spoladea recurvalis is one of the most destructive insect pests of amaranth, a leafy vegetable in both Asia and Africa. The present study characterized the pheromone biosynthesis-activating neuropeptide (DH-PBAN) and pheromone/odorant binding proteins in S. recurvalis. The open reading frame of 600 base pairs encodes a 200-amino acid protein possessing five neuropeptide motifs (DH, PBAN, α-, β-, and γ- subesophageal ganglion neuropeptides) and shares a characteristic conserved C-terminal pentapeptide fragment FXPRL. The full-length genome of Spre-DH-PBAN was 4,295 bp in length and comprised of six exons interspersed by five introns. Sequence homology and phylogenetic analysis of Spre-DH-PBAN have high similarity to its homologs in Crambidae of Lepidopteran order. We quantitatively measured the relative expression level (qRT_PCR) of Spre-DH-PBAN gene, the binding proteins such as odorant binding proteins (OBPs) and pheromone binding protein (PBPs) at different developmental stages. The results confirmed their role in recognition and chemoreception of sex pheromone components, and they were distinct, tissue- and sex-specific. This is the first report on the molecular analysis of PBAN gene and binding proteins, which can improve the understanding of molecular mechanisms of growth, development, and reproductive behavior of S. recurvalis, and may become effective targets for controlling this insect.

Insulin-like peptides of the legume pod borer, Maruca vitrata, and their mediation effects on hemolymph trehalose level, larval development, and adult reproduction

Insulin-like peptides (ILPs) of insects mediate various physiological processes including hemolymph sugar level, immature growth, female reproduction, and lifespan. In target cells of ILPs, insulin/insulin-like growth factor signaling (IIS) is highly conserved in animals. IIS in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), is known to be involved in maintaining hemolymph trehalose levels and promoting larval growth. However, ILPs in M. vitrata have not been reported yet. This study predicted two ILP genes of Mv-ILP1 and Mv-ILP2 from transcriptome of M. vitrata. Mv-ILP1 and Mv-ILP2 shared high sequence homologies and domain architecture with Drosophila ILPs. Both ILPs exhibited similar expression patterns in most developmental stages, showing high expression levels in adult stage. In the larval stage, Mv-ILP1 and Mv-IlP2 were expressed mostly in the brain and fat body. However, in the adult stage, both ILP genes were expressed more in the abdomen than those in the head containing the brain. RNA interference (RNAi) of either Mv-ILP1 or Mv-ILP2 during larval stage resulted in significant malfunctioning in regulating hemolymph trehalose titers. RNAi-treated larvae also exhibited significant retardation of larval growth. RNAi treatment in adult stage interfered with the ovarian development of females. These results suggest that Mv-ILP1 and Mv-ILP2 play crucial roles in mediating larval growth and adult reproduction.

Transcriptome analysis in the beet webworm, Spoladea recurvalis (Lepidoptera: Crambidae)

The beet webworm, Spoladea recurvalis Fabricius, is a destructive pest on vegetable crops in tropics and subtropics; its main host plant is amaranth. It has become imperative to develop non-chemical methods to control S. recurvalis on amaranth. However, the lack of molecular information about this species has hindered the development of novel pest management strategies. In this study, high-throughput RNA sequencing covering de novo sequence assemblies, functional annotation of transcripts, gene function classification and enrichment was performed on S. recurvalis. Illumina sequencing generated a total of 120 435 transcript contigs ranging from 201 to 22 729 bases with a mean length of 688 bases. The assembled transcripts were subjected to Basic Local Alignment Search Tool-X (BLASTX) to obtain the annotations against non-redundant, Swiss-Prot, Clusters of Orthologous Groups (COG) and Kyoto Encyclopedia of Genes and Genomes (KEGG) protein databases. A subset of 58 225 transcript sequences returned hits from known proteins in the National Center for Biotechnology Information database, and the majority of the transcript sequences had the highest number of hits for Danaus plexippus (50.43%). A total of 1217 Gene Ontology-level 3 annotations were assigned to 51 805 transcripts, while 39 650 transcripts were predicted as functional protein-coding genes in the COG database and 20 037 transcripts were enriched to KEGG pathways. We identified 40 putative genes related to pheromone production and reception in S. recurvalis, with the expression of one gene between 0.29 and 1141.79 fragments per kilo base per million (FPKM) reads. The transcriptome sequence of S. recurvalis is a first step toward offering a comprehensive genomic resource which would enable better understanding of molecular mechanisms to enable development of effective pest management practices for this species.

Identification and expression profiling of pheromone biosynthesis activating neuropeptide in Chlumetia transversa (Walker)

Insect neuropeptides (NPs) in the pyrokinin/pheromone biosynthesis-activating neuropeptide (PBAN) family are actively involved in many essential endocrine functions. These peptides are potential targets in the search for novel insect control agents. This is the first report on the cloning and sequence determination of Chlumetia transversa (Walker) PBAN (Ct-PBAN) using rapid amplification of cDNA ends. The open reading frame of Ct-PBAN was 588bp in length and encoded 195 amino acids, which were assembled into five putative neuropeptides (diapause hormone homolog, α-neuropeptide, β-neuropeptide, PBAN, and γ-neuropeptide). These peptides were amidated at C-terminus and shared the conserved pentapeptide motif FXPR (or K) L. Moreover, Ct-PBAN had high homology to PBANs in Helicoverpa zea (84.1%), Helicoverpa armigera (83.5%), Helicoverpa assulta (83%), and Heliothis virescens (82.6%). Phylogenetic analysis showed that Ct-PBAN was closely related to its orthologs in the family Noctuidae. In addition, real-time quantitative polymerase chain reaction assays showed that the expression of Ct-PBAN peaked in the female head and was also detected at high levels in 1-d-old adults. These results suggested that Ct-PBAN is associated with sex pheromone biosynthesis in female C. transversa and could be used for developing C. transversa control systems based on molecular techniques.

