The light cycle controls the hatching rhythm in Bombyx mori via negative feedback loop of the circadian oscillator

Circadian rhythm regulation in the sea cucumber Apostichopus japonicus: Insights into clock gene expression, photoperiod susceptibility, and neurohormone signaling

Sea cucumber Apostichopus japonicus displays the typical circadian rhythms. This present study investigated the molecular regulation of clock genes, as well as monoamines and melatonin, in multiple tissues of A. japonicus, responding to the photoperiod. In order to determine their pivotal role in circadian rhythms, the crucial clock genes, namely AjClock, AjArnt1, AjCry1, and AjTimeless, were identified and a comprehensive analysis of their expressions across various tissues in adult A. japonicus was conducted, revealing the potential existence of central and peripheral oscillators. Results demonstrated that the tissues of polian vesicle and nerve ring exhibited significant clock gene expression associated with the orchestration of circadian regulation, and that environmental light fluctuations exerted influence on the expression of these clock genes. However, a number of genes, such as AjArnt1 and AjCry1, maintained their circadian rhythmicity even under continuous light conditions. Moreover, we further investigated the circadian patterns of melatonin (MT), serotonin (5-HT), and dopamine (DA) secretion in A. japonicus, data that underscored the tissue-specific regulatory differences and the inherent adaptability to dynamic light environments. Collectively, these findings will provide the molecular mechanisms controlling the circadian rhythm in echinoderms and the candidate tissues playing the role of central oscillators in sea cucumbers.

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.

Knockout of cryptochrome 1 disturbs the locomotor circadian rhythm and development of Plutella xylostella

Cryptochrome 1 (CRY1) functions as a light-responsive photoreceptor, which is crucial for circadian rhythms. The identity and function of CRY1 in Plutella xylostella remain unknown. In this study, cry1 was cloned and identified in P. xylostella. Then, a cry1-knockout strain (Cry1-KO) of P. xylostella with a 2-bp deletion was established from the strain Geneva 88 (G88) using the CRISPR/Cas9 technology. No daily temporal oscillation of cry1 was observed in G88 and Cry1-KO, and cry1 mean daily transcription of Cry1-KO was lower than that of G88. Both G88 and Cry1-KO demonstrated rhythmic locomotion under the light/dark condition with Cry1-KO being more active than G88 in the daytime, whereas Cry1-KO completely lost rhythmicity under constant darkness. The developmental period of pre-adult of Cry1-KO was longer than that of G88; the lifespan of the Cry1-KO male adult was shorter than that of G88; the fecundity of Cry1-KO was lower than that of G88; and Cry1-KO showed lower intrinsic rate of increase (r), net reproduction rate (R₀ ), finite increase rate (λ), and longer mean generation time (T) than G88. Our results indicate that cry1 is involved in the regulation of locomotor circadian rhythm and development in P. xylostella, providing a potential target gene for controlling the pest and a basis for further investigation on circadian rhythms in lepidopterans.

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.

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

The bivoltine strain of the domestic silkworm, Bombyx mori, exhibits a facultative diapause phenotype that is determined by maternal environmental conditions during embryonic and larval development. Although a recent study implicated a circadian clock gene period (per) in circadian rhythms and photoperiod-induced diapause, the roles of other core feedback loop genes, including timeless (tim), Clock (Clk), cycle (cyc), and cryptochrome2 (cry2), have to be clarified yet. Therefore, the aim of this study was to elucidate the roles of circadian clock genes in temperature-dependent diapause induction. To achieve this, per, tim, Clk, cyc, and cry2 knockout (KO) mutants were generated, and the percentages of diapause and non-diapause eggs were determined. The results show that per, tim, Clk, cyc, and cry2 regulated temperature-induced diapause by acting upstream of cerebral γ-aminobutyric acid (GABA)ergic and diapause hormone signaling pathways. Moreover, the temporal expression of the clock genes in wild-type (wt) silkworms was significantly different from that of thermosensitive transient receptor potential ankyrin 1 (TRPA1) KO mutants during embryonic development. Overall, the findings of this study provide target genes for regulating temperature-dependent diapause induction in silkworms.

