Timing Of Diapause Research Articles

Investigating the physiological ecology of mesopelagic zooplankton in the Scotia sea (Southern ocean) using lipid and stable isotope signatures

The mesopelagic zooplankton community plays an important role in the cycling and sequestration of carbon via the biological pump. However, little is known about the physiology and ecology of key taxa found within this region, hindering our understanding of their influence on the pathways of energy and organic matter cycling. We sampled the eight most abundant zooplankton (Calanoides acutus, Rhincalanus gigas, Paraeuchaeta spp., Chaetognatha, Euphausia triacantha, Thysanoessa spp., Themisto gaudichaudii and Salpa thompsoni) from within the mesopelagic zone in the Scotia Sea during a sinking diatom bloom and investigated their physiological ecology using lipid biomarkers and stable isotopic signatures of nitrogen. Data suggest that the large calanoid copepods, C. acutus and R. gigas, were in, or emerging from, a period of metabolic inactivity during the study period (November 15th – December 15th, 2017). Abundant, but decreasing lipid reserves in the predominantly herbivorous calanoid copepods, suggest these animals may have been metabolising previously stored lipids at the time of sampling, rather than deriving energy solely from the diatom bloom. This highlights the importance of understanding the timing of diapause of overwintering species as their feeding is likely to have an impact on the turnover of particulate organic matter (POM) in the upper mesopelagic. The δ15N signatures of POM became enriched with increasing depth, whereas all species of zooplankton except T. gaudichaudii did not. This suggests that animals were feeding on fresher, surface-derived POM, rather than reworked particles at depth, likely influencing the quantity and quality of organic matter leaving the upper mesopelagic. Our study highlights the complexity of mesopelagic food webs and suggests that the application of broad trophic functional types may lead to an incorrect understanding of ecosystem dynamics.

Key roles of insulin receptor InR1 in initiating reproductive diapause in males of the cabbage beetle Colaphellus bowringi (Coleoptera: Chrysomelidae).

Reproductive diapause serves as a valuable strategy enabling insects to survive unfavorable seasonal conditions. However, forcing insects into diapause when the environment is conducive to their well-being can cause them to miss out on seasonal opportunities for reproduction. This outcome not only reduces insect populations but also minimizes crop losses caused by insect feeding. Therefore, altering the timing of diapause initiation presents a potential strategy for managing pests. In this study, we examined the possible role of the Insulin Receptor 1 (InR1) in controlling reproductive diapause entry in the male cabbage beetle, Colaphellus bowringi. Compared to short-day (SD) conditions, long-day (LD) conditions led to reproductive diapause of C. bowringi males, characterized by arrested gonad development, increased Triglyceride (TG) accumulation, and upregulated expression of diapause protein 1 and genes associated with lipogenesis and stress tolerance. Upon employing RNA interference to knock down InR1 under SD conditions, males destined for reproduction were compelled into diapause, evidenced by arrested gonadal development, accumulation of TG, and elevated expression of diapause-related genes. Intriguingly, despite the common association of the absence of juvenile hormone (JH) with reproductive diapause in females, the knockdown of InR1 in males did not significant affect the expression of JH biosynthesis and JH response gene. The study highlight InR1 is a key factor involved in regulating male reproductive diapause in C. bowringi. Consequently, targeting insulin signaling could be a viable approach to perturb diapause timing, offering a promising strategy for managing pests with reproductive diapause capabilities. © 2024 Society of Chemical Industry.

