Time- and temperature-dependent dynamics of prothoracicotropic hormone and ecdysone sensitivity co-regulate pupal diapause in the green-veined white butterfly Pieris napi

Diapause, a general shutdown of developmental pathways, is a vital adaptation allowing insects to adjust their life cycle to adverse environmental conditions such as winter. Diapause in the pupal stage is regulated by the major developmental hormones prothoracicotropic hormone (PTTH) and ecdysone. Termination of pupal diapause in the butterfly Pieris napi depends on low temperatures; therefore, we study the temperature-dependence of PTTH secretion and ecdysone sensitivity dynamics throughout diapause, with a focus on diapause termination. While PTTH is present throughout diapause in the cell bodies of two pairs of neurosecretory cells in the brain, it is absent in the axons, and the PTTH concentration in the haemolymph is significantly lower during diapause than during post diapause development, indicating that the PTTH signaling is reduced during diapause. The sensitivity of pupae to ecdysone injections is dependent on diapause stage. While pupae are sensitive to ecdysone during early diapause initiation, they gradually lose this sensitivity and become insensitive to non-lethal concentrations of ecdysone about 30 days into diapause. At low temperatures, reflecting natural overwintering conditions, diapause termination propensity after ecdysone injection is precocious compared to controls. In stark contrast, at high temperatures reflecting late summer and early autumn conditions, sensitivity to ecdysone does not return. Thus, here we show that PTTH secretion is reduced during diapause, and additionally, that the low ecdysone sensitivity of early diapause maintenance is lost during termination in a temperature dependent manner. The link between ecdysone sensitivity and low-temperature dependence reveals a putative mechanism of how diapause termination operates in insects that is in line with adaptive expectations for diapause.

Seasonal occurrence and development of three closely related Oligonychus species (Acari: Tetranychidae) and their associated natural enemies on fagaceous trees

We compared the life cycles and diapause attributes among three closely related spider mites, Oligonychus castaneae on Castanea crenata, and O. gotohi and O. amiensis on Lithocarpus edulis. The lower thermal thresholds from egg to egg were 10.5, 8.5 and 8.9 °C, respectively, and the thermal constants were 177.8, 229.5 and 232.5 degree-days, respectively. The cumulative hatching rates of diapause eggs of O. castaneae and O. gotohi increased as the season progressed in and after early-to-mid January, which indicates diapause termination. In contrast, O. amiensis showed higher hatching rates in December and January, but hatchability gradually decreased in and after February because some of the eggs died from the cold. Oligonychus castaneae and O. gotohi females produced diapause eggs in response to the short photoperiod in late September to early October and in early-to-late October, respectively, which corresponded to the times predicted by the critical photoperiods (at 15 °C) of 12 h 15 min and 11 h 15 min for the respective species. Oligonychus castaneae showed at least a single population peak over the 3-year observation period, but the time of peak population varied from mid-July to mid-September. The population of O. gotohi was higher between November and May when diapause eggs were present on host plants in early winter and the first-generation females laid eggs on leaves in spring. The population of O. amiensis, which is a non-diapause species, was only high between September and December, because eggs were laid on leaves in autumn to winter and then gradually disappeared and/or died during winter. Natural enemies were observed as the number of spider mites declined, and the density suppression effect by natural enemies was confirmed in the field.

Structural properties and cellular expression of AfrLEA6, a group 6 late embryogenesis abundant protein from embryos of Artemia franciscana

Late embryogenesis abundant (LEA) proteins are intrinsically disordered proteins (IDPs) commonly found in anhydrobiotic organisms and are frequently correlated with desiccation tolerance. Herein we report new findings on AfrLEA6, a novel group 6 LEA protein from embryos of Artemia franciscana. Assessment of secondary structure in aqueous and dried states with circular dichroism (CD) reveals 89% random coil in the aqueous state, thus supporting classification of AfrLEA6 as an IDP. Removal of water from the protein by drying or exposure to trifluoroethanol (a chemical de-solvating agent) promotes a large gain in secondary structure of AfrLEA6, predominated by α-helix and exhibiting minimal β-sheet structure. We evaluated the impact of physiological concentrations (up to 400 mM) of the disaccharide trehalose on the folding of LEA proteins in solution. CD spectra for AfrLEA2, AfrLEA3m, and AfrLEA6 are unaffected by this organic solute noted for its ability to drive protein folding. AfrLEA6 exhibits its highest concentration in vivo during embryonic diapause, drops acutely at diapause termination, and then declines during development to undetectable values at the larval stage. Maximum cellular titer of AfrLEA6 was 10-fold lower than for AfrLEA2 or AfrLEA3, both group 3 LEA proteins. Acute termination of diapause with H2O2 (a far more effective terminator than desiccation in this Great Salt Lake, UT, population) fostered a rapid 38% decrease in AfrLEA6 content of embryos. While the ultimate mechanism of diapause termination is unknown, disruption of key macromolecules could initiate physiological signaling events necessary for resumption of development and metabolism.

