Circadian Clock Genes Regulate Temperature-Dependent Diapause Induction in Silkworm Bombyx mori

Endocrine and enzymatic shifts during insect diapause: a review of regulatory mechanisms

Insect diapause is a vital survival strategy that enables insects to enter a state of suspended development, allowing them to withstand unfavorable environmental conditions. During diapause, insects significantly lower their metabolic rate and build up energy reserves, which they gradually utilize throughout this period. The regulation of diapause involves a complex interaction of hormones and enzymes. Juvenile hormones (JHs) affect adults and larvae differently; in adults, the absence of JH typically triggers diapause, while in larvae, the presence of JH encourages this state. Ecdysteroids, which regulate molting and metamorphosis, are carefully controlled to prevent premature development. Reduced signaling of insulin-like peptides enhances stress resistance and promotes energy storage. Several enzymes play crucial roles in the metabolic adjustments necessary for diapause. These adjustments include the degradation of JH, the ecdysteroidogenic pathway, and the metabolism of fatty acids, glycogen, cryoprotectants, and stress responses. Understanding diapause's molecular and biochemical mechanisms is essential for fundamental entomological research and practical applications. Despite recent advances, many aspects of diapause regulation, especially the interactions among hormonal pathways and the role of enzymes, remain poorly understood. This review analyzes approximately 250 papers to consolidate current knowledge on the enzymatic and hormonal regulation of diapause. It offers a comprehensive overview of key processes based on recent studies and suggests future research directions to fill gaps in our understanding of this significant biological phenomenon. The review also lays the groundwork for enhancing pest control strategies and ecological conservation by deepening our understanding of diapause mechanisms.

BmSV2A and BmSV2B Are Involved in Regulating GABAergic Neuron-Related Gene Expression in the Silkworm, Bombyx mori

In insects, the number of life cycles varies inter- and intra-specifically, and it is widely accepted that the variation in the number of life cycles is an adaptive response to diverse environmental conditions. However, the molecular mechanism that underlies the variety and plasticity in the number of life cycles is largely unknown. In the silkworm, Bombyx mori, the Voltinism (V) locus has three alleles, V1(univoltine; dominant), V2 (bivoltine; standard), and V3 (polyvoltine; recessive), which are known to generate variation in the number of life cycles in a year under natural conditions, with obligatory diapause for the V1 allele, facultative diapause for V2, and non-diapause for V3. Here, we further confirm that the γ-aminobutyric acid (GABA)ergic neuron signal pathway modulates progeny diapause via controlling diapause hormone release. A population genetic analysis (Fst) revealed that the synaptic vesicle glycoprotein 2A and 2B (BmSV2A and BmSV2B) genes, tightly related to the transport of neurotransmitters, are located in the V locus. Importantly, using the CRISPR/Cas9 editing technique, we have discovered that the BmSV2A and BmSV2B genes increased or modified the expression of GABAergic neuron signal pathway genes, respectively. These results demonstrate that BmSV2A and BmSV2B, positioned within the V locus, could be involved in voltinism control via the GABAergic neuron signal pathway.

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.

Comparative analysis of locomotor behavior and head diurnal transcriptome regulation by PERIOD and CRY2 in the diamondback moth

Earth's rotation shapes a 24-h cycle, governing circadian rhythms in organisms. In mammals, the core clock genes, CLOCK and BMAL1, are regulated by PERIODs (PERs) and CRYPTOCHROMEs (CRYs), but their roles remain unclear in the diamondback moth, Plutella xylostella. To explore this, we studied P. xylostella, which possesses a simplified circadian system compared to mammals. In P. xylostella, we observed rhythmic expressions of the Pxper and Pxcry2 genes in their heads, with differing phases. In vitro experiments revealed that PxCRY2 repressed monarch butterfly CLK:BMAL1 transcriptional activation, while PxPER and other CRY-like proteins did not. However, PxPER showed an inhibitory effect on PxCLK/PxCYCLE. Using CRISPR/Cas9, we individually and in combination knocked out Pxper and Pxcry2, then conducted gene function studies and circadian transcriptome sequencing. Loss of either Pxper or Pxcry2 eliminated the activity peak after lights-off in light-dark cycles, and Pxcry2 loss reduced overall activity. Pxcry2 was crucial for maintaining endogenous rhythms in constant darkness. Under light-dark conditions, 1 098 genes exhibited rhythmic expression in wild-type P. xylostella heads, with 749 relying on Pxper and Pxcry2 for their rhythms. Most core clock genes lost their rhythmicity in Pxper and Pxcry2 mutants, while Pxcry2 sustained rhythmic expression, albeit with reduced amplitude and altered phase. Additionally, rhythmic genes were linked to biological processes like the spliceosome and Toll signaling pathway, with these rhythms depending on Pxper or Pxcry2 function. In summary, our study unveils differences in circadian rhythm regulation by Pxper and Pxcry2 in P. xylostella. This provides a valuable model for understanding circadian clock regulation in nocturnal animals.

