Diapause Induction Research Articles

Diapause Induced by Temperature and Photoperiod Affects Fatty Acid Compositions and Cold Tolerance of Phthorimaea Operculella (Lepidoptera: Gelechiidae).

To find out the potential condition for diapause induction in the potato tuber moth Phthorimaea operculella (Zeller), combination of constant temperatures (15, 20, and 30°C) and photoperiods (8, 9, 10, 11, 12, 14, and 16 h) were employed from egg to adult emergence. In addition, changes in the total phospholipid fatty acid content and cold tolerance of non-diapausing and diapausing pupae were examined. The critical daylength for diapause induction were 12.43 h at 20°C and lower temperatures that can induce in 50% of population. Moreover, the composition of total phospholipid fatty acids in the pupae revealed seven major fatty acids in both non-diapausing and diapausing pupae: oleic (26-32%), palmitic (21-29%), linoleic (18-21%), palmitoleic (4-10%), stearic (9%), linolenic (7-8%), and pentadecanoic acids (3-5%) with an increase in proportion of unsaturated fatty acids in diapause state. In contrast to increase of oleic acid (C18:1) from 26 to 32% in non-diapausing to diapausing pupae, a decrement trend from 29 to 21% in palmitic acid (C16:0) was observed at the same state. Additionally, supercooling point was observed to be significantly lower in diapausing (-22.6°C) than in non-diapausing pupae (-18.5°C) and the fresh weight of diapausing pupae was found to be significantly higher than non-diapausing ones. The significance of these findings would allow us a better understanding of interrelationship between diapause and cold tolerance.

Instar- and Stage-Specific Photoperiodic Diapause Response of Lygus hesperus (Hemiptera: Miridae)

The western tarnished plant bug (Lygus hesperus Knight) (Hemiptera: Miridae) is a polyphagous pest of numerous western crops. This pest overwinters in a relatively short duration adult diapause, but many details regarding diapause induction and maintenance remain unstudied. Instar-specific responses to a switch from short (10-h) to long (14-h) days, long to short days, and a temporary (4-d) switch from short to long days were compared with responses of insects maintained under respective short- and long-day photoperiods. Influences of short days received during the nymphal stage on subsequent adult reproductive development under long days were also examined. Substantial diapause responses were observed only for insects switched to short days by 4th instar. Few insects that switched from short to long days exhibited diapause characters by Day 10 of adulthood. When 2nd instars were temporarily switched from short to long days no effect on the diapause response was observed, but the response was diminished when the switch occurred beginning at 4th instar. Reproductive development of adults reared under short days and switched to long days was modestly delayed compared with insects reared from egg to adult under long days. These results indicate the diapause response is elicited by short day cues received prior to the 5th instar, and that continued exposure to short days is necessary to maintain the adult diapause. These findings, combined with earlier reports of diapause termination in the field, suggest that southern populations of L. hesperus may express a more dynamic and complex overwintering strategy than was previously recognized.

The effect of Wolbachia on diapause, fecundity, and clock gene expression in Trichogramma brassicae (Hymenoptera: Trichogrammatidae).

The short day lengths of late summer in moderate regions are used to induce diapause in various insects. Many studies have shown the maternal effect of photoperiod on diapause induction of Trichogramma wasps, but there is no study to show the relationship between photoperiodic regimes and clock genes in these useful biological control agents. Here, we investigated the role of photoperiods on diapause, fecundity, and clock gene expression (clk, cyc, cry2, per, and timeout) in asexual and sexual Trichogramma brassicae as a model insect to find any differences between two strains. Asexual strain was infected by Wolbachia, an endosymbiont bacterium. The diapause percentage was significantly higher under short days (8h in sexual and 12h in the asexual T. brassicae), although the diapause percentage of the sexual strain was significantly higher than the asexual one in all the photoperiods. The ANOVA revealed no significant changes between different photoperiods in the clock gene expression in the sexual strain but significant photoperiodic changes in clk, cyc, and timeout in the asexual strain. Our results showed that the mRNA levels of clock genes of asexual T. brassicae were significantly lower than those of sexual strain. The fecundity was significantly higher in the asexual strain. These results suggest that Wolbachia infection makes disturbance on the clock gene expression which consequently reduces the percentage of diapause but increases the fecundity in asexual T. brassicae.

