Photoperiodic Induction Of Diapause Research Articles

Temporal dynamics of the egg bank of Aedes aegypti (Diptera: Culicidae) in the winter-spring transition in a temperate region.

In temperate regions, the populations of Aedes aegypti (L.) (Diptera: Culicidae) remain in the egg stage during the winter. In Buenos Aires, Argentina, a temperate region in the southern limit of Ae. aegypti distribution, the start of the next reproductive season and the rate of increase of the adult population depend on the egg bank that remains after the winter. This study aimed to analyze the mortality, field-hatching, and survival of eggs exposed to field conditions representative of those that occur in nature. In addition, the post-exposure hatching response of the eggs was assessed. Four egg batches were exposed to natural conditions starting in mid-winter and were recovered progressively after 3, 6, 9, and 12 weeks. One egg batch (initial control) was not exposed in the field and remained under laboratory conditions. After the exposure period, the recovered intact, collapsed, and hatched eggs were counted. Intact eggs were immersed three times in the laboratory to study their hatching response. Progressive increases in the proportion of lost (presumably by predation), dead, and hatched eggs in successive egg batches were recorded. Field-hatching was recorded from late winter onwards. The first hatchings occurred in conditions probably not favorable to complete development into reproductive adults. A progressive decrease in live eggs was observed, with 51% of the recovered eggs alive after 12 weeks of exposure. In the laboratory, the hatching response in the first immersion was low for the initial control and for the eggs exposed for 3 weeks, and increased for successive cohorts. The results confirm that the survival of Ae. aegypti eggs in the winter-spring transition ensures persistence throughout the next favorable season in Buenos Aires City. The observed inhibition to hatch of the first batches might relate to a photoperiod-induced diapause, as observed in previous studies.

Semi-field and surveillance data define the natural diapause timeline for Culex pipiens across the United States

Reproductive diapause serves as biological mechanism for many insects, including the mosquito Culex pipiens, to overwinter in temperate climates. While Cx. pipiens diapause has been well-studied in the laboratory, the timing and environmental signals that promote diapause under natural conditions are less understood. In this study, we examine laboratory, semi-field, and mosquito surveillance data to define the approximate timeline and seasonal conditions that contribute to Cx. pipiens diapause across the United States. While confirming integral roles of temperature and photoperiod in diapause induction, we also demonstrate the influence of latitude, elevation, and mosquito population genetics in shaping Cx. pipiens diapause incidence across the country. Coinciding with the cessation of WNV activity, these data can have important implications for mosquito control, where targeted efforts prior to diapause induction can decrease mosquito populations and WNV overwintering to reduce mosquito-borne disease incidence the following season.

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.

A Comparative Study of Circadian Rhythmicity and Photoperiodism in Closely Related Species of Blow Flies: External Coincidence, Maternal Induction, and Diapause at Northern Latitudes.

This review compares adult locomotor activity rhythms and photoperiodic induction of diapause in 3 common species of blow fly, Calliphora vicina, Lucilia sericata, and Protophormia terraenovae. Activity rhythms were broadly similar in all 3 species, although P. terraenovae is much less sensitive to constant light inducing arrhythmicity. Photoperiodic induction of diapause, on the other hand, varies more widely between species. C. vicina and L. sericata overwinter in a larval diapause induced by autumnal short days (long nights) acting both maternally and directly upon the larvae. P. terraenovae, on the other hand, shows an adult (reproductive) diapause induced by short daylength and low temperature experienced by the larvae. In the Nanda-Hamner protocol, C. vicina shows 3 clear peaks of high diapause incidence in cycle lengths close to 24, 48, and 72 h, without dampening and therefore suggesting a photoperiodic mechanism based on a self-sustained circadian oscillator acting in a clock of the external coincidence type. Entrainment of the locomotor activity rhythm to extended Nanda-Hamner photocycles, as well as to LD cycles close to the limits of the primary range of entrainment, demonstrates that overt circadian rhythmicity may act as ‘hands’ of the otherwise covert photoperiodic system, as suggested by Bünning, nearly 8 decades ago. In 24 h LD cycles, both locomotor activity rhythms and the photoperiodic oscillator are set to constant phase (CT 12) at light-off, so that the photoperiodic clock measures changes in nightlength by the coincidence (or not) of dawn light with a ‘photoinducible phase’ late in the subjective night (at about CT 21.5 h) as photoperiod changes with the seasons. Apparent differences between quantitative and qualitative photoperiodic responses are discussed.