Differentially expressed microRNAs in brains of adult females may regulate the maternal block of diapause in Sarcophaga bullata

The maternal regulation of diapause is one type of phenotypic plasticity where the experience of the mother leads to changes in the phenotype of her offspring that impact how well-suited they will be to their future environment. Sarcophaga bullata females with a diapause history produce offspring that cannot enter diapause even if they are reared in a diapause inducing environment. Accumulating evidence suggests that microRNAs regulate diapause and, possibly, maternal regulation of diapause. We found significant differences in the abundances of several microRNAs (miR-125-5p, miR-124-3p, miR-31-5p, and miR-277-3p) in brains dissected from adult female S. bullata that had experienced diapause compared to females with no diapause history. We also found moderate differences in the mRNA expression of the circadian-clock related genes, clock, clockwork orange, and period. MiR-124-3p and miR-31-5p are part of a gene network that includes these circadian clock-related genes. Taken together our results suggest the maternal block of diapause in S. bullata is regulated, at least in part, by a network that includes microRNAs and the circadian clock.

Knockout of cryptochrome 1 disrupts circadian rhythm and photoperiodic diapause induction in the silkworm, Bombyx mori

Most insects enter diapause, a state of physiological dormancy crucial for enduring harsh seasons, with photoperiod serving as the primary cue for its induction, ensuring proper seasonal timing of the process. Although the involvement of the circadian clock in the photoperiodic time measurement has been demonstrated through knockdown or knockout of clock genes, the involvement of clock gene cryptochrome 1 (cry1), which functions as a photoreceptor implicated in photoentrainment of the circadian clock across various insect species, remains unclear. In bivoltine strains of the silkworm, Bombyx mori, embryonic diapause is maternally controlled and affected by environmental conditions experienced by mother moths during embryonic and larval stages. Previous research highlighted the role of core clock genes, including period (per), timeless (tim), Clock (Clk) and cycle (cyc), in photoperiodic diapause induction in B. mori. In this study, we focused on the involvement of cry1 gene in B. mori photoperiodism. Phylogenetic analysis and conserved domain identification confirmed the presence of both Drosophila-type cry (cry1) and mammalian-type cry (cry2) genes in the B. mori genome, akin to other lepidopterans. Temporal expression analysis revealed higher cry1 gene expression during the photophase and lower expression during the scotophase, with knockouts of core clock genes (per, tim, Clk and cyc) disrupting this temporal expression pattern. Using CRISPR/Cas9-mediated genome editing, we established a cry1 knockout strain in p50T, a bivoltine strain exhibiting clear photoperiodism during both embryonic and larval stages. Although the wild-type strain displayed circadian rhythm in eclosion under continuous darkness, the cry1 knockout strain exhibited arrhythmic eclosion, implicating B. mori cry1 in the circadian clock feedback loop governing behavior rhythms. Females of the cry1 knockout strain failed to control photoperiodic diapause induction during both embryonic and larval stages, mirroring the diapause phenotype of the wild-type individuals reared under constant darkness, indicating that B. mori CRY1 contributes to photoperiodic time measurement as a photoreceptor. Furthermore, photoperiodic diapause induction during the larval stage was abolished in a cry1/tim double-knockout strain, suggesting that photic information received by CRY1 is relayed to the circadian clock. Overall, this study represents the first evidence of cry1 involvement in insect photoperiodism, specifically in diapause induction.

Photoperiodism of Diapause Induction in the Silkworm, Bombyx mori

The silkworm Bombyx mori exhibits a photoperiodic response (PR) for embryonic diapause induction. This article provides a comprehensive review of literature on the silkworm PR, starting from early works on population to recent studies uncovering the molecular mechanism. Makita Kogure (1933) conducted extensive research on the PR, presenting a pioneering paper on insect photoperiodism. In the 1970s and 80s, artificial diets were developed, and the influence of nutrition on PR was well documented. The photoperiodic photoreceptor has been investigated from organ to molecular level in the silkworm. Culture experiments demonstrated that the photoperiodic induction can be programmed in an isolated brain (Br)-subesophageal ganglion (SG) complex with corpora cardiaca (CC)-corpora allata (CA). The requirement of dietary vitamin A for PR suggests the involvement of opsin pigment in the photoperiodic reception, and a cDNA encoding an opsin (Boceropsin) was cloned from the brain. The effector system concerning the production and secretion of diapause hormone (DH) has also been extensively investigated in the silkworm. DH is produced in a pair of posterior cells of SG, transported to CC by nervi corporis cardiaci, and ultimately released into the hemolymph. Possible involvement of GABAergic and corazonin (Crz) signal pathways was suggested in the control of DH secretion. Knockout (KO) experiments of GABA transporter (GAT) and circadian clock genes demonstrated that GAT plays a crucial role in PR through circadian control. A model outlining the PR mechanism, from maternal photoperiodic light reception to DH secretion, has been proposed.

