Studies of sex pheromone production under neuroendocrine control by analytical and morphological means in the oriental armyworm, Pseudaletia separata, Walker (Lepidoptera: Noctuidae)

The Mythimna separata general odorant binding protein 2 (MsepGOBP2) is involved in the larval detection of the sex pheromone (Z)-11-hexadecenal

BACKGROUND

Mythimna separata is a notorious pest causing crop damages at the larval stage. Gaining insight into larval olfaction mechanisms would provide knowledge for olfaction-based management of M. separata larvae.

RESULTS

In the present research, (Z)-11-hexadecenal (Z11-16: Ald), a major component of M. separata sex pheromone, was found to attract early-instar larvae of M. separata in a food context. Using a fluorescent binding assay, we found that M. separata general odorant binding protein 2 (MsepGOBP2) exhibited high binding affinity to Z11-16: Ald. Further, silencing of MsepGOBP2 resulted in a sharp reduction of the response to Z11-16: Ald, which could not be mitigated by increasing the concentration of Z11-16: Ald. Additionally, we employed molecular dynamics-based approaches to unravel the interaction details between MsepGOBP2 and Z11-16: Ald, specifically the binding of Z11-16: Ald to MsepGOBP2.

CONCLUSION

Z11-16: Ald is attractive to early-instar larvae of M. separata, and MsepGOBP2 is identified to be indispensable in the larval detection of Z11-16: Ald. These results could aid in the development of olfaction-based methods for controlling M. separata in the larval stage. © 2023 Society of Chemical Industry.

Redundant actions of neuropeptides encoded by the dh-pban gene for larval color pattern formation in the oriental armyworm Mythimnaseparata

The pyrokinin (PK)/pheromone biosynthesis-activating neuropeptide (PBAN) family, which is defined by a conserved C-terminal pentapeptide (FXPRLamide), is involved in many physiological processes in insects. In the oriental armyworm Mythimna separata, the larvae exhibit a variety of color patterns in response to changes in population density, which are caused by melanization and a reddish coloration hormone (MRCH), which is a member of the FXPRLamide neuropeptides. Interestingly, in some lepidopteran insects, MRCH is known as a PBAN, which activates the pheromone gland to produce sex pheromones. PBAN is encoded by a single gene, dh-pban, which encodes additional FXPRLamide neuropeptides, such as the diapause hormone (DH) and subesophageal ganglion neuropeptides (SGNPs). To determine the roles of the dh-pban gene, which produces multiple types of FXPRLamide neuropeptides after post-transcriptional cleavage of the precursor protein, we performed CRISPR/Cas9-mediated targeted mutagenesis in M. separata. We demonstrated that knockout armyworm larvae lost density-dependent cuticular melanization and retained yellow body color, even when reared under crowded conditions. Moreover, our rescue experiments using the synthetic peptides showed that not only PBAN but also β- and γ-SGNPs significantly induce the cuticular melanization in a dose dependent manner. Taken together, our results provide genetic evidence that neuropeptides encoded by the single dh-pban gene act redundantly to control density-dependent color pattern formation in M. separata.

Lipid droplets in the pheromone glands of bombycids: Effects of larval diet on their size and pheromone titer

In addition to the blend ratio, the quantity of sex pheromone components secreted by female moths may affect the efficient attraction of conspecific males. The present study using the silkmoth Bombyx mori, which has bombykol as its pheromone component, demonstrated that pheromone titer, body weight, and lipid droplet (LD) diameter in the pheromone gland were affected by the larval diet. Although the artificial diet contained approximately 11-fold more total fatty acids than mulberry leaf, the pheromone titer in the group fed the artificial diet (group AD) was approximately 2-fold higher than that of the group fed mulberry (group M). The diameter of LDs, which store the pheromone-precursor fatty acyl, E10,Z12-16:Acyl, was also larger in the AD group. The relatively small increase in sex pheromone titer by feeding on a fatty-acid-rich diet may be partly attributable to the storage of excess precursors in the LDs. We detected LDs in the pheromone glands of Trilocha varians, the closest non-congener of B. mori available in Bombycidae. T. varians uses bombykal and bombykyl acetate as sex pheromone components, which are biosynthesized via the same precursor fatty acyl as that of B. mori. The presence of LDs in T. varians suggests that the storage and mobilization mechanisms of the pheromone precursor fatty acyl via LDs may be conserved in bombycids.