The expression of circadian clock genes in Daphnia magna diapause

Diapause is a mechanism necessary for survival in arthropods. Often diapause induction and resurrection is light-dependent and therefore dependent on the photoperiod length and on the number of consecutive short-days. In many organisms, including the microcrustacean Daphnia magna, one functional entity with the capacity to measure seasonal changes in day-length is the circadian clock. There is a long-standing discussion that the circadian clock also controls photoperiod-induced diapause. We tested this hypothesis in D. magna, an organism which goes into a state of suspended animation with the shortening of the photoperiod. We measured gene expression of clock genes in diapause-destined embryos of D. magna in the initiation, resting and resurrection phases and checked it against gene expression levels of continuously developing embryos. We demonstrate that some genes of the clock are differentially expressed during diapause induction but not during its maintenance. Furthermore, the photoreceptor gene cry2 and the clock-associated gene brp are highly expressed during induction and early diapause, probably in order to produce excess mRNA to prepare for immediate resurrection. After resurrection, both types of embryos show a similar pattern of gene expression during development. Our study contributes significantly to the understanding of the molecular basis of diapause induction, maintenance and termination.

Grandmaternal temperature effect on diapause induction in Trichogramma telengai (Hymenoptera: Trichogrammatidae)

Adaptive grandmaternal thermal effect, wherein the grandmaternal thermal environment affects the induction of progeny diapause two generations later, has not been studied in any insect system. We have studied this effect in the parthenogenetic egg parasitoid Trichogramma telengai Sor. (Hymenoptera: Trichogrammatidae) under laboratory conditions. The grandmaternal generation developed at temperatures from 18 to 30 °C combined with short (L:D = 12:12) or long (L:D = 18:6) photoperiod. The maternal generation developed at the same two photoperiods combined with low (20 °C) or high (30 °C) temperature. The progeny generation developed at 14 °C and L:D = 12:12. The grandmaternal temperature response was consistent with an adaptive response (low temperature induces facultative prepupal winter diapause two generations later) and rather strong (ranged up to 20–25%). The experiments suggested that both grandmaternal and maternal thermal effects are based on the influence of temperature on photoperiodic induction of diapause. However, experiments also revealed substantial differences between the grandmaternal and maternal thermal responses. In particular, the grandmaternal thermal response was observed only at short-day grandmaternal photoperiod, whereas the maternal thermal response was independent both of maternal and grandmaternal photoperiods. Although under natural conditions the adaptive value of the grandmaternal thermal response in T. telengai is most probably low, this effect should be considered in physiological models of diapause induction and can be important for mass rearing of Trichogramma species.

The Effects of Photoperiod on Diapause Induction in Hypena opulenta (Lepidoptera: Erebidae), a Biological Control Agent Against Invasive Swallow-Worts in North America.

Many insects exhibit a short-day diapause response, whereby diapause is induced when daylength falls below a critical threshold. This response is an adaptation to ensure synchrony between periods of insect activity, and the availability of resources, but it can cause problems when organisms are moved to new locations, where early or late-induced diapause can prove a barrier to establishment. We explored the role of photoperiod in diapause induction in Hypena opulenta, a recently introduced classical biological control agent for invasive swallow-worts in North America. We conducted four experimental cage releases as well as a growth chamber experiment to determine the threshold photoperiod for diapause induction in H. opulenta. We determined that the critical photoperiod for inducing diapause in 50% of H. opulenta is 15 h 35 min, which the moth only experiences in the Ottawa release site around summer solstice. This may lead to univoltinism, premature diapause, and poor establishment at some North American release sites. Our results can inform practical aspects of the biological control program for H. opulenta, such as fine-tuning methodologies for stockpiling diapausing pupae in the laboratory and narrowing down the optimal time window for releases at a given location. Additionally, our results will be important for the development of a temperature-based phenology model to more accurately predict voltinism in H. opulenta across the invasive range of swallow-worts in North America.

Latitudinal Variation in Circadian Rhythmicity in Nasonia vitripennis.

Many physiological processes of living organisms show circadian rhythms, governed by an endogenous clock. This clock has a genetic basis and is entrained by external cues, such as light and temperature. Other physiological processes exhibit seasonal rhythms, that are also responsive to light and temperature. We previously reported a natural latitudinal cline of photoperiodic diapause induction in the parasitic wasp Nasonia vitripennis in Europe and a correlated haplotype frequency for the circadian clock gene period (per). To evaluate if this correlation is reflected in circadian behaviour, we investigated the circadian locomotor activity of seven populations from the cline. We found that the proportion of rhythmic males was higher than females in constant darkness, and that mating decreased rhythmicity of both sexes. Only for virgin females, the free running period (τ) increased weakly with latitude. Wasps from the most southern locality had an overall shorter free running rhythm and earlier onset, peak, and offset of activity during the 24 h period, than wasps from the northernmost locality. We evaluated this variation in rhythmicity as a function of period haplotype frequencies in the populations and discussed its functional significance in the context of local adaptation.