Effects of inter-subspecies hybridization of non-native Monochamus alternatus alternatus and native M. a. endai (Coleoptera: Cerambycidae) on the induction of larval diapause and adult body size

To evaluate the effects of Taiwanese Monochamus alternatus alternatus (Coleoptera: Cerambycidae) (T) on native M. a. endai populations (J) and pine wilt disease epidemics in Japan, the biological traits of reciprocally crossed lines between two subspecies were compared with parental lines. No significant differences were observed in egg viability among four crosses in the F1 generation. Hybridized lines had greater survival rates than inbred lines of the J × J cross. The incidence of larval diapause was lower in the inbred lines of M. a. alternatus (0.176 and 0.361) than in the inbred lines of M. a. endai (1.000). Hybridized lines showed a high diapause incidence of 0.896–1.000 in the F1 and F2 generations, indicating that diapause induction was dominant over nondiapause induction. Large numbers of ovarioles dominated over small numbers of ovarioles. Hybridized lines revealed greater adult body sizes than the inbred lines of parental subspecies in the F1 generation (heterosis), but not in the F2 generation. Heterosis in adult body size may contribute to pine wilt disease epidemics because of the greater oviposition rate and initial load of pathogenic nematodes. Two unlinked loci are estimated to control diapause induction. Gene introgression and the dynamics of nondiapause-inducing alleles are discussed.

Inheritance of diapause regulation in the multicoloured Asian ladybird Harmonia axyridis (Coleoptera: Coccinellidae)

It is known that females from native populations of the multicoloured Asian ladybird, Harmonia axyridis, have a strong photoperiodic response (enter reproductive diapause under short photoperiods), whereas the proportion of diapausing females from invasive populations is less dependent on day length. The aim of the present study was to determine the mode of inheritance of these differences. The experiments were conducted with two laboratory populations of H. axyridis, the High Diapause (HD) population originated from Irkutsk (Southern Siberia, ca 52.3°N, 104.3°E) and the Low Diapause (LD) population originated from Sochi (North Caucasus, ca 43.6°N, 39.6°E). Reciprocal first generation hybrids and reciprocal backcrosses were investigated. Under a strong diapause-inducing photoperiod (12 h) 100% of the females from the HD population and about 70% of those from the LD population entered diapause. First generation hybrids, as well as their backcrosses with individuals from the HD population, showed almost 100% diapause. Among the backcrosses with the LD population the percentage of diapausing females was widely variable (70-100%) but close to the average between the two populations. The comparison of reciprocal crosses did not reveal any significant difference. We conclude that photoperiodic induction of diapause in the populations of H. axyridis studied is most probably determined by several genes (although one of them evidently plays a leading role with diapause being dominant over non-diapause) and that male and female genotypes are equally important in the determination of female reproductive diapause.

Comparison of diapause and non-diapause induced Orius sauteri (Hemiptera: Anthocoridae) based on a two-sex life table in the laboratory

The flower bug Orius sauteri (Poppius) is commonly used as an effective biocontrol agent for controlling small pest populations in greenhouses in China. The impact of two temperature and photoperiod regimes (26°C, L16:D8 h; 21°C, L8:D16 h) on the life table of O. sauteri was studied in the laboratory using an age-stage, two-sex life-table method. The results showed that the low-temperature short-photoperiod regime resulted in significant increases of the developmental times of the egg, the first to fifth nymphal stages, and the total preadult stage. Female adult longevity was significantly increased, whereas male adult longevity was significantly decreased, under this regime. Furthermore, the adult preoviposition period (APOP), the total preoviposition period (TPOP), and the sex ratio were all increased under this regime. Fecundity decreased, whereas the mean generation time (T) increased, as temperature and photoperiod decreased. The intrinsic rate of increase (r), finite rate of increase (λ), and net reproductive rate (Ro) all increased as the temperature decreased under diapause induction. Based on the estimated data, the higher r value (0.06d−1) occurred under the 26°C, L16:D8 h regime, compared with 0.03d−1, under the 21°C, L8:D16 h regime. Thus, the results showed that reproductive diapause slowed O. sauteri population growth, about 50% reduction based on r. There is significant interest in using O. sauteri as a biological control agent against non-diapausing small pests, such as thrips, in greenhouses under short photoperiods. However, based on the current results, managers should consider maintaining a high temperature and long photoperiod to prevent reproductive diapause induction in O. sauteri. Such conditions should not have a detrimental effect on the greenhouse crops.