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.

Mutation of the clock gene timeless disturbs diapause induction and adult emergence rhythm in Helicoverpa armigera

BACKGROUND

Circadian rhythms are physical and behavioral changes that follow the 24-h cycle of Earth's light and temperature and are regulated by clock genes. Timeless (Tim) has been identified as a canonical clock gene in some insects, however, its functions have been little studied in lepidopteran pests.

RESULTS

To investigate Tim (HaTim) gene function in Helicoverpa armigera, an important lepidopteran pest, we obtained the HaTim mutant using the CRISPR/Cas9 gene editing system. Our results showed that the transcript levels of HaTim rhythmically peaked at night in heads of the wild larvae and adult, and the diel expression of HaTim was sensitive to photoperiod and temperature. The expression rhythms of other clock genes, such as HaPer, HaCry1, HaCry2 and HaCwo, were disturbed in the HaTim mutant larvae, as that stage is a sensitivity period for diapause induction. Fifth-instar wild-type larvae could be induced to pupate in diapause under a short-day photoperiod and low temperature, however, fifth-instar HaTim mutant larvae could not be induced under the same conditions. In addition, the emergence of wild-type adults peaked early at night, but the rhythm was disturbed in the HaTim mutant with arrhythmic expression of some clock genes, such as HaPer, HaCry1 and HaCwo in adults.

CONCLUSION

Our results suggest that the clock gene Tim is involved in diapause induction and adult emergence in H. armigera, and is a potential target gene for controlling pest. © 2023 Society of Chemical Industry.

Molecular characterization of TRPA1 and its function in temperature preference in Eriocheir sinensis

Chinese mitten crab (Eriocheir sinensis) is an economically important aquaculture species, and its growth and development are regulated by temperature, but the molecular mechanisms of the responses to temperature remain unclear. Herein, we identified TRPA1 from E. sinensis, a member of the TRP family of heat receptor potential channels, performed RACE cloning and bioinformatics analysis, and investigated the effect of TRPA1 on temperature responses and molting by real-time PCR and RNA interference (RNAi). The open reading frame of Es-TRPA1 is 3660 bp, and the encoded protein has a molecular weight of 136.91 kDa, and is expressed in embryos and juveniles. RNAi-mediated silencing decreased Es-TRPA1 expression in juvenile crabs, molting rate was decreased, mortality was increased, and crabs avoided cold areas (4 °C) much less than control juvenile crabs. The results suggest that Es-TRPA1 is involved in regulating temperature adaptation and molting processes in E. sinensis. The findings lay a foundation for further exploration of temperature regulation mechanisms in E. sinensis and other crustaceans.

Knockouts of positive and negative elements of the circadian clock disrupt photoperiodic diapause induction in the silkworm, Bombyx mori

Diapause is one of the most important traits that have sustained insects to thrive. To survive harsh seasons, most insects can arrest their development and enter diapause. The photoperiod is the signal that indicates insects the proper timing to enter diapause. Circadian clock genes are shown to be involved in photoperiodic diapause induction in various insect species. The silkworm, Bombyx mori, enters diapause at the embryonic stage. In bivoltine strains, diapause determination is under maternal control and affected by temperature and photoperiodic conditions that mothers experienced during embryonic and larval stages. Two independent studies showed that knocking out the core clock gene, period, perturb photoperiodic diapause induction in B. mori. However, whether the circadian clock as whole or individual clock genes are responsible for the photoperiodic diapause induction remains unknown. In this study, using CRISPR/Cas9 we knocked out negative (period and timeless) and positive elements (Clock and cycle) in p50T, a bivoltine strain which exhibits photoperiodic diapause induction during both embryonic and larval stages. The temporal expression patterns of clock genes changed in each core clock gene knockout strain, suggesting disruption of normal feedback loops produced by circadian clock genes. Furthermore, the ability of female moths to appropriately produce diapause or non-diapause eggs in response to photoperiod in both embryonic and larval stages was lost in all knockout strains. Our results indicate the involvement of circadian clock in photoperiodic diapause induction in B. mori.