The Circadian Clock in Lepidoptera

With approximately 160,000 identified species of butterflies and moths, Lepidoptera are among the most species-rich and diverse insect orders. Lepidopteran insects have fundamental ecosystem functions as pollinators and valuable food sources for countless animals. Furthermore, Lepidoptera have a significant impact on the economy and global food security because many species in their larval stage are harmful pests of staple food crops. Moreover, domesticated species such as the silkworm Bombyx mori produce silk and silk byproducts that are utilized by the luxury textile, biomedical, and cosmetics sectors. Several Lepidoptera have been fundamental as model organisms for basic biological research, from formal genetics to evolutionary studies. Regarding chronobiology, in the 1970s, Truman's seminal transplantation experiments on different lepidopteran species were the first to show that the circadian clock resides in the brain. With the implementation of molecular genetics, subsequent studies identified key differences in core components of the molecular circadian clock of Lepidoptera compared to the dipteran Drosophila melanogaster, the dominant insect species in chronobiological research. More recently, studies on the butterfly Danaus plexippus have been fundamental in characterizing the interplay between the circadian clock and navigation during the seasonal migration of this species. Moreover, the advent of Next Generation Omic technologies has resulted in the production of many publicly available datasets regarding circadian clocks in pest and beneficial Lepidoptera. This review presents an updated overview of the molecular and anatomical organization of the circadian clock in Lepidoptera. We report different behavioral circadian rhythms currently identified, focusing on the importance of the circadian clock in controlling developmental, mating and migration phenotypes. We then describe the ecological importance of circadian clocks detailing the complex interplay between the feeding behavior of these organisms and plants. Finally, we discuss how the characterization of these features could be useful in both pest control, and in optimizing rearing of beneficial Lepidoptera.

Involvement of the Clock Gene Period in the Photoperiodism of the Silkmoth Bombyx mori

We established a knockout strain of a clock gene, period (per), by using TALEN in a bivoltine strain (Kosetsu) of Bombyx mori (Insecta, Lepidoptera), and examined the effect of per knockout on the circadian rhythm and photoperiodism. The generated per knockout allele was considered to be null, because a new stop codon was present in the insertion allele. The wild type (Kosetsu) showed clear circadian rhythms in eclosion and hatching, whereas the per knockout strain showed arrhythmic eclosion and hatching under constant darkness. In this strain, moreover, temporal expression changes of clock genes per and timeless were disrupted. The wild type showed a clear long-day response for induction of embryonic diapause: when larvae were reared under long-day and short-day conditions at 25°C, adults produced nondiapause and diapause eggs, respectively. However, the per knockout strain lost the sensitivity to photoperiod and laid nondiapause eggs under both conditions. We conclude that per plays an important role both in circadian rhythms and in photoperiodism of B. mori, indicating the involvement of the circadian clock consisting of per in the photoperiodism.

CRISPR/Cas9-based knockout reveals that the clock gene timeless is indispensable for regulating circadian behavioral rhythms in Bombyx mori

Circadian rhythms, which are ubiquitous and adaptive, occur across all species, from microbes to humans, in which they organize and modify behavior and physiology. timeless (tim) is a canonical clock gene. The core composition of the Drosophila melanogaster endogenous circadian clock has been extensively investigated; however, in lepidopteran insects, including Bombyx mori, the mechanism is complicated and little is known regarding the participation of tim in the negative feedback loop responsible for behavioral activities. To arrive at a comprehensive understanding of the role of tim in the B. mori endogenous circadian clock, we exploited the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 gene editing system. We attempted to elucidate the functions of tim in the circadian clock of B. mori using Bmtim mutants. The knockouts affected two circadian behavioral activities: adult emergence and embryo hatching rhythms. Quantitative real-time polymerase chain reaction results confirmed that tim-knockouts induced relative reductions in the expression levels, and thereby the oscillation amplitudes, of Bmper and Bmclk messenger RNAs during both the photophase and scotophase. Additionally, the daily rhythmic expression of Bmdbt was upregulated in the photophase and downregulated in the scotophase in a tim-knockout. Our study reveals that tim is integral to the B. mori circadian clock and may be involved in regulating eclosion and hatching rhythms.

Potential targets for controlling Bactrocera dorsalis using cuticle- and hormone-related genes revealed by a developmental transcriptome analysis

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), is an important agricultural pest and has developed resistance to many insecticides. To investigate vital genes participating in metamorphosis for development of additional control strategies, a comprehensive transcriptome analysis covering ten developmental stages of B. dorsalis was performed.