On the photoperiodic induction of diapause in first generation hybrids of Harmonia axyridis (Pall.) (Coleoptera, Coccinellidae)

Ladybirds Harmonia axyridis (Pallas) belonging to the autochthonous and invasive populations from Irkutsk and Sochi that differed in the photoperiodic induction of diapause were crossed in laboratory conditions. The individuals from the Irkutsk population show a strong induction of diapause under short day conditions, while individuals from the Sochi population exhibit very weak photoperiodic induction of diapause. The F 1 hybrids obtained from reciprocal crosses demonstrated a photoperiodic reaction similar to that of the parental individuals of the Irkutsk population. The obtained data suggest that the strong photoperiodic induction of diapause is dominant and the effects of male and female genotypes on progeny phenotype are equal.

Diapause induction and termination in Indian strains ofTrichogramma chilonis(Hymenoptera: Trichogrammatidae)

AbstractThe role of temperature in diapause induction was studied as a mode of long-term storage ofTrichogramma chilonis(Ishii) (Hymenoptera: Trichogrammatidae). Three different strains of this widely used biocontrol agent (T. chilonisNilgiris strain,T. chilonisKodaikanal strain, andT. chilonis15 °C strain) reared on the factitious hostCorcyra cephalonica(Stainton) (Lepidoptera: Pyralidae) were used for this experiment. ExceptT. chilonislaboratory strain, all the other strains could successfully undergo diapause at their pre-pupal stage. Maximum percentage of healthy pre-pupae were recorded in the three strains by providing a pre-storage temperature of 10 °C for 35 days with eight hours of photophase wherein 75–87% could enter into diapause. Further, at a maintenance temperature of 5 °C with 24 hours of scotophase, diapause could be maintained. Diapause could be terminated after six months of storage with 23–36% of adult emergence. However, there was significant reduction in longevity and parasitism rate of the emerged adults. Considering superior biological parameters, 95 days of storage (including pre-storage duration) could provide around 60% adult emergence. Successful long-term storage ofT. chilonisstrains through diapause induction can facilitate commercial insectaries in stockpiling this biocontrol agent for large-scale field releases. This is the first study on successful induction and termination of diapause inT. chilonisstrains and evaluating their performance attributes.

Intraspecific maternal competition induces summer diapause in insect parasitoids.

Organisms often live in unpredictable environments and have to adopt life history strategies that optimize their fitness under these conditions. According to bet-hedging theory, individuals can reduce variation in fitness outcomes by investing in different strategies at the same time. For arthropods, facultative summer diapause enables survival during dry and hot periods of the year, and can be triggered by a decrease in resource abundance. However, the effect of resource depletion on diapause induction has never been disentangled from the effect of the perception of the presence of competitors. Using two solitary parasitoid species of cereal aphids as a model system, Aphidius avenae (Haliday) and Aphidius rhopalosiphi (De Stefani-Perez) (Hymenoptera: Braconidae), we tested whether (i) low absolute host density and/or (ii) high levels of parasitoid females' competition lead to maternal-induced summer diapause in parasitoid offspring. Under summer-like climatic conditions, emerging parasitoid females were (i) reared alone and exposed to different host densities (from 5 to 130 aphids), or (ii) reared together with competing females (from 2 to 20 females) and then exposed individually to 50 aphids. For both parasitoid species, low aphid densities did not induce summer diapause. However, the incidence of summer diapause increased up to a maximum of 11% with increasing levels of competition experienced by female parasitoids. More than 60% of the females produced both diapausing and nondiapausing offspring after being kept at the two highest competition densities. Such a "spreading-the-risk" strategy has likely evolved to optimize parasitoid fitness by preventing the following generation from exposure to low populations of suitable hosts and high mortality from superparasitism. These results provide the first experimental evidence of direct maternal competition-induced diapause in insects, and may change the way we apprehend the evolution of arthropod seasonal ecology, by considering intraspecific competition.

