Circadian clock regulates developmental time through ecdysone and juvenile hormones in Bombyx mori

Knockdown of farnesyl pyrophosphate synthetase gene TuFPPS by RNAi extends quiescent period and halts molting of the spider mite, Tetranychus urticae

In insects, farnesyl pyrophosphate synthase (FPPS), a pivotal enzyme in the mevalonate pathway, is crucial in the juvenile hormone (JH)/methyl farnesoate (MF) biosynthesis pathway, particularly regulating the molting process. However, its role in the post-embryonic development and molting in Acari remains unclear. This study therefore investigated the function of TuFPPS in the deutonymphal stage of the two-spotted spider mite, Tetranychus urticae, an important pest in agriculture. Real time quantitative PCR (qPCR) revealed a significant increase in TuFPPS expression in the middle of the quiescent period (28 h). RNA interference (RNAi)-mediated knockdown of TuFPPS caused developmental delays, notably extending the quiescent period and delaying the molting process. Ultimately, the treated mites failed to molt and died with 80% mortality at 108 h before molting, while > 90% of the mites in the control successfully molted. To investigate the potential functions of TuFPPS during the quiescent period and molting process in T. urticae, we performed a genome-wide RNA sequencing following RNAi knockdown of TuFPPS. Four genes were identified based on the thresholds of|FC| ≥ 3 and FDR < 0.05, and these are involved in carbohydrate metabolism, fatty acid and lipid metabolism, amino acid metabolism, and ABC transporters. Altogether, our data suggest that TuFPPS plays a critical role in regulating energy transport, intracellular digestion, and energy supply, as seen in ametabolous insects. These findings lay the foundation for further studies of the molecular mechanism of JH/MF in post-embryonic development and molting in mites, and also identify a potential target for pest control, as demonstrated in T. urticae.

The circadian clock affects starvation resistance through the pentose phosphate pathway in silkworm, Bombyx mori

Disruption of the circadian clock can affect starvation resistance, but the molecular mechanism is still unclear. Here, we found that starvation resistance was significantly reduced in the core gene BmPer deficient mutant silkworms (Per-/-), but the mutant's starvation resistance increased with larval age. Under natural physiological conditions, the weight of mutant 5th instar larvae was significantly increased compared to wild type, and the accumulation ability of triglycerides and glycogen in the fat bodies was upregulated. However, under starvation conditions, the weight consumption of mutant larvae was increased and cholesterol utilization was intensified. Transcriptome analysis showed that beta-oxidation was significantly upregulated under starvation conditions, fatty acid synthesis was inhibited, and the expression levels of genes related to mitochondrial function were significantly changed. Further investigations revealed that the redox balance, which is closely related to mitochondrial metabolism, was altered in the fat bodies, the antioxidant level was increased, and the pentose phosphate pathway, the source of reducing power in cells, was activated. Our findings suggest that one of the reasons for the increased energy burden observed in mutants is the need to maintain a more robust redox balance in metabolic tissues. This necessitates the diversion of more glucose into the pentose phosphate pathway to ensure an adequate supply of reducing power.

Enhanced silk production and pupal weight in Bombyx mori through CRISPR/Cas9-mediated circadian Clock gene disruption

The domesticated silkworm, Bombyx mori, is crucial for global silk production, which is a significant economic activity supporting millions of livelihoods worldwide. Beyond traditional silk production, the growing demand for insect larvae in cosmetics, biomedical products, and animal feed underscores the need to enhance B. mori productivity. This study investigates the role of the circadian clock gene Clock in B. mori using CRISPR/Cas9-mediated mutagenesis to establish the ClkΔ29 knock-out mutant strain. Dysregulation of the circadian clock in ClkΔ29 was demonstrated by altered temporal transcriptional profiles of core circadian clock genes in adult heads and disrupted circadian-controlled behaviors, including adult eclosion and egg hatching rhythms under constant darkness. By analysing larval development timing, as well as the weights of late instar larvae, pupae, and cocoon components in ClkΔ29 mutants and in ClkΔ1922 silkworms (carrying an independently generated Clk- null allele), we showed that CLK contributes to physiological processes regulating B. mori development and growth. Importantly, ClkΔ29 mutants reared on a standard sericulture diet exhibited significant increases in key economic traits, with silk production increasing by up to 7%, and pupal weight increasing by up to 25% compared to wild-type controls. This study highlights the potential of circadian clock gene manipulation to significantly enhance sericultural productivity. Future research should focus on elucidating the molecular mechanisms driving these phenotypes and determining whether they result from circadian clock functions or pleiotropic effects of B. mori Clk. These findings provide a foundation for advancing sustainable sericulture and developing new commercial applications for silkworm-derived products.