Functional identification of an FMRFamide-related peptide gene on diapause induction of the migratory locust, Locusta migratoria L

FMRFamide-related peptides (FaRPs) are a type of neuropeptide, which participate in a variety of physiological processes in insects. Previous study showed that myosuppressin, being a member of FaRPs, initiated pupal diapause in Mamestra brassicae. We presumed that FaRPs genes might play a critical role in photoperiodic diapause induction of L. migratoria. To verify our hypothesis, flrf, a precursor gene of FaRP from L. migratoria, was initially cloned under long and short photoperiods that encoded by flrf gene identified from central nervous system (CNS). Phylogenetic analysis showed that the protein encoded by L. migratoria flrf gene, clustered together with Nilaparvata lugens (Hemiptera: Delphacidae) with 100% bootstrap support, was basically an FMRFamide precursor homologue. We noticed the availability of -RFamide peptides (GSERNFLRFa, DRNFIRFa) under short photoperiod only, which suggested their functions related to photoperiodic diapause induction. RNAi and quantitative real-time PCR (qRT-PCR) results further confirmed that the flrf gene promoted locust's diapause.

Molecular identification and diapause-related functional characterization of a novel dual-specificity kinase gene, MPKL, in Locusta migratoria.

Diapause is an important overwintering strategy enabling Locustamigratoria to survive under stressed conditions. We identified a novel dual-specificity kinase gene that is differentially expressed between long and short day-treated L.migratoria. To determine its function on photoperiodic diapause induction, we cloned the specific gene. Interestingly, phylogenetic analysis shows that this dual-specificity kinase is of the mycetozoa protein kinase-like (MPKL) type and may have been transferred horizontally from Mycetozoa to L.migratoria. RNA interference results confirm that MPKL promotes photoperiodic diapause induction of L.migratoria. Furthermore, MPKL significantly inhibits Akt and FOXO (i.e. forkhead box protein O) phosphorylation levels in ovaries, and also enhances reactive oxygen species, superoxide dismutase and catalase activities, whereas peroxidase activity is decreased under both photoperiodic regimes. The findings of the present study offer insight into the molecular mechanism responsible for dual-specificity kinase-induced diapause in insects.

Photoperiodic modulation of diapause induction and termination in Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae), an egg parasitoid of the invasive emerald ash borer

Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is a solitary and parthenogenetic egg parasitoid from China being introduced into North America (NA) as a biological control agent of the emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), an extremely invasive and destructive pest of ash trees (Fraxinus spp.). Oobius agrili is being released over a broad geographical range in NA where photoperiod varies considerably during the season of parasitoid-host activity. We conducted laboratory studies to determine 1) if photoperiod-induced diapause is modulated maternally, grand-maternally, and/or directly in immature parasitoids; 2) interactions of maternal adult age and photoperiod exposure on diapause induction; 3) the critical day length for diapause induction; 4) the critical age at which photoperiod-induced diapause is modulated in developing larvae; 5) the effects of photoperiod and length of chill on diapause termination; and, 6) the effects of photoperiod on O. agrili biology across a latitudinal gradient. We found that photoperiod exposure of O. agrili larvae developing inside host eggs directly induced diapause, and maternal or grand-maternal photoperiod treatments did not affect diapause induction in their progeny. However, older adults that experienced diapause as larvae produced more progeny that entered diapause when their progeny were exposed to short-day photoperiods. All progeny produced by adults that developed from nondiapaused larvae entered diapause when exposed to short-day photoperiods. Diapause response to photoperiod declined dramatically after larvae were 6–7 days old. The critical day length for diapause induction was between 14.25 and 14.5 h of daylight (at 25 °C). Photoperiod and duration of chill affected diapause termination of diapausing O. agrili larvae. The cumulative number of degree days (base 10 °C) required for adult emergence was highest for the combination of 12 h light:12 h dark (12L:12D) photoperiod combined with the shortest chill period, and lowest for 14.5L:9.5D and 16L:8D combined with the longest chill period. We discuss the effects of photoperiod on parasitoid-host synchrony, population dynamics, and fitness of O. agrili across the geographical area where it is being released.