MicroRNAs are differentially abundant during Aedes albopictus diapause maintenance but not diapause induction.

Diapause is a programmed dormancy that allows organisms to tolerate predictable periods of unfavourable conditions by temporarily halting development and reducing metabolism. Diapause is widespread amongst insects and is crucial for allowing organisms to coordinate their growth and reproduction with favourable environmental conditions. Although the adaptive significance of diapause is well understood, the molecular mechanisms underpinning diapause remain unresolved. We performed high-throughput sequencing to investigate the role of microRNAs (miRNAs) in the diapause of the Asian tiger mosquito, Aedes albopictus. We first investigated miRNAs in diapause induction by characterizing maternally provisioned miRNAs in mature oocytes of Ae.albopictus under diapause-inducing and diapause-averting conditions. Second, we investigated miRNAs in diapause maintenance by characterizing miRNAs in diapause and nondiapause pharate larvae. We identified 162 miRNAs, 152 previously known and 10 putatively novel. We identified no differentially abundant miRNAs in mature oocytes and seven differentially abundant miRNAs in pharate larvae. The predicted targets of differentially abundant miRNAs include genes affecting several processes related to diapause maintenance including ecdysone regulation, immune response, lipid metabolism and regulation of development. Our results suggest that Ae.albopictus does not maternally provision a unique set of miRNAs during diapause induction but miRNAs are a component of diapause maintenance in this species.

Dedicated issue: photoperiodic induction of diapause and seasonal morphs

Whether transitions from warm to cold in temperate regions or dry to wet in areas nearer the equator, the changing of the seasons is a major stress for most organisms and as such, a major organizer of organismal lifecycles. To mitigate stressful periods and to exploit periods with high resource availability that are favorable for growth and development, insects and other arthropods have evolved a suite of seasonal adaptations including programmed seasonal dormancy, diapause. Yet, climate change, urbanization, and other alterations in land use are increasingly exposing insects and other organisms to changes in seasonality, especially decreased winter cold periods, lengthened growing seasons, and increased climatic variability on the seasonal shoulders in the fall and spring where warm and cold spells are frequently interspersed. Furthermore, increasing globalization has led to many insects expanding into new areas with very different patterns of seasonality than in their home ranges, whether due to inadvertent introductions of invasive species or managed introductions of beneficial pollinators or natural enemies. We must continue to expand our understanding of insect seasonal responses to meet these increasing challenges in seasonal adaption in our ever-changing world. Building greater understanding of the seasonal responses of both pest and beneficial insects will require an inherently interdisciplinary approach that includes a range of expertise from studying organismal behavior and field ecology to identifying the genetic architecture and regulatory physiology of diapause. A recent symposium at the XXV International Congress of Entomology held in Orlando, Florida, USA from 25–30 September 2016 titled ‘Photoperiodic Induction of Diapause and Seasonal Morphs’ provided the forum for such a broad and interdisciplinary discussion. Many symposium participants and several additional investigators generously agreed to contribute papers to this Dedicated Issue of Physiological Entomology. Reflective of the breadth in approaches and perspectives within the sub-field of insect seasonal biology, this issue contains a range of article types. One particularly broad article leverages the rapidly accumulating transcriptomic datasets available for a wide variety of insects and performs a meta-analysis to test the degree to which a common ‘genetic toolkit’ underlies insect diapause responses. Another article reviews the classic literature on diapause in spider mites and puts forth a modern perspective on the genetics of spider mite reproductive diapause with a focus on the essential role of carotenoid compounds. Several more primary data articles follow that range in their approach and perspectives from whole-brain transcriptomics, to focused transcript abundance work on clock genes and photoperiodic responses, and even classical genetic architecture of diapause inheritance patterns. Additional articles cover a gamut of topics from evolutionary ecology of bet-hedging strategies in diapause length and the timing of diapause termination to endocrine changes associated with progression through diapause development, again reflecting the integrative nature of studying insect photoperiodism and seasonal responses. We thank all of our contributors for their timely submissions and efforts that have made it possible to continue the long tradition of both symposia on photoperiodism and seasonal biology at the International Congress of Entomology and producing a published series of papers allied with that session. We expect that this issue will serve to stimulate discussion of past, contemporary, and future issues in the study of insect seasonal biology for both specialist practitioners within the field and among insect biologists in general.