Functional Characterization of Sex Pheromone Neurons and Receptors in the Armyworm, Mythimna separata (Walker)

Pheromone receptors (PRs) of moths are expressed on the dendritic membrane of odorant receptor neurons (ORNs) housed in the long trichoid sensilla (TS) of antennae and are essential to sex pheromone reception. The function of peripheral neurons of Mythimna separata in recognizing sex pheromones is still unclear. In this study, electroantennogram recordings were performed from male and female antennae of M. separata, and showed that the major component of sex pheromones, (Z)-11-hexadecenal (Z11-16:Ald), evoked the strongest response of male antennae with significant differences between sexes. Single sensillum recording was used to record responses of neurons housed in TS of male M. separata. The results revealed four types of TS with three neurons housed in each type, based on profiles of responses to sex pheromone components and pheromone analogs. ORN-B of type-I TS was specifically tuned to the major sex pheromone component Z11-16:Ald; ORN-Bs in type-III and type-IV TSs were, respectively, activated by minor components (Z)-11-hexadecen-1-yl acetate (Z11-16:OAc) and hexadecenal (16:Ald); and ORNs in type-II TS were mainly activated by the sex pheromone analogs. We further cloned full-length sequences of six putative PR genes and an Orco gene. Functional characterization of PRs in the Xenopus oocyte system demonstrated that male antennae-biased MsepPR1 responded strongly to (Z)-9-tetradecenal (Z9-14:Ald), suggesting that MsepPR1 may be expressed in type-II TS. MsepPR6 was exclusively tuned to (Z)-9-tetradecen-1-yl acetate (Z9-14:OAc). MsepPR2 and MsepPR4 showed no responses to any tested components. Female antennae-biased MespPR5 was broadly tuned to Z9-14:Ald, Z9-14:OAc, Z11-16:Ald, and (Z)-11-hexadecen-1-ol (Z11-16:OH). Our results further enriched the sex pheromone recognition mechanism in the peripheral nervous system of moth M. separata.

Dissecting sex pheromone communication of Mythimna separata (Walker) in North China from receptor molecules and antennal lobes to behavior

The Oriental armyworm, Mythimna separata, has been described to emit geographic population-specific sex pheromones, with either Z11-16:Ald or Z11-16:Ac as the major component. Using a comprehensive set of electrophysiological, behavioral, and genetic analyses, we study the sex pheromone communication of M. separata in North China from pheromone receptors and antennal lobe to behavior. GC-EAD results show that Z11-16:Ald is the only compound eliciting electrophysiological responses in pheromone gland extracts. Further in vivo optical imaging studies reveal that Z11-16:Ald activates the cumulus of the MGC and show dose-dependent responses. The wind tunnel tests demonstrate that Z11-16:Ald alone is sufficient to induce the entire sequence of male sexual behaviors. Transcriptome and q-PCR results show that MsepOR3 is specifically and abundantly expressed in male antennae. By using the Xenopus oocytes and two-electrode voltage-clamp recording, we finally validate that the oocytes expressing MsepOR3/ORco gave dose dependent responses to Z11-16:Ald. We suggest single Z11-16:Ald could be used for monitoring the population of M. separata in North China.

The regulation of Δ11-desaturase gene expression in the pheromone gland of Mamestra brassicae (Lepidoptera; Noctuidae) during pheromonogenesis

Cabbage moth (Mamestra brassicae) females produce sex pheromones to attract conspecific males. In our M. brassicae colony, the pheromone blend is composed of Z11-hexadecenyl acetate (Z11-16Ac) and hexadecyl acetate (16Ac) in a 93:7 ratio. A fatty acyl Δ11-desaturase is involved in the production of the main pheromone component. The release of Pheromone Biosynthesis Activating Neuropeptide (PBAN) regulates the pheromone production in the pheromone gland (PG). We cloned a cDNA encoding the MambrΔ11-desaturase and analyzed its expression profile over time in M. brassicae tissues. Transcript levels of the Δ11-desaturase in larvae, pupal PGs, fat body, brain and muscle tissues were <0.1% of that in female PGs, whereas expression in male genitalia was 2%. In the PGs of virgin females the expression level increased continuously from eclosion to the end of the 1st day when it reached a plateau without further significant fluctuation up to the 8th day. In contrast, we recorded a characteristic daily rhythmicity in pheromone production with a maximum around 200 ng Z11-16Ac/PG. In some experiments, females were decapitated to prevent PBAN release and thereby inhibit pheromone production, which remarkably increased after treatment with Mambr-Pheromonotropin. Further experiments revealed that mating resulted in a significant suppression of pheromone production. However, expression of the Δ11-desaturase was not affected by any of these interventions, suggesting that it's not regulated by PBAN. Fluorescent microscopy was used to study the potential role of lipid droplets during pheromone production, however, no lipid droplets were identified indicating that pheromonogenesis is regulated via de novo fatty acid synthesis.