Variation in photoperiod response corresponds to differences in circadian light sensitivity in northern and southern Nasonia vitripennis lines

The circadian clock times physiological and behavioural processes and resets on a daily basis to synchronize with the environment. The involvement of the circadian clock in photoperiodic time measurement synchronising annual rhythms is still under debate and different models have been proposed explaining their integration. Insects overcome unfavourable conditions in diapause, a form of dormancy. A latitudinal cline in diapause induction in the parasitoid wasp Nasonia vitripennis as well as a difference in circadian light sensitivity between north and south provide us with additional evidence that the circadian system of Nasonia is involved in photoperiodic time measurement and that latitude-specific seasonality drives adaptive evolution in photoperiodism partly through adaptation responses in the circadian system. We tested diapause induction in a range of T-cycles and photoperiods and found diapause induction in short photoperiods in all T-cycles in the northern line but in the southern line, diapause only occurred in T-cycles close to 24 h. Due to a lower light sensitivity in the southern line, a wider distribution of phase angles of entrainment can be expected at a specific T-cycle duration, while the range of entrainment will decrease. Taking these oscillator properties into account, our data can be explained by an external coincidence model involving a single oscillator with a light-sensitive phase that drives annual timing of diapause in Nasonia vitripennis.

A range‐expanding butterfly is susceptible to cold and long winters but shows no signs of local adaptation to winter conditions

Abstract Numerous species shift or expand their ranges poleward in response to climate change. Even when expanding species follow their climatic niches, expanding range margin populations are likely to face unfamiliar environmental conditions and thus natural selection for local adaptation. The wall brown butterfly (Lasiommata megera) has expanded northward in Sweden in the years 2000–2020, most likely as a result of climate change, and has previously been shown to have evolved local adaptations to northern daylength conditions. This evolution has occurred despite hypothesised genetic constraints to adaptation at range margins. We studied local adaptation to winter conditions in four of the previously‐studied L. megera populations, using a common garden laboratory experiment with a warm and short, an intermediate, and a cold and long winter treatment. We compared the winter and post‐winter survival of caterpillars from two southern core range and two northern range margin populations in Sweden. During the experiment, we measured metabolic rates of a subset of diapausing caterpillars to test whether populations differ in metabolic suppression during diapause. Further, we measured supercooling points, which reflect lower lethal temperature in L. megera, of the same subset of caterpillars. We also compared supercooling points between L. megera and three closely related species with more northern distributions. Few individuals survived the coldest treatment all the way to successful adult emergence, so L. megera seems susceptible to cold winters. Individuals of northern descent did not survive cold winters any better than individuals from southern populations. Similarly, there were no signs of local adaptation in metabolic rates or supercooling points. The comparison among species did not reveal any clear relationship between geographical distribution and supercooling point. Although northern winters probably exert strong selection on L. megera, we provide comprehensive evidence for the lack of local adaptation to winter conditions. This contrasts with the previous finding of quickly evolved local adaptation in diapause timing, highlighting the need to consider how traits associated with different seasons differ in how they may evolve and facilitate climate change‐induced range expansions. Read the free Plain Language Summary for this article on the Journal blog.

Divergence in cell cycle progression is associated with shifted phenology in a multivoltine moth: the European corn borer, Ostrinia nubilalis.

Evolutionary change in diapause timing can be an adaptive response to changing seasonality, and even result in ecological speciation. However, the molecular and cellular mechanisms regulating shifts in diapause timing remain poorly understood. One of the hallmarks of diapause is a massive slowdown in the cell cycle of target organs such as the brain and primordial imaginal structures, and resumption of cell cycle proliferation is an indication of diapause termination and resumption of development. Characterizing cell cycle parameters between lineages differing in diapause life history timing may help identify molecular mechanisms associated with alterations of diapause timing. We tested the extent to which progression of the cell cycle differs across diapause between two genetically distinct European corn borer strains that differ in their seasonal diapause timing. We show the cell cycle slows down during larval diapause with a significant decrease in the proportion of cells in S phase. Brain-subesophageal complex cells slow primarily in G0/G1 phase whereas most wing disc cells are in G2 phase. Diapausing larvae of the earlier emerging, bivoltine E-strain (BE) suppressed cell cycle progression less than the later emerging, univoltine Z-strain (UZ) individuals, with a greater proportion of cells in S phase across both tissues during diapause. Additionally, resumption of cell cycle proliferation occurred earlier in the BE strain than in the UZ strain after exposure to diapause-terminating conditions. We propose that regulation of cell cycle progression rates ultimately drives differences in larval diapause termination, and adult emergence timing, between early- and late-emerging European corn borer strains.