Temperature-dependent development of diapausing larvae of Chilo partellus (Swinhoe) (Lepidoptera: Crambidae)

Temperature-dependent development rate, percent diapause induction (hibernation at low temperature and aestivation at high temperature), and survival of diapausing larvae of Chilo partellus (Swinhoe, 1885) were examined on 13 constant temperatures ranging from 8 to 40°C. Development of hibernating and aestivating larvae occurred from 10 to 25°C and 27–38°C, respectively. However, no development occurred at 8°C and 40°C. To determine actual thermal conditions that affect development and trigger both kind of diapause (hibernation and aestivation), various thermal parameters were estimated by fitting the development rate data to two linear (Ordinary equation and Ikemoto & Takai) models and thirteen non-linear models. The lower thermal thresholds (Tmin) for development of diapausing larvae of C. partellus were calculated as 9.60°C and 10.29°C using the ordinary linear model and Ikemoto & Takai model, respectively. Similarly, the thermal constants (K) estimated using the ordinary linear model was 333.33 degree-days and that estimated with Ikemoto & Takai model was 338.92 degree-days. Among the non-linear models, Lactin-2 followed by Lactin-1 were found to be the best as these models estimated the critical temperatures (Tmin, Tmax and Topt) similar to those of observed values. Conclusively, the Ikemoto & Takai linear model and Lactin-2 followed by Lactin-1 non-linear models are useful and efficient for describing temperature-dependent development and estimating the temperature thresholds of diapausing larvae of C. partellus. Our findings provided fundamental information for estimation of thermal requirement and temperature based development models for diapausing larvae of C. partellus. This information will be highly useful for predicting the occurrence, seasonal emergence, number of generations and population dynamics of C. partellus.

Transcriptome analysis of the brain under diapause and nondiapause conditions in the blowfly Protophormia terraenovae

AbstractThe blow fly Protophormia terraenovae responds to photoperiod and temperature to control reproductive diapause. Differentially expressed genes (DEGs) in the brain are investigated under diapause‐inducing and diapause‐averting conditions in search of potential genes for diapause induction. Total RNA is extracted from female brains 1–2.5 days after adult emergence under an LD 18 : 6 h photocycle at 25 °C as nondiapause samples and 3, 3.5, 7 and 7.5 days after adult emergence under an LD 12 : 12 h photocycle at 20 °C as diapause samples. From our transcriptome analysis, 626 350 654 reads are assembled into 126 231 contigs, from which 46 844 contigs (37.1% of the total) are annotated as protein‐coding genes. Among them, the expression levels in 549 contigs are specifically up‐regulated under diapause‐inducing (44.1%) or diapause‐averting (55.9%) conditions. Using quantitative real‐time reverse transcriptase‐polymerase chain reaction analysis, higher expression levels under diapause‐inducing conditions are found in a contig encoding angiotensin‐converting enzyme (ACE). Expression levels of the contig significantly increase 3 days after changing the photoperiod to diapause‐inducing short days. Rabbit antisera against 17 amino acid peptides included in P. terraenovae‐ACE stain one pair of cells lateral to the oesophageal foramen. These results suggest that the expression of a small number of genes is associated with the regulation of diapause in response to photoperiod and temperature, and ACE in the immunoreactive cells might respond to short days, which is why it is involved in diapause induction.

Disruption of a horizontally transferred phytoene desaturase abolishes carotenoid accumulation and diapause in Tetranychus urticae

Carotenoids underlie many of the vibrant yellow, orange, and red colors in animals, and are involved in processes ranging from vision to protection from stresses. Most animals acquire carotenoids from their diets because de novo synthesis of carotenoids is primarily limited to plants and some bacteria and fungi. Recently, sequencing projects in aphids and adelgids, spider mites, and gall midges identified genes with homology to fungal sequences encoding de novo carotenoid biosynthetic proteins like phytoene desaturase. The finding of horizontal gene transfers of carotenoid biosynthetic genes to three arthropod lineages was unprecedented; however, the relevance of the transfers for the arthropods that acquired them has remained largely speculative, which is especially true for spider mites that feed on plant cell contents, a known source of carotenoids. Pigmentation in spider mites results solely from carotenoids. Using a combination of genetic approaches, we show that mutations in a single horizontally transferred phytoene desaturase result in complete albinism in the two-spotted spider mite, Tetranychus urticae, as well as in the citrus red mite, Panonychus citri Further, we show that phytoene desaturase activity is essential for photoperiodic induction of diapause in an overwintering strain of T. urticae, consistent with a role for this enzyme in provisioning provitamin A carotenoids required for light perception. Carotenoid biosynthetic genes of fungal origin have therefore enabled some mites to forgo dietary carotenoids, with endogenous synthesis underlying their intense pigmentation and ability to enter diapause, a key to the global distribution of major spider mite pests of agriculture.