De novo characterization of transcriptome and gene expression dynamics in epidermis during the larval-pupal metamorphosis of common cutworm

Larval cuticle is degraded and replaced by the pupal counterpart during larval-pupal metamorphosis in the holometabolous insects. In addition to the extrinsic transformation, the epidermis goes through significant changes at molecular levels. To elucidate the intrinsic mechanism of epidermal metamorphosis, the dynamics of chitin content in the cuticle was examined in an important agricultural lepidopteran, the common cutworm, and the transcriptome was analyzed using Illumina sequencing technology. Gene expression profiles during the metamorphosis were further studied by both the digital gene expression (DGE) system and real-time quantitative PCR. The results showed that the chitin content decreased in prepupae and then increased in pupae. A total of 58 million sequencing reads were obtained and assembled into 70,346 unigenes. Over 9000 unigenes were identified to express differentially during the transformation process. As compared with the 6th instar feeding larvae, the most significant changes took place in the proteasome and metabolic pathways in prepupae and pupae, respectively. The cytochrome P450s, VHDLs, chitinase, serine protease and genes involved in sex pheromone biosynthesis changed their mRNA levels remarkably. Three chitinolytic enzymes (chitinase, β-N-acetylglucosaminidase and chitin deacetylase) showed distinct mRNA expression patterns, the former two enzymes revealed the highest expression in prepupae, however the latter one showed its climax mRNA level in pupae. The gene expression patterns suggest that chitinase and β-N-acetylglucosaminidase may be responsible for the degradation of larval cuticles, whereas chitin deacetylase may help to degrade the pupal counterparts. Gene expression dynamics also implied that the chitin of pupal cuticle might be formed by recycling of the degraded chitin of larval cuticle rather than through de novo synthesis. The 20E-induced nuclear receptors seem to be important factors regulating chitin metabolic enzymes during the cuticle remodeling. Our data provide a comprehensive resource for exploring the molecular mechanism of epidermal metamorphosis in insects.

Re-Evaluation of the PBAN Receptor Molecule: Characterization of PBANR Variants Expressed in the Pheromone Glands of Moths

Sex pheromone production in most moths is initiated following pheromone biosynthesis activating neuropeptide receptor (PBANR) activation. PBANR was initially cloned from pheromone glands (PGs) of Helicoverpa zea and Bombyx mori. The B. mori PBANR is characterized by a relatively long C-terminus that is essential for ligand-induced internalization, whereas the H. zea PBANR has a shorter C-terminus that lacks features present in the B. mori PBANR critical for internalization. Multiple PBANRs have been reported to be concurrently expressed in the larval CNS of Heliothis virescens. In the current study, we sought to examine the prevalence of multiple PBANRs in the PGs of three moths and to ascertain their potential functional relevance. Multiple PBANR variants (As, A, B, and C) were cloned from the PGs of all species examined with PBANR-C the most highly expressed. Alternative splicing of the C-terminal coding sequence of the PBAN gene gives rise to the variants, which are distinguishable only by the length and composition of their respective C-terminal tails. Transient expression of fluorescent PBANR chimeras in insect cells revealed that PBANR-B and PBANR-C localized exclusively to the cell surface while PBANR-As and PBANR-A exhibited varying degrees of cytosolic localization. Similarly, only the PBANR-B and PBANR-C variants underwent ligand-induced internalization. Taken together, our results suggest that PBANR-C is the principal receptor molecule involved in PBAN signaling regardless of moth species. The high GC content of the C-terminal coding sequence in the B and C variants, which makes amplification using conventional polymerases difficult, likely accounts for previous "preferential" amplification of PBANR-A like receptors from other species.