The moulting arthropod: a complete genetic toolkit review

Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as protection from the environment and predation. This ubiquitous yet evolutionarily variable feature has been instrumental in facilitating the adoption of a variety of lifestyles and the exploitation of ecological niches across all environments. Throughout the radiation that produced the more than one million described modern species, adaptability afforded by segmentation and exoskeletons has led to a diversity that is unrivalled amongst animals. However, because of the limited extensibility of exoskeleton chitin and cuticle components, they must be periodically shed and replaced with new larger ones, notably to accommodate the growing individuals encased within. Therefore, arthropods grow discontinuously by undergoing periodic moulting events, which follow a series of steps from the preparatory pre-moult phase to ecdysis itself and post-moult maturation of new exoskeletons. Each event represents a particularly vulnerable period in an arthropod's life cycle, so processes must be tightly regulated and meticulously executed to ensure successful transitions for normal growth and development. Decades of research in representative arthropods provide a foundation of understanding of the mechanisms involved. Building on this, studies continue to develop and test hypotheses on the presence and function of molecular components, including neuropeptides, hormones, and receptors, as well as the so-called early, late, and fate genes, across arthropod diversity. Here, we review the literature to develop a comprehensive overview of the status of accumulated knowledge of the genetic toolkit governing arthropod moulting. From biosynthesis and regulation of ecdysteroid and sesquiterpenoid hormones, to factors involved in hormonal stimulation responses and exoskeleton remodelling, we identify commonalities and differences, as well as highlighting major knowledge gaps, across arthropod groups. We examine the available evidence supporting current models of how components operate together to prepare for, execute, and recover from ecdysis, comparing reports from Chelicerata, Myriapoda, Crustacea, and Hexapoda. Evidence is generally highly taxonomically imbalanced, with most reports based on insect study systems. Biases are also evident in research on different moulting phases and processes, with the early triggers and late effectors generally being the least well explored. Our synthesis contrasts knowledge based on reported observations with reasonably plausible assumptions given current taxonomic sampling, and exposes weak assumptions or major gaps that need addressing. Encouragingly, advances in genomics are driving a diversification of tractable study systems by facilitating the cataloguing of putative genetic toolkits in previously under-explored taxa. Analysis of genome and transcriptome data supported by experimental investigations have validated the presence of an "ultra-conserved" core of arthropod genes involved in moulting processes. The molecular machinery has likely evolved with elaborations on this conserved pathway backbone, but more taxonomic exploration is needed to characterise lineage-specific changes and novelties. Furthermore, linking these to transformative innovations in moulting processes across Arthropoda remains hampered by knowledge gaps and hypotheses based on untested assumptions. Promisingly however, emerging from the synthesis is a framework that highlights research avenues from the underlying genetics to the dynamic molecular biology through to the complex physiology of moulting.

Effects of pyriproxyfen on development and hormone of the aphis, Aphis craccivora (Hemiptera: Aphididae)

Pyriproxyfen (PPF) has been shown to affect the pupal stage and ecdysone levels in holometabolous insects, such as silkworms and mealworms. It remains unknown whether it affects hemimetabolous insects with their hormone levels in insects lacking a pupal stage. In this laboratory study, bioassays were conducted to investigate the effects of varying doses of PPF on Aphis craccivora Koch (Hemiptera: Aphididae). Ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was used to determine the types and titers of juvenile hormone (JH) and 20-hydroxyecdysone (20E). Additionally, the effects of PPF on A. craccivora reproduction and molting, as well as its influence on relevant gene expression, were examined. The results revealed LC50 and LC90 values of 3.84 and 7.49 mg/l for PPF, respectively, after 48 h of exposure. The results demonstrated a significant reduction in the titer of JH III and a significant increase in the titer of 20E following treatment with PPF. However, there was no significant decrease observed in the titer of JH III skipped bisepoxide (JH SB3). A sublethal concentration of PPF was found to inhibit Krüppel homolog 1 (kr-h1) gene expression and reduce aphid reproduction, but it did not significantly impact ecdysone receptor expression and aphid molting. The results of this study demonstrate that PPF exhibits a lethal effect on aphids, thereby providing an effective means of control. Additionally, sublethal concentrations of PPF have been found to inhibit the JH in aphids, resulting in a decline in their reproductive ability and achieving the desired control objectives.