Adaptive Differences in Circadian Clock Gene Expression Patterns and Photoperiodic Diapause Induction in Nasonia vitripennis.

Day length (photoperiod) and temperature oscillate daily and seasonally and are important cues for season-dependent behavior. Larval diapause of the parasitoid Nasonia vitripennis is maternally induced following a certain number of days (switch point) of a given critical photoperiod (CPP). Both the switch point and the CPP follow a latitudinal cline in European N. vitripennis populations. We previously showed that allelic frequencies of the clock gene period correlate with this diapause induction cline. Here we report that circadian expression of four clock genes-period (per), cryptochrome-2 (cry-2), clock (clk), and cycle (cyc)-oscillates as a function of photoperiod and latitude of origin in wasps from populations from the extremes of the cline. Expression amplitudes are lower in northern wasps, indicating a weaker, more plastic clock. Northern wasps also have a later onset of activity and longer free-running rhythms under constant conditions. RNA interference of per caused speeding up of the circadian clock, changed the expression of other clock genes, and delayed diapause in both southern and northern wasps. These results point toward adaptive latitudinal clock gene expression differences and to a key role of per in the timing of photoperiodic diapause induction of N. vitripennis.

Distinct Physiological Mechanisms Induce Latitudinal and Sexual Differences in the Photoperiodic Induction of Diapause in a Fly

Many temperate insects enter diapause (dormancy) for overwintering in response to short days (long nights). A latitudinal cline in the critical day lengths for the photoperiodic induction of diapause has been reported in various insect species. However, the physiological mechanisms underlying this cline have remained elusive. We approached this issue in the flesh fly Sarcophaga similis, in which the photoperiodic time measurement system meets the "external coincidence model." In this model, measuring day lengths depends on whether the photoinducible phase (φi), determined by a circadian clock, is exposed to light or not. First, we detected a clear latitudinal cline in the critical day lengths of flies collected from 4 localities at different latitudes. The phase positions of the φi, which can be verified by night interruption photoperiods, also showed a clear latitudinal cline. This result supports the hypothesis that the latitudinal cline in the critical day length is produced by the difference in the phase positions of the φi among different strains. A sexual difference in the critical day length for photoperiodic induction has also been detected in various species. In this study, a sexual difference in the critical day length was observed in the southern strains but there was no sexual difference in the phase positions of the φi. This result indicates that both sexes measure photoperiods in the same manner. Males are less sensitive than females to the light pulse given at the φi, suggesting a quantitative difference in the photoperiodic time measurement and counter systems. This study clearly reveals that distinct mechanisms induce latitudinal and sexual differences in the critical day length for the photoperiodic induction of diapause in a fly.

Circadian clock genes link photoperiodic signals to lipid accumulation during diapause preparation in the diapause-destined female cabbage beetles Colaphellus bowringi

Many organisms have evolved a series of adaptions, such as dormancy or diapause in insects that enable them to withstand seasonally adverse conditions. In insects, photoperiodic signals received during the diapause induction phase have irreversible effect on diapause initiation. Insects continue to be exposed to diapause-inducing photoperiod after the diapause induction phase during diapause preparation before they enter diapause. However, how photoperiodic signals experienced during the diapause preparation phase (DPP) regulate diapause remains largely unclear. In this paper, we investigate this in the cabbage beetle Colaphellus bowringi. The cabbage beetle is in many respects an ideal experimental model in which to investigate the effect of photoperiodic signals on the DPP because it has facultative reproductive diapause induced by long-day length and has differentiable diapause induction and preparation phases. We found that the lipid content of female cabbage beetles decreased after diapause-destined (DD) individuals were exposed to a diapause-inhibiting photoperiod during the DPP. Two circadian clock negative regulators, per and tim, were probably involved in the photoperiodic response of beetles during the DPP. Per and tim presented obvious oscillation of circadian rhythm and photoperiodic response during the DPP in DD females and knock-down of these genes in DD females caused their lipid content to decrease. Per and tim probably promote lipid accumulation by regulating the expression of genes that regulate lipogenesis and lipolysis. Moreover, decreased lipid accumulation caused by exposure to different photoperiods during the DPP was independent of juvenile hormone. In summary, these results suggest that photoperiodic signals received during the DPP influence lipid accumulation in DD insects. DD insects still have some ability to monitor photoperiodic changes during the DPP and per and tim are probably involved in regulating physiological responses to photoperiodic signals during diapause preparation. These results shed light on the relationship between photoperiodic signals and diapause preparation, and may provide new insights on both how to better utilize insects as resources and for pest management.

