Isolation and characterization of three diapause-associated transcripts from the Colorado potato beetle, Leptinotarsa decemlineata

Diapause differentially modulates the transcriptomes of fat body and flight muscle in the Colorado potato beetle

Many temperate insects, such as the Colorado potato beetle, enter diapause in winter, during which they arrest their development, suppress their metabolic rate and have high stress tolerance. Diapause phenotypes can be transcriptionally regulated, however many studies to date report only whole animal gene expression rather than tissue-specific processes during diapause. We used RNA-seq to measure gene expression in fat body and flight muscle of diapausing and non-diapausing beetles. We used differential expression and GO enrichment analyses to evaluate longstanding hypotheses about the mechanisms that drive arrested development, changes in energy metabolism, and increased stress tolerance during diapause. We found evidence of G2/M cell cycle arrest, juvenile hormone catabolism, increased antioxidant metabolism, epigenetic modification, transposable element regulation, and cytoskeletal remodeling in both the fat body and flight muscle of diapausing beetles. Beetles differentially modulated the fat body and flight muscle transcriptomes during diapause with fat body playing a larger role in the hypoxia response and immunity, whereas flight muscle had higher abundance of transcripts related to the chaperone response and proteostasis. Our transcriptome provides evidence for distinct roles and responses of fat body and flight muscle during diapause in the Colorado potato beetle, and we provide testable hypotheses for biological processes that appear to drive diapause phenotypes in insects.

Global Transcriptome Sequencing Reveals Molecular Profiles of Summer Diapause Induction Stage of Onion Maggot, Delia antiqua (Diptera: Anthomyiidae)

The onion maggot, Delia antiqua, is a worldwide subterranean pest and can enter diapause during the summer and winter seasons. The molecular regulation of the ontogenesis transition remains largely unknown. Here we used high-throughput RNA sequencing to identify candidate genes and processes linked to summer diapause (SD) induction by comparing the transcriptome differences between the most sensitive larval developmental stage of SD and nondiapause (ND). Nine pairwise comparisons were performed, and significantly differentially regulated transcripts were identified. Several functional terms related to lipid, carbohydrate, and energy metabolism, environmental adaption, immune response, and aging were enriched during the most sensitive SD induction period. A subset of genes, including circadian clock genes, were expressed differentially under diapause induction conditions, and there was much more variation in the most sensitive period of ND- than SD-destined larvae. These expression variations probably resulted in a deep restructuring of metabolic pathways. Potential regulatory elements of SD induction including genes related to lipid, carbohydrate, energy metabolism, and environmental adaption. Collectively, our results suggest the circadian clock is one of the key drivers for integrating environmental signals into the SD induction. Our transcriptome analysis provides insight into the fundamental role of the circadian clock in SD induction in this important model insect species, and contributes to the in-depth elucidation of the molecular regulation mechanism of insect diapause induction.

Early transcriptional events linked to induction of diapause revealed by RNAseq in larvae of drosophilid fly, Chymomyza costata

Background

Diapause is a developmental alternative to direct ontogeny in many invertebrates. Its primary adaptive meaning is to secure survival over unfavourable seasons in a state of developmental arrest usually accompanied by metabolic suppression and enhanced tolerance to environmental stressors. During photoperiodically triggered diapause of insects, the ontogeny is centrally turned off under hormonal control, the molecular details of this transition being poorly understood. Using RNAseq technology, we characterized transcription profiles associated with photoperiodic diapause induction in the larvae of the drosophilid fly Chymomyza costata with the goal of identifying candidate genes and processes linked to upstream regulatory events that eventually lead to a complex phenotypic change.

Results

Short day photoperiod triggering diapause was associated to inhibition of 20-hydroxy ecdysone (20-HE) signalling during the photoperiod-sensitive stage of C. costata larval development. The mRNA levels of several key genes involved in 20-HE biosynthesis, perception, and signalling were significantly downregulated under short days. Hormonal change was translated into downregulation of a series of other transcripts with broad influence on gene expression, protein translation, alternative histone marking by methylation and alternative splicing. These changes probably resulted in blockade of direct development and deep restructuring of metabolic pathways indicated by differential expression of genes involved in cell cycle regulation, metabolism, detoxification, redox balance, protection against oxidative stress, cuticle formation and synthesis of larval storage proteins. This highly complex alteration of gene transcription was expressed already during first extended night, within the first four hours after the change of the photoperiodic signal from long days to short days. We validated our RNAseq differential gene expression results in an independent qRT-PCR experiment involving wild-type (photoperiodic) and NPD-mutant (non-photoperiodic) strains of C. costata.

Conclusions

Our study revealed several strong candidate genes for follow-up functional studies. Candidate genes code for upstream regulators of a complex change of gene expression, which leads to phenotypic switch from direct ontogeny to larval diapause.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1907-4) contains supplementary material, which is available to authorized users.

Photoperiodic effects on diapause-associated gene expression trajectories in European Leptinotarsa decemlineata populations

Behavioural and physiological changes during diapause, an important strategy of insects for surviving harsh seasonal conditions, have been intensively studied. The genetic and molecular mechanisms underpinning diapause development are less well known. We took a candidate gene approach to study prediapause gene expression patterns in the Colorado potato beetle (Leptinotarsa decemlineata), an invasive insect that has rapidly spread northwards to high seasonality environments. Newly eclosed beetles originating from southern (Italy) and northern (Russia) Europe were reared under short- [12 h light (L):12 h dark (D)] and long-day (18L:6D) photoperiods for 10 days. This time period includes the sensitive period for the photoperiodic induction and initiation of diapause. Gene expression trajectories of 12 diapause-related genes (regulatory, metabolic and stress-resistance) were analysed from 0-, 5- and 10-day-old beetles. Gene expression differences increased with age, deviating significantly between populations and photoperiods in 10-day-old beetles. The gene expression profiles, particularly those related to energy metabolism and stress-resistance, indicate that beetles originating from Russia also prepare for diapause under the long-day photoperiod and show qualitative differences in the diapausing phenotype. Our study shows that population-dependent differences seen in behavioural and physiological traits connected with diapause in L. decemlineata are also evident in the expression trajectories of diapause-related genes.