Significance of the clock gene period in photoperiodism in larval development and production of diapause eggs in the silkworm Bombyx mori

Many insects living in seasonal environments sense seasonal changes from photoperiod and appropriately regulate their development and physiological activities. Genetic researches have indicated the importance of a circadian clock system in photoperiodic time-measurement for photoperiodic regulations. However, most previous studies have focused on the effects on a single photoperiodic phenotype, without elucidating whether the circadian clock is involved in the core photoperiodic mechanism or only in the production of one target phenotype, such as diapause. Here, we focused on two different phenotypes in a bivoltine Kosetsu strain of the silkworm Bombyx mori, namely, embryonic diapause and larval development, and examined their photoperiodic responses and relationship to the circadian clock gene period. Photoperiod during the larval stage clearly influenced the induction of embryonic diapause and duration of larval development in the Kosetsu strain; short-day exposure leaded to the production of diapause eggs and shortened the larval duration. Genetic knockout of period inhibited the short-day-induced embryonic diapause. Conversely, in the period-knockout silkworms, the larval duration was shortened, but the photoperiodic difference was maintained. In conclusion, our results indicate that the period gene is not causally involved in the photoperiodic response of larval development, while that is essential for the short-day-induced embryonic diapause.

Insights on reproduction-related genes in the striped fruit fly, Zeugodacus scutellata (Hendel) (Diptera: Tephritidae)

The striped fruit fly, Zeugodacus scutellata is a significant pest in East and Southeast Asia by damaging Cucurbitaceae blossoms and fruits. To control this pest, a novel strategy to suppress the gene(s) associated with sexually dimorphic phenotypes has been devised and implemented in a laboratory scale. However, comprehensive transcriptomic analysis related to this sex differentiation of Z. scutellata was necessary to determine effective target genes for the genetic control. We performed de novo assembly of the transcript obtained by paired-end sequencing using an Illumina HiSeq platform and let to 217,967 unigenes (i.e., unique genes) with a minimum length of 200 bp. The female produced 31, 604, 442 reads with 97.93% of Q20, 94.76% of Q30, and the male produced 130, 592, 828 reads with 97.93% of Q20 and 94.76 of Q30%. The differentially expressed genes were used to predict genetic factors associated with sex differentiation, which included Rho1, extra-macrochaetae (emc), hopscotch (hop), doublesex (dsx), sex-lethal (sxl), transformer-2 (tra-2), testis-specific serine/threonine-protein kinase (tssk1), tektin1 (tkt1) and 2 (tkt2), odorant binding proteins (OBPs), fruitless (fru), vitellogenin receptor, and hormone receptors in Z. scutellata. In addition, this transcriptome analysis provides the additional gene associated with sex determination and mating behaviors, which would be applied to develop a novel sterile insect technique against Z. scutellata.

The clock gene Cryptochrome 1 is involved in the photoresponse of embryonic hatching behavior in Bombyx mori

The hatching of insect eggs is a classic circadian behavior rhythm controlled by the biological clock. Its function is considered to impose a daily rhythm on the embryo, allowing it to hatch within a permissible time window. However, the molecular pathways through which the clock affects embryonic hatching behavior remain unclear. Here, we utilized a clock gene Cryptochrome1 (Cry1) knockout mutant to dissect the pathways by which the circadian clock affects embryonic hatching rhythm in the silkworm. In the Cry1 mutant, the embryo hatching rhythm was disrupted. Under the constant light or constant dark incubation conditions, mutant embryos lost their hatching rhythm, while wild-type embryos hatch exhibiting free-running rhythm. In the light-dark cycle (LD), the hatching rhythm of CRY1-deficient silkworms could not be entrained by the LD photoperiod during the incubation period. The messenger RNA levels and enzymatic activities of Cht and Hel in the mutant embryos were significantly reduced at circadian time 24 (CT24). Transcriptome analysis revealed significant differences in gene expression at CT24 between the Cry1 knockout mutant and the wild-type, with 2616 differentially expressed genes identified. The enriched Gene Ontology pathway includes enzyme activity, energy availability, and protein translation. Short neuropeptide F signaling was reduced in the CT24 embryonic brain of the mutant, the expression of the neuropeptide PTTH was also reduced and the rhythm was lost, which further affects ecdysteroid signaling. Our results suggested that the silkworm circadian clock affects neuropeptide-hormone signaling as well as physiological functions related to hatching, which may regulate the hatching rhythm.