Genetic variation underlying local adaptation of diapause induction along a cline in a butterfly.

Diapause is a life history strategy allowing individuals to arrest development until favourable conditions return, and it is commonly induced by shortened day length that is latitude specific for local populations. Although understanding the evolutionary dynamics of a threshold trait like diapause induction provides insights into the adaptive process and adaptive potential of populations, the genetic mechanism of variation in photoperiodic induction of diapause is not well understood. Here, we investigate genetic variation underlying latitudinal variation in diapause induction and the selection dynamics acting upon it. Using a genomewide scan for divergent regions between two populations of the butterfly Pararge aegeria that differ strongly in their induction thresholds, we identified and investigated the patterns of variation in those regions. We then tested the association of these regions with diapause induction using between-population crosses, finding significant SNP associations in four genes present in two chromosomal regions, one with the gene period, and the other with the genes kinesin, carnitine O-acetyltransferase and timeless. Patterns of allele frequencies in these two regions in population samples along a latitudinal cline suggest strong selection against heterozygotes at two genes within these loci (period, timeless). Evidence for additional loci modifying the diapause decision was found in patterns of allelic change in relation to induction thresholds over the cline, as well as in backcross analyses. Taken together, population-specific adaptations of diapause induction appear to be due to a combination of alleles of larger and smaller effect size, consistent with an exponential distribution of effect sizes involved in local adaption.

The Effect of Temperature and Photoperiod on Diapause Induction in Pupae of Scrobipalpa ocellatella (Lepidoptera: Gelechiidae).

Scrobipalpa ocellatella (Boyd) (Lepidoptera: Gelechiidae) is one of the most important pests of sugar beet that causes quantitative and qualitative yield loss in the late summer. To locate the position for diapause induction, combinations of constant temperatures at 15, 18, 20, and 25°C and day lengths of 8, 10, 12, 14, 16, and 24 h were studied from egg to adult emergence. The incidence of diapause peaked at 15 and 18°C, with the day lengths of 12 and 11 h, whereas low temperatures did not improve the effects of short photoperiods (day lengths of 8 and 10 h) in diapause induction. The results showed that the critical day length for diapause induction was 12.8 h at overall 15 and 18°C. It was observed that the third instar larvae were the most sensitive stage to the inductive photoperiod (12:12 [L:D] h). The non-24-h light-dark experiment showed that the nigh length is more important than the day lengths measurement. In a set of 24-h light-dark cycles at 2:12 (L:D) h, a 1-h light pulse declined diapause induction markedly 1 h after scotophase. Field monitoring of the S. ocellatella for 2 yr (2015 and 2016) showed that the 50% of larvae enter winter pupal diapause in early September and this proportion increases in response to a decrease in the day lengths and temperature. From this study, it was concluded that low temperature acts in conjunction with short-day photoperiod in diapause induction of S. ocellatella.

Gene expression profiling across ontogenetic stages in the wood white (Leptidea sinapis) reveals pathways linked to butterfly diapause regulation.

In temperate latitudes, many insects enter diapause (dormancy) during the cold season, a period during which developmental processes come to a standstill. The wood white (Leptidea sinapis) is a butterfly species distributed across western Eurasia that shows photoperiod-induced diapause with variation in critical day-length across populations at different latitudes. We assembled transcriptomes and estimated gene expression levels at different developmental stages in experimentally induced directly developing and diapausing cohorts of a single Swedish population of L.sinapis to investigate the regulatory mechanisms underpinning diapause initiation. Different day lengths resulted in expression changes of developmental genes and affected the rate of accumulation of signal molecules, suggesting that diapause induction might be controlled by increased activity of monoamine neurotransmitters in larvae reared under short-day light conditions. Expression differences between light treatment groups of two monoamine regulator genes (DDC and ST) were observed already in instar III larvae. Once developmental pathways were irreversibly set at instar V, a handful of genes related to dopamine production were differentially expressed leading to a significant decrease in expression of global metabolic genes and increase in expression of genes related to fatty acid synthesis and sequestration. This is in line with a time-dependent (hour-glass) model of diapause regulation where a gradual shift in the concentration of monoamine neurotransmitters and their metabolites during development of larvae under short-day conditions leads to increased storage of fat, decreased energy expenditures, and ultimately developmental stasis at the pupal stage.

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.

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.