Chilling effect on termination of reproductive diapause in Listronotus maculicollis (Coleoptera: Curculionidae)

Diapausing adults of the annual bluegrass weevil, Listronotus maculicollis, were collected from their hibernating sites at different times in autumn and winter, and subjected to different conditions to observe diapause termination by dissecting and measuring the reproductive organs. When diapausing weevils were maintained under laboratory cold conditions (10 h light at 6 °C:14 h dark at 4 °C) from early December to late March, the sizes of reproductive organs of both sexes increased or fluctuated slightly, and very few females had developing oocytes, suggesting that most adults did not resume development during the chilling period. When diapausing weevils (chilled for 40-83 days) were transferred to warm conditions (LD 14:10 and 21 °C) for different lengths of time, reproductive organ sizes in both sexes increased as chilling period prolonged, implying that chilling played an important role in diapause termination. Under field conditions, an apparent peak of reproductive development was observed on January 07 when 80% of males and 53% of females had resumed growth of reproductive organs. Diapausing weevils collected in September without chilling did not develop successfully despite exposure to warm conditions. In contrast, 87% of males and 93% of females collected from the field on January 21 had initiated reproductive development after 5 days of exposure to warm conditions, indicating the necessity of chilling for diapause termination. Male and female reproductive organ sizes increased faster and to a greater final size the longer the preceding chilling period was. The prolonged chilling period in the field resulted in more synchronized and advanced development in L. maculicollis when exposed to warm conditions.

Effect of temperature on diapause termination and post-diapause development in Eotetranychus smithi (Acari: Tetranychidae)

Previous studies on the spider mite Eotetranychus smithi Pritchard & Baker have shown that diapause in eggs is induced by low temperature alone and that females developed at ≤ 17.5 °C laid diapause eggs, regardless of the photoperiod. In this study, diapause eggs were kept at 5 °C and a photoperiod of 16L:8D for 0-120 days and then maintained at 25 °C to know the effect of chilling on diapause termination. Diapause eggs mostly hatched when they were maintained at 25 °C after chilling for 30-90 days at 5 °C, which suggests that diapause termination is favored by low temperatures. To clarify the hatching conditions after diapause termination, diapause eggs kept at 5 °C for 45 days were subsequently maintained at various constant temperatures (from 15 to 25 °C) under a long-day photoperiod (16L:8D). The hatchability at all temperatures tested was high (> 90%) and did not significantly differ among the high temperatures. Duration of embryonic development was shorter with increasing warming temperature after chilling. The lower thermal threshold (t) and thermal constant (k) for post-diapause egg development were 10.5 °C and 76.9 degree-days, respectively. Females, which developed from diapause eggs that were chilled at 5 °C for 45 days and then maintained at 15 °C, laid only non-diapause eggs, which indicates that they were prevented from re-entering diapause even under diapause-inducing conditions (15 °C). Thus, temperature is the main factor to control diapause termination and post-diapause development, which has also been found for other spider mites that enter diapause at the egg stage.

Effects of temperature, soil moisture and photoperiod on diapause termination and post-diapause development of the wheat blossom midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae)