Monitoring diapause development in the Colorado potato beetle, Leptinotarsa decemlineata, under field conditions using molecular biomarkers

A multiplex PCR protocol was developed using five diapause-regulated genes to monitor diapause development of the Colorado potato beetle under field conditions. A total of 870 beetles from the Red River valley of North Dakota and Minnesota, USA, were screened for three consecutive years. Out of the 32 possible expression profiles, eight could be arranged in chronological order of diapause development. These eight profiles account for over 92% of the beetles surveyed. Intra-population variation in diapause phenotypes was observed in the field. Some beetles were already in the diapause initiation phase in June when the day length was greater than 17 h. Inter-seasonal variation in the timing of diapause development was also noted. The greatest differences were before the day length decreased to less than 15 h. Anomalies in the results, e.g., the presence of the diapause maintenance phase profiles in beetles collected on the potato plants, argue that laboratory results are not always equivalent with what is observed under field conditions.

Characterization of gene expression patterns during the initiation and maintenance phases of diapause in the Colorado potato beetle, Leptinotarsa decemlineata

Using differential display, 55 differentially regulated transcripts were isolated from the Colorado potato beetle, Leptinotarsa decemlineata (Say). The insert sizes of the clones ranged from 114 to 795bp. Fourteen of the transcripts were confirmed by northern blot analysis to be differentially regulated transcripts with respect to the diapause initiation and maintenance phases. Based on Blast search results, these 14 transcripts were assigned putative identities and placed into four broad categories of proteins: unknown function, defensive, structural/glycine-rich, and digestive. The transcripts were highly expressed for the first 13-15 days postemergence during the diapause initiation and early diapause maintenance phases and were then substantially down-regulated. These down-regulated transcripts were also highly expressed for the first seven days postemergence in nondiapausing adults and their expression became more variable on day 9 or 11 in most individuals examined. The glycine-rich protein transcripts were all down-regulated by day 11 in the nondiapausing adults. A comparison of the transcript expression patterns between diapause initiation phase and nondiapausing adults showed that elevated levels of expression of the glycine-rich transcripts and two transcripts with unknown functions persisted for approximately four days longer in the diapause-programmed beetles.

A chitinase structurally related to the glycoside hydrolase family 48 is indispensable for the hormonally induced diapause termination in a beetle

Two proteins (APAP I and II) of the glycoside hydrolase family 48 (Family GH48) were isolated from the active adults of the leaf beetle Gastrophysa atrocyanea. Full-length and cDNAs were sequenced. APAP I expression and function were examined in detail. The protein has a chitinase but not a glucanase and cellobiohydrolase activity. It is expressed in the feeding stages, including beetles whose diapause was terminated with a juvenile hormone agonist. Suppression of the APAP I expression by means of RNA interference prevented the hormonal termination of diapause.

Eco-physiological phases of insect diapause

Insect diapause is a dynamic process consisting of several successive phases. The conception and naming of the phases is unsettled and, sometimes, ambiguous in the literature. In this paper, the ontogeny of diapause was reviewed and the most often used terms and the best substantiated phases were highlighted, explained and re-defined. The aim was to propose relatively simple and generally applicable terminological system. The phases of diapause induction, preparation, initiation, maintenance, termination and post-diapause quiescence were distinguished. The specific progression through diapause phases in each species, population (genotype), or even individual, is based on (thus far largely unknown) physiological processes, the actual expression of which is significantly modified by diverse environmental factors. Thus, such phases are eco-physiological in their nature.

Expression profiling of a diapause-specific peptide (DSP) of the leaf beetle Gastrophysa atrocyanea and silencing of DSP by double-strand RNA

A diapause-specific peptide (DSP) composed of 41 amino acid residues including 6 cysteines, has been isolated from diapausing adults of the leaf beetle Gastrophysa atrocyanea. In this study, DSP was found to be expressed primarily in diapausing adults and to a minor extent in pupae, but not in eggs, larvae, or post-diapausing adults. DSP was not induced by bacterial or fungal challenge. DSP-less adults were generated by the injection of double-stranded RNA (dsRNA) corresponding to the dsp gene into pre-diapausing adults. Gene silencing induced by dsRNA was found to be a useful tool for the analysis of DSP in diapausing adults. DSP-less adults showed similar burrowing behavior and oxygen consumption as control insects suggesting that DSP is not essential for the normal onset and maintenance of diapause.

Insect diapause-specific peptide from the leaf beetle has consensus with a putative iridovirus peptide

Diapause and hibernation during periods of environmental adversity are essential features of the life cycle in many organisms, yet the molecular basis for these events differs among animals. We have identified an endogenous diapause/hibernation-specific peptide, from the leaf beetle Gastrophysa atrocyanea. This peptide provides antifungal activity, acts as a N-type voltage-gated Ca2+ channel blocker, and has a new consensus sequence with an unknown polypeptide encoded in the insect iridescent virus. These results indicate that the diapause-specific peptide may be utilized as a probe to analyze and compare functional and evolutional aspects of the life cycles of insects and iridoviruses.