Characterization and cytotoxic activity of apoptosis-inducing pierisin-5 protein from white cabbage butterfly

In this study, caspase-dependent apoptosis-inducing pierisin-5 gene was identified and characterized from cabbage white butterfly, Pieris canidia. A thousand-fold increase in expression of pierisin-5 gene was observed from second to third instar larvae, gradually decreasing before pupation. Pierisin-5 was purified from the fifth-instar larvae and was found to exhibit cytotoxicity against HeLa and HepG2 human cancer cell lines. Pierisin-5 showed growth inhibition and several morphological changes such as cell shrinkage, chromatin condensation and apoptotic body formation with programmed cell death in HeLa and HepG2 cells. Moreover, DNA fragmentation was observed after gel electrophoresis analysis. Caspase substrate assay showed further cleavage of Ac-DEVD-pNA, suggesting the activation of Caspase-3. Flow cytometry analysis revealed the cell cycle arrest at G1 phase and increased the percentage of apoptotic cells in cancer cell lines treated with pierisin-5. These findings suggest that pierisin-5 could significantly induce apoptosis in cancer cell lines and is mediated by activation of caspase-3 in the mitochondrial pathway. Phylogenetic analysis using pierisin proteins from Pierid butterflies, ADP-ribosylating toxins from bacteria, human, rat, and mouse indicated the possibility of horizontal transfer of pierisin genes from bacteria to butterflies. The single copy of pierisin gene unlike other insect toxin genes also supports lateral transfer.

Identification and expression of a new member of the pyrokinin/pban gene family in the sand fly Phlebotomus papatasi

The major family of neuropeptides (NPs) derived from the pk (pyrokinin)/pban (pheromone biosynthesis activating neuropeptide) gene are defined by a common FXPRL-NH2 or similar sequence at the C-termini. This family of peptides has been found in all insect groups investigated to date and is implicated in regulating various physiological functions, including pheromone biosynthesis and diapause, but other functions are still largely unknown in specific life stages. Here we identify two isoforms of pk/pban cDNA encoding the PBAN domain from the sand fly Phlebotomus papatasi. The two pk/pban isoforms have the same sequence except for a 63 nucleotide difference between the long and short forms, and contain no alternative mRNA splicing site. Two NP homologues, DASGDNGSDSQRTRPPFAPRLamide and SLPFSPRLamide are expected, however, sequence corresponding to the diapause hormone was not found in the P. papatasi pk/pban gene. The PBAN-like amino acid sequence homologue SNKYMTPRL is conserved in the gene, but there is no cleavage site for processing a functional peptide. Characterizing the expression of the isoforms in developmental stages and adults indicates that the short form is differentially transcribed depending on the life stage. The P. papatasi pk/pban gene is the only known pk/pban gene with two transcriptional isoforms and from examination of endoproteolytic cleavage sites is expected to produce fewer peptides than most of the pk/pban genes elucidated to date; only Drosophila melanogaster is simpler with a single NP detected by mass spectroscopy. A phylogenetic analysis showed P. papatasi pk/pban grouped more closely with other nematoceran flies rather than higher flies.

Identification and RNA Interference of the Pheromone Biosynthesis Activating Neuropeptide (PBAN) in the Common Cutworm Moth Spodoptera litura (Lepidoptera: Noctuidae)

Spodoptera litura F. is one of the most destructive insect pests of many agricultural crops and notorious for developing insecticide resistance. Developing environmental friendly control methods such as novel pheromone and RNAi-related control strategies is imperative to control this pest. In the present study, the full-length cDNA encoding the diapause hormone and pheromone biosynthesis activating neuropeptide (DH-PBAN) was identified and characterized in S. litura. This 809-bp transcript contains a 573-nucleotide ORF encoding a 191-amino acid protein, from which five putative neuropeptides, including PBAN, DH, and α-, β-, and γ-subesophageal ganglion neuropeptides, were derived. Phylogenetic analysis showed that both the whole protein and each of the five neuropeptides have high similarities to those of DH-PBANs from other insect orders particularly Lepidoptera. Females treated with TKYFSPRLamide (the active core fragment of PBAN) produced significantly more four types of pheromone compounds (A; B; C; D) than controls. RNA interference by injection of PBAN dsRNA significantly reduced the relative expression levels of this gene in adult females (approximately reduced by 60%). As a consequence, females treated with PBAN dsRNA produced significantly less four types of pheromone compounds (A; B; C; D) than controls. These results suggest that PBAN function in activating sex pheromone biosynthesis and the RNAi of DH-PBAN gene can be induced by the injection of dsRNA into the body cavity in S. litura. This study suggests the possibility of novel pheromone-related pest control strategies based on RNAi techniques.