Sitodiplosis mosellana, one of the most important wheat pests, goes through larval diapause in a cocooned form. It is univoltine, but some individuals exhibit prolonged diapause. In this study, we documented diapause termination rate of cocooned larvae at different diapausing periods and time required for adult emergence when they were brought to 25°C from the field in northern China. We found that field larvae all entered diapause by June, but none terminated diapause until late September when the daily average temperature dropped to below 20°C. Furthermore, termination rate increased significantly as diapausing larvae underwent increasing chilling duration, reaching >95% from early December to early March. Our results suggest that chilling was necessary for diapause to terminate and that field diapause termination ended in early December. To explore low temperature and duration required, we cold-treated field diapausing larvae of different periods at different low temperatures for various lengths of time prior to the 25°C incubation. Chilling at 4-8°C for 60-90days resulted in a higher termination rate (>90%) and shorter adult emergence time in general. Additionally, we investigated the combined effect of temperature (18-30°C) and soil moisture (20-60%, on dry weight basis) as well as the effect of photoperiod (24:0-0:24L:D) on post-diapause development. While photoperiod did not affect adult emergence, soil moisture ranging from 30 to 50% and temperature from 22 to 26°C resulted in the highest adult emergence rates (>46%) within relatively short time (<18days). Mortality and/or prolonged diapause rate drastically increased when incubation conditions were outside the optimal range, especially at 30°C. These findings provide new insight into the diapause process of S. mosellana, and information will be useful for development of field forecasting and laboratory rearing techniques of this pest insect.

Cloning and expression of retinoblastoma-binding protein 4 gene in embryo diapause termination and in response to salinity stress from brine shrimp Artemia sinica

Retinoblastoma binding protein 4 (RBBP4) is a nuclear protein with four WD-repeat sequences and thus belongs to a highly conserved subfamily of proteins with such domains. This retinoblastoma-binding protein plays an important role in nucleosome assembly and histone modification, which influences gene transcription and regulates cell cycle and proliferation. Artemia sinica (brine shrimp) undergoes an unusual diapause process under stress conditions of high salinity and low temperature. However, the role of RBBP4 in diapause termination of embryo development in A. sinica remains unknown. Here, the full-length cDNA of the As-rbbp4 gene was obtained from A. sinica and found to contain 1411 nucleotides, including a 1281 bp open reading frame (ORF), 63 bp 5'-untranslated region (UTR) and a 67-bp 3'-UTR, which encodes a 427 amino acid (48 kDa) protein. Bioinformatic analysis indicated As-RBBP4 to be mainly located in the nucleus, with a theoretical isoelectric point of 4.79. Protein sequence domain analysis showed that As-RBBP4 is a conserved protein, especially in the WD40 domain. No specificity in expression of this gene was observed in tissues or organs by in situ hybridization. Real-time quantitative PCR and Western blot analyses of As-RBBP4 gene and protein expression, respectively, showed notably high levels at 10 h and a subsequent downward trend. Obvious trends in upregulation of As-RBBP4 were observed under conditions of low temperature and high salinity stress. As-E2F1 and As-CyclinE also presented similar trends as that of As-RBBP4 in Western blots. Analysis of the RBBP4 expression in early embryonic development of A. sinica indicated that this protein plays an important role in diapause termination and cell cycle regulation.

Emergence of the overwintering generation of peach fruit moth (Carposina sasakii) depends on diapause and spring soil temperatures

Survival rate and emergence timing of the overwintering generation of many temperate agricultural pests is expected to affect their population dynamics and damage potential. However the impact of fluctuating winter and spring conditions on the successful emergence of insects post-diapause is generally poorly known. Here we characterize diapause responses in the peach fruit moth (PFM) pest, Carposina sasakii Matsumura, which overwinters at the larval stage in soil. Temperatures at a depth of 5 cm fluctuated markedly in early spring during the critical PFM post-diapause period (late December to mid-April). By removing outdoor larval samples over this period, we show that the completion of diapause for PFM in northern China starts from late January and continues until March. This extended developmental period is accompanied by an ongoing loss of cold resistance. Temperature conditions experienced in the field were associated with cold tolerance and emergence times, and reduced cold tolerance was associated with shorter emergence time. Cryoprotectants declined from late December, and levels were associated with changes in the supercooling point (SCP) of the larvae, but both correlated weakly to survival under cold stress during the post-diapause period. These findings suggest that diapause stage and soil temperatures should be taken into account when predicting field dynamics of soil-dwelling overwintering insects based on degree day accumulation models and other approaches.