Rapid brain development and reduced neuromodulator titres correlate with host shifts in Rhagoletis pomonella.

Host shifts are considered a key generator of insect biodiversity. For insects, adaptation to new host plants often requires changes in larval/pupal development and adult behavioural preference toward new hosts. Neurochemicals play key roles in both development and behaviour and therefore provide a potential source for such synchronization. Here, we correlated life-history timing, brain development and corresponding levels of 14 neurochemicals in Rhagoletis pomonella (Diptera: Tephritidae), a species undergoing ecological speciation through an ongoing host shift from hawthorn to apple fruit. These races exhibit differences in pupal diapause timing as well as adult behavioural preference with respect to their hosts. This difference in behavioural preference is coupled with differences in neurophysiological response to host volatiles. We found that apple race pupae exhibited adult brain morphogenesis three weeks faster after an identical simulated winter than the hawthorn race, which correlated with significantly lower titres of several neurochemicals. In some cases, particularly biogenic amines, differences in titres were reflected in the mature adult stage, when host preference is exhibited. In summary, life-history timing, neurochemical titre and brain development can be coupled in this speciating system, providing new hypotheses for the origins of new species through host shifts.

Combining photoperiod and thermal responses to predict phenological mismatch for introduced insects.

A wide variety of organisms use the regular seasonal changes in photoperiod as a cue to align their life cycles with favorable conditions. Yet the phenological consequences of photoperiodism for organisms exposed to new climates are often overlooked. We present a conceptual approach and phenology model that maps voltinism (generations per year) and the degree of phenological mismatch that can arise when organisms with a short-day diapause response are introduced to new regions or are otherwise exposed to new climates. Our degree-day-based model combines continent-wide spatialized daily climate data, calculated date-specific and latitude-specific day lengths, and experimentally determined developmental responses to both photoperiod and temperature. Using the case of the knotweed psyllid Aphalara itadori, a new biological control agent being introduced from Japan to North America and Europe to control an invasive weed, we show how incorporating a short-day diapause response will result in geographic patterns of attempted voltinism that are strikingly different from the potential number of generations based on degree-days alone. The difference between the attempted and potential generations represents a quantitative measure of phenological mismatch between diapause timing and the end of the growing season. We conclude that insects moved from lower to higher latitudes (or to cooler climates) will tend to diapause too late, potentially resulting in high mortality from inclement weather, and those moved from higher to lower latitude (to warmer climates) may be prone to diapausing too early, therefore not fully exploiting the growing season and/or suffering from insufficient reserves for the longer duration in diapause. Mapped output reveals a central region with good phenology match that shifts north or south depending on the geographic source of the insect and its corresponding critical photoperiod for diapause. These results have direct relevance for efforts to establish populations of classical biocontrol agents. More generally, our approach and model could be applied to a wide variety of photoperiod- and temperature-sensitive organisms that are exposed to changes in climate, including resident and invasive agricultural pests and species of conservation concern.

Portable locomotion activity monitor (pLAM): A cost‐effective setup for robust activity tracking in small animals