Color polymorphism, development, and reproductive diapause of the multicolored Asian ladybird Harmonia axyridis (Pallas) (Coleoptera, Coccinellidae) females

Preimaginal development and maturation of females of two laboratory strains originated from two populations of the multicolored Asian ladybird Harmonia axyridis were investigated under laboratory conditions. Females of the autochthonous population from Irkutsk (Siberia) enter diapause under the short day conditions, whereas in females of the invasive population from Sochi (the Caucasus) photoperiodic induction of diapause is weak. The two populations also differ in the proportions of morphs: the population from Sochi is represented only by the morph succinea, whereas the population from Irkutsk is represented by the morphs succinea и axyridis with the predominance of the last one, which in our study was separated into two phenotypes, differed in the degree of melanization. Rearing of adults under the short day (12 h) conditions induced reproductive diapause in females of all the studied morphs and phenotypes of the Irkutsk population, whereas under the long day (18 h) conditions females of the morph succinea showed a slightly lower tendency to diapause. Evidently, the interpopulation differences in the responses regulating the seasonal cycle of the multicolored Asian ladybird are determined by genes that are not involved (or almost not involved) in the determination of the color polymorphism. The interpopulation differences in size and weight were quite substantial, whereas the differences between morphs and phenotypes were statistically significant in only some of our experiments. In combination with the literature data, these results suggest that the genes determining the color polymorphism in H. axyridis may have a pleiotropic effect on other traits including important eco-physiological parameters but this effect is relatively weak and manifests itself only against a particular genetic background and / or under particular environmental conditions.

A molecular‐genetic understanding of diapause in spider mites: current knowledge and future directions

AbstractDuring unfavourable conditions, many arthropods have the ability to enter into diapause and synchronize their development and reproduction to seasonal patterns. Diapause or winter hibernation in insects and mites is set off by a number of cues, with photoperiod being the most well‐defined and strongest signal. This review focuses on the current knowledge of ‘‐omics’ data and the genetics of diapause in the two‐spotted spider mite Tetranychus urticae, a member of the family Tetranychidae (Arthropoda: Chelicerata: Arachnida: Acari). This species is a serious polyphagous pest and females undergo a reproductive facultative diapause when immature stages are exposed to long nights. Winter hibernation induces different physiological processes characterized by a metabolic suppression, different energy use, increased stress tolerance and the production of cryoprotectants, all initiated by a complex signal transduction pathway. Keto‐carotenoids are known to cause the deeply orange colour typical for diapausing females. Furthermore, research with colour mutants of T. urticae has shown the need for carotenoids with respect to the induction of diapause, even though the molecular‐genetic mechanisms underlying these colour phenotypes are still unknown. In addition, marked latitudinal variation in diapause incidence among populations has been observed in nature, with modes of inheritance ranging from recessive to dominant, as well as monogenic to polygenic. We end by highlighting the emerging opportunities for functional studies that aim to unravel the complex factors underlying diapause in spider mites.

Geographic Variation of Diapause Induction and Termination in the Spider Mite Tetrαnychus urticαe: Α mini-review

In eight strains of the spider mite Tetranychus urticae Koch (Acari: Tetranychidae), originating from different localities in Europe, the critical daylength for diapause induction and termination was almost the same in each strain but varied with the latitudinal origin of the strains; critical daylength was shorter in strains originating from lower latitudes and longer in those from higher latitudes. Diapause intensity, measured as the period of chilling required for diapause termination under a short day photoperiod (LD 10:14) and 19°C, again varied with the latitudinal origin of each strain, being higher the more northern the origin of the strain. An exception were two mountain strains which showed a longer critical daylength and a deeper diapause than expected on the basis of their latitudinal origin. The number of long-day (LD 17:7) cycles required for 50% diapause termination after a certain period of chilling was higher in the northern and lower in the southern strains. These results indicate that geographic strains of T urticae may differ considerably in their diapause attributes, which may be explained as an adaptation to local climatic conditions. The great plasticity of the diapause response may, among other factors, have been responsible for the wide distribution of this mite species.