Diapause hormone in the Helicoverpa/Heliothis complex: A review of gene expression, peptide structure and activity, analog and antagonist development, and the receptor

This review summarizes recent studies focusing on diapause hormone (DH) in the Helicoverpa/Heliothis complex of agricultural pests. Moths in this complex overwinter in pupal diapause, a form of developmental arrest used to circumvent unfavorable seasons. DH was originally reported in the silkmoth Bombyx mori, a species that relies on DH to induce an embryonic diapause. But, in the case of Helicoverpa/Heliothis, levels of dh transcripts and DH peptides are more abundant in nondiapausing pupae than in diapausing individuals, and DH effectively terminates diapause within a specific temperature range. A structure activity relationship study indicated that the active core of DH is the C-terminal hepta-peptide, LWFGPRLa. We designed and synthesized a first generation of DH agonists and identified two agonists (PK-2Abf and PK-Etz) that were nearly 50- and 13-fold more potent than the native hormone. These studies revealed two structural characteristics of DH and its agonists that are essential for interaction with the receptor: a trans-Pro configuration to form a type I β-turn and a hydrophobic moiety involved in ligand binding. Modification of DH at the active core yielded a potent DH antagonist (DH-Jo, acetyl-GLWA[Jo]RLa) as well as an agonist (DH-2Abf-K). Three compounds (Decyl-1963, Dodecyl-1967, Heptyl-1965) were identified as agents capable of penetrating the cuticle of young pupae and thereby preventing the entry into diapause. DH receptor cDNA was cloned and an effective in vitro high throughput screen system was established for future use. This work sets the stage for further development of DH analogs and antagonists that have the potential to disrupt insect diapause as a tool for pest management.

Molecular identification and expression analysis of a diapause hormone receptor in the corn earworm, Helicoverpa zea

Diapause hormone (DH) is an insect neuropeptide that is highly effective in terminating the overwintering pupal diapause in members of the Helicoverpa/Heliothis complex of agricultural pests, thus DH and related compounds have promise as tools for pest management. To augment our development of effective DH analogs and antagonists that could be used as diapause disruptors this study focuses on the cloning and identification of the DH receptor (DHR) in the corn earworm, Helicoverpa zea. The full-length dhr cDNA contains 2153 nucleotides encoding 511 amino acids. Our results suggest there are at least two splicing variants of Hezea-DHR. Hydrophobicity analysis and sequence alignment indicate that Hezea-DHR has 7 transmembrane regions and a highly conserved C-terminal region that is also present in related receptors. Hezea-DHR has 95%, 82% and 79% identity to a partial DHR sequence from Heliothis virescens, a full-length DHR in Orgyia thyellina, and DHR-1 in Bombyx mori, but only 45-49% identity to pheromone biosynthesis activating neuropeptide receptor (PBANR). Expression of dhr mRNA remained low in whole body extracts throughout diapause and in young nondiapausing pupae, but was distinctly elevated as development ensued in pharate adults 7 days after pupation. The highest expression of dhr mRNA we noted was in the ovary. A DHR fusion protein with enhanced-green fluorescent protein was successfully expressed heterologously in X. laevis oocytes, as verified by fluorescent imaging and Western blots, but an electrophysiological assay failed to detect receptor-ligand binding activity, which suggests that an essential cofactor and/or accessory protein is required for functional activity of the DHR.

Identification of the glycerol kinase gene and its role in diapause embryo restart and early embryo development of Artemia sinica

Glycerol kinase (GK) catalyzes the rate-limiting step in glycerol utilization by transferring a phosphate from ATP to glycerol, yielding glycerol 3-phosphate, which is an important intermediate for both energy metabolism and glycerolipid production. Artemia sinica has an unusual diapause process under stress conditions of high salinity, low temperature and lack of food. In the process, diapause embryos of A. sinica (brine shrimp) accumulate high concentrations of glycerol as a cryoprotectant to prevent low temperature damage to embryos. Upon embryo restart, glycerol is converted into glucose and other carbohydrates. Therefore, GK plays an important role in the diapause embryo restart process. However, the role of GK in diapause termination of embryo development in A. sinica remains unknown. In the present study, a 2096 bp full-length cDNA of gk from A. sinica (As-gk) was obtained, encoding putative 551 amino acids, 60.6 kDa protein. As a crucial enzyme in glycerol uptake and metabolism, GK has been conserved structurally and functionally during evolution. The expression pattern of As-gk was investigated by quantitative real-time PCR and Western blotting. Expression locations of As-gk were analyzed using in situ hybridization. As-gk was widely distributed in the early embryo and several main parts of Artemia after differentiation. The expression of As-GK was also induced by stresses such as cold exposure and high salinity. This initial research into the expression pattern and stress response of GK in Artemia provides a sound basis for further understanding of the function and regulation of genes in early embryonic development in A. sinica and the stress response.