Abstract Characterising the frequency and timing of biological processes such as locomotion, eclosion or foraging, is often needed to get a complete picture of a species' ecology. Automated trackers are an invaluable tool for high‐throughput collection of activity data and have become more accurate and efficient with advances in computer vision and deep learning. However, tracking activity of small and fast flying animals remains a hurdle, especially in a field setting with variable light conditions. Commercial activity monitors can be expensive, closed source and generally limited to laboratory settings. Here, we present a portable locomotion activity monitor (pLAM), a mobile activity detector to quantify small animal activity. Our setup uses inexpensive components, builds upon open‐source motion tracking software, and is easy to assemble and use in the field. It runs off‐grid, supports low‐light tracking with infrared lights and can implement arbitrary light cycle colours and brightnesses with programmable LEDs. We provide a user‐friendly guide to assembling pLAM hardware, accessing its pre‐configured software and guidelines for using it in other systems. We benchmarked pLAM for insects under various laboratory and field conditions, then compared results to a commercial activity detector. They offer broadly similar activity measures, but our setup captures flight and bouts of motion that are often missed by beam breaking activity detection. pLAM can automate laboratory and field monitoring of activity and timing in a wide range of biological processes, including circadian rhythm, eclosion and diapause timing, pollination and flower foraging, or pest feeding activity. This low cost and easy setup allows high‐throughput animal behaviour studies for basic and applied ecology and evolution research.

Simulation of Diapause Induction in Spider Mites (Tetranychus urticae and T. kanzawai) by Reproducing Field Environments in the Laboratory

‌Backgroud: Diapause as the basic mechanism, helps insects and mites to synchronize their life cycles with local seasonal changes. Field tests are desirable to determine the timing of diapause accurately, but the environments are variable, making it challenging to validate the reproducibility of results. Materials and Methods: We designed an environmental simulator that reproduces the field variation in the laboratory based on the installation of 20 years of climate data in the Hokkaido region. We computed a regression equation to predict the light intensity and temperature between the laboratory simulator and field data and its accuracy was clearly demonstrated. Findings: Photoperiodic reaction curves of T. urticae and T. kanzawai indicated a generic short-day reaction type with critical photoperiods at 18.0°C of approximately 13.5 h for T. urticae and 12.5 h for T. kanzawai. Diapause of T. urticae and T. kanzawai in the field was induced on 22 and 23 September, respectively, which is close to the dates of diapause induction estimated by using critical photoperiods (19 and 30 September, respectively). Little variation between observed and estimated critical photoperiods of the mentioned species may be due to minor deviations in temperature in the simulator. Conclusion: The present study demonstrates the importance of factoring in variable temperatures in the field for accurate prediction of the timing of diapause induction in spider mites. Our experimental system may also be useful in pest mite forecasting by predicting its diapause termination and spring emergence in any geographic area where environmental data are available.

Do genetic differences in growth thermal reaction norms maintain genetic variation in timing of diapause induction?

AbstractAn optimal timing for diapause induction through the sexual production of dormant propagules is expected in organisms with temporary populations. Yet, empirical studies often find high within‐population genetic variation in the sexual production of such propagules, suggesting that this is a common feature of such organisms.Here, we hypothesize that genetic variation in the propensity to produce dormant propagules,Pd, is maintained by a genotype‐by‐environment interaction in clonal reproductive rates, where fast‐growing genotypes within an environment should delay diapause relative to slow‐growing genotypes. From this, we derive two predictions. First, if reaction norms of clonal reproduction cross between two environments, the genetic correlation ofPdbetween these environments should be negative. Second, the correlation between plasticity values of clonal reproduction andPdshould be negative.We tested these predictions by quantifying ephippia production in genotypes of a population of the facultative sexual cladoceranDaphnia magnaat two temperatures. The population biomass at the onset of ephippia production was used as a measure ofPd, whereas juvenile somatic growth rate was used as a proxy for clonal reproductive rate. Plasticity for both measurements was derived from thermal reaction norms.Our results did not support either prediction, as neither the genetic correlation ofPdbetween environments, nor the correlation between plasticity values of growth andPdwere found to be significant.Our results suggest that genetic variation in the timing of diapause is not maintained by genetic differences in thermal clonal reproduction reaction norms. We propose as an alternative hypothesis that if there is variation across years in how the environment deteriorates over a season, fluctuating selection may favor genotypes with differentPdbetween years.