RESULTS

There were 2132, 952, 1062, 2301 and 1333 differentially expressed genes identified during hatching, 1st-instar larval molting, 2nd-instar larval molting, pupariation and emergence, respectively. Further expression analyses indicated that genes in hormone- (20-hydroxyecdysone and juvenile hormone) and cuticle- (chitin and cuticle protein) related pathways were essential for metamorphosis in B. dorsalis. Among chitinase (Cht) genes, BdCht-5, -8 and -10 were differentially expressed during larval-larval, larval-pupal and pupal-adult moltings. However, BdCht7 was differentially expressed during egg-larval and larval-larval moltings. Knockdown of BdCht7 at the 1st-instar larval stage disrupted normal development of larvae and was lethal to B. dorsalis. Among cuticle protein (CP) genes, 15 genes (BdCPLCG-1, BdCPLCP-2, BdCPAP1-B2, BdRR1-21, BdRR1-31, BdRR2-15, BdRR2-26, BdRR2-30, BdRR2-32, BdTweedle-9, BdTweedle-24, BdRR2-10, BdCPAP3-C1, BdRR1-34 and BdRR1-41) were differentially expressed during four of five types of moltings. Among hormone-relative genes, BdJHBP-4, -9 and -13 were differentially expressed during all five types of moltings, whereas BdJHBP-5, -12 and BdHR4 were differentially expressed during four of five types of moltings.

CONCLUSION

This study reveals critical genes involved in development and metamorphosis of B. dorsaslis, and BdCht7 is dispensable for larval survival. It also provides comprehensive transcriptome information for finding more molecular targets to control this pest. © 2020 Society of Chemical Industry.

Light-exposure at night impairs mouse ovary development via cell apoptosis and DNA damage

The alternation of light and dark rhythm causes a series of physiological, biochemical and metabolic changes in animals, which also alters the growth and development of animals, and feeding, migration, reproduction and other behavioral activities. In recent years, many studies have reported the effects of long-term (more than 6 weeks) illumination on ovarian growth and development. In the present study, we observed the damage, repair and apoptosis of ovarian DNA in a short period of illumination. The results showed that, in short time (less than 2 weeks) illumination conditions, the 24-h light treatment caused the reduction of total ovarian follicle number and down-regulation of circadian clock related genes. Furthermore, the changed levels of serum sex hormones were also detected after 24-h light exposure, of which the concentrations of LH (luteinizing hormone), FSH (follicle-stimulating hormone) and E2 (estradiol) were increased, but the concentration of PROG (progesterone) was decreased. Moreover, 24-h light exposure increased the expression of DNA damage and repair related genes, the number of TUNEL and RAD51 positive cells. These results indicated that 24-h light exposure for 4, 8 and 12 days increased DNA damage and cell apoptosis, thereby affecting the development of ovary.

Involvement of the Clock Gene period in the Circadian Rhythm of the Silkmoth Bombyx mori

In Lepidoptera, the roles of period ( per) and the negative feedback involving this gene in circadian rhythm are controversial. In the present study, we established a per knockout strain using TALEN in Bombyx mori, and compared eclosion and hatching rhythms between the per-knockout and wild-type strains to examine whether per is actually involved in these rhythms. The generated per knockout allele was considered null, because it encoded an extensively truncated form of PERIOD (198 aa due to a 64-bp deletion in exon 7, in contrast to 1113 aa in the wild-type protein). In this per knockout strain, circadian rhythms in eclosion and hatching were disrupted. Under LD cycles, however, a steep peak existed at 1 h after lights-on in both eclosion and hatching, and was considered to be produced by a masking effect—a direct response to light. In the per-knockout strain, temporal expression changes of per and timeless ( tim) were also lost. The expression levels of tim were continuously high, probably due to the loss of negative feedback by per and tim. In contrast, the expression levels of per were much lower in the per knockout strain than in the wild type at every time point. From these results, we concluded that per is indispensable for circadian rhythms, and we suggest that the negative feedback loop of the circadian rhythm involving per functions for the production of behavioral rhythms in B. mori.

Cryptochrome Regulates Circadian Locomotor Rhythms in the Small Brown Planthopper Laodelphax striatellus (Fallén)

Most living organisms have developed internal circadian clocks to anticipate the daily environmental changes. The circadian clocks are composed of several transcriptional-translational feedback loops, in which cryptochromes (CRYs) serve as critical elements. In insects, some CRYs act as photopigments to control circadian photoentrainment, while the others act as transcriptional regulators. We cloned and characterized two cryptochrome genes, the Drosophila-like (lscry1) and vertebrate-like (lscry2) genes, in a rice pest Laodelphax striatellus. Quantitative real-time PCR showed that lscry1 and lscry2 expressed ubiquitously from nymph to adult stages as well as in different tissues. The transcript levels of lscry2 fluctuated in a circadian manner. Constant light led to arrhythmic locomotor activities in L. striatellus. It also inhibited the mRNA oscillation of lscry2 and promoted the transcription of lscry1. Knockdown of lscry1 or lscry2 by RNA interference (RNAi) reduced the rhythmicity of L. striatellus in constant darkness, but not in light dark cycles. These results suggested that lscry1 and lscry2 were putative circadian clock genes of L. striatellus, involved in the regulation of locomotor rhythms.