The role of circadian clock genes in the photoperiodic timer of the linden bug Pyrrhocoris apterus during the nymphal stage

AbstractMany insects survive seasonal adversities during diapause, a form of programmed developmental and metabolic arrest. Photoperiodically regulated entry into diapause allows multivoltine insect species to optimize the number of generations. The molecular mechanism of the photoperiodic timer is unknown in insects. In the present study, we take advantage of the robust reproductive diapause response in the linden bug Pyrrhocoris apterus and explore the fifth‐instar nymphal stage, which is the most photoperiod‐sensitive stage. The nymphs display daily changes in locomotor activity during short days; this differs from the activity observed during long days. We find evidence of cyclical expression of the circadian clock genes, per and cyc, in nymphal heads; in addition, per expression is also photoperiod‐dependent. The RNA interference‐mediated knockdown of the two circadian clock genes, Clk and cyc, during the nymphal stage results in reproductive arrest in adult females. Furthermore, Clk and cyc knockdown induces the expression of the storage protein hexamerin in the fat body, whereas the expression of vitellogenin diminishes. Taken together, these data support the involvement of circadian clock genes in photoperiodic timer and/or diapause induction.

Sex‐linked inheritance of diapause induction in the butterfly Pieris napi

AbstractMany temperate insects survive harsh environmental conditions, such as winter, by entering a state of developmental arrest. This diapause state is predominantly induced by photoperiod. The photoperiod varies with latitude and has led to local adaptation in the photoperiodic induction of diapause in many insects. To understand the rapid evolution of the photoperiodic threshold, it is important to investigate and understand the underlying genetic mechanisms. In the present study, the genetic basis of photoperiodic diapause induction is investigated in the green‐veined white butterfly Pieris napi (Lepidoptera, Pieridae) by assaying diapause induction in a range of conditions for a Swedish and Spanish population. Furthermore, the inheritance of diapause induction is assessed in reciprocal F1 hybrids and backcrosses between the two populations. The southern population shows a clear photoperiodic threshold determining diapause or direct development, whereas the northern populations show a high incidence of diapause, regardless of photoperiod. The hybrid crosses reveal that the inheritance of diapause induction is strongly sex‐linked, and that diapause incidence in the genetic crosses is highly dependent on photoperiod. This emphasizes the importance of assaying a range of conditions in diapause inheritance studies. The results indicate a strongly heritable diapause induction with a major component on the Z‐chromosome, as well as a minor effect of the autosomal background.

Adaptive significance of precocious pupation in the bean blister beetle, Epicauta gorhami (Coleoptera: Meloidae), a hypermetamorphic insect

Larvae of the bean blister beetle, Epicauta gorhami Marseul (Coleoptera: Meloidae), feed on grasshopper eggs in soil and undergo hypermetamorphosis. They normally enter diapause as a pseudopupa at the fifth instar, a form characteristic of hypermetamorphosis for meloid beetles. However, fourth-instar larvae exposed to long days and high temperature avoid pseudopupal diapause and pupate directly from the fourth instar. Fourth-instar larvae also tend to pupate precociously with a smaller body size if they are deprived of food. In these larvae, the critical day-length controlling induction of pseudopupal diapause becomes shorter than that for fully-fed larvae. In this study, we examined how the reaction norm of food-deprived E. gorhami larvae functions in nature by rearing insects from the egg stage outdoors in different seasons with manipulation of the food supply. The results indicated that most fully-fed larvae entered pseudopupal diapause, whereas food-deprived larvae tended to pupate precociously without entering diapause, especially early in the season. The resulting smaller adults reproduced early in the autumn and their progeny attained the pseudopupal stage before winter, indicating that the reaction norm may have an adaptive role in controlling seasonal development in the face of food shortages, producing a bivoltine life cycle.