Timing of Calanus finmarchicus diapause in stochastic environments

In environments with strong seasonality, many herbivorous zooplankton remain active only during the productive season and undergo a period of inactivity and suppressed development termed ‘diapause’ during the unproductive season. The ability to time the diapause entry and exit in response to the seasonality of the environment is thus essential for their survival. However, timing of diapause may become challenging when environmental conditions vary stochastically across shorter and longer timescales, and particularly when zooplankton lack external cues to predict these variations. In this study, we used a novel individual-based model to study the emerging patterns of diapause timing of the high-latitude marine herbivorous copepod Calanus finmarchicus under shorter- (6-h) and longer-term (interannual) environmental stochasticity. The model simulated growth, development, survival and reproduction (income breeding) of a C. finmarchicus population over multiple calendar years and traced the emergence of behavioral responses and life history strategies. The emergent timing of diapause entry and exit were robust to shorter-term environmental stochasticity, which was manifested through morphological (i.e., body and energy reserve sizes) and behavioral plasticity (i.e., diel vertical migration). Longer-term stochastic variations of temperature and food environments altered the timing of diapause entry, which occurred earlier in warmer years with higher growth potential and vice versa. Irrespective of the modelled environmental variability, diapause exit occurred asynchronously throughout the year. This appeared to be a consequence of a diversified bet hedging strategy, where parents spread the starvation mortality risk of ascending to the upper pelagial at food-deprived times of the year among their offspring. This was a potent strategy, particularly in simulations where the timing of the algal bloom varied stochastically between years, since a fraction of the population was present in the upper pelagial year-round and those that coincided with the emergence of the pelagic primary production survived and produced the next generation.

Contrast in Post-Chill Eclosion Time Strategies Between Two Specialist Braconid Wasps (Hymenoptera: Braconidae) Attacking Rhagoletis Flies (Diptera: Tephritidae) in Western North America.

Parasitoids comprise a speciose insect group, displaying a wide array of life history strategies. In the Pacific Northwest of the United States, the tephritid fruit flies Rhagoletis tabellaria (Fitch) and Rhagoletis indifferens Curran infest red osier dogwood, Cornus sericea L. (Cornaceae), and bitter cherry, Prunus emarginata (Douglas ex Hooker) Eaton (Rosaceae), respectively. The flies are parasitized by different braconid wasps at different life stages; Utetes tabellariae (Fischer) oviposits into R. tabellaria eggs, whereas Diachasma muliebre (Muesebeck) oviposits into R. indifferens larvae feeding in cherries. Because Rhagoletis only have one major generation a year and the wasps attack temporally distinct fly life stages, we predicted that eclosion times of U. tabellariae should more closely follow that of its host than the larval-attacking D. muliebre. As predicted, U. tabellariae eclosed on average 6.0-12.5 d later than R. tabellaria, whereas D. muliebre eclosed on average 32.1 d after R. indifferens. Unexpectedly, however, longer chill duration differentially affected the systems; longer overwinters minimally influenced eclosion times of R. tabellaria and U. tabellariae but caused earlier eclosion of both R. indifferens and D. muliebre. Results imply that in temperate regions, diapause timing in braconid wasps evolves in response to both host life stage attacked and fly eclosion characteristics, possibly reflecting differential effects of winter on host plant fruiting phenology. Differences in phenological sensitivity of the lower host plant trophic level to variation in environmental conditions may have cascading effects, sequentially and differentially affecting eclosion times in higher frugivore (fly) and parasitoid (wasp) trophic levels.

Urban warming and artificial light alter dormancy in the flesh fly.

Seasonal changes in temperature and day length are distinct between rural and urban areas due to urban warming and the presence of artificial light at night. Many studies have focused on the impacts of these ubiquitous signatures on daily biological events, but empirical studies on their impacts on insect seasonality are limited. In the present study, we used the flesh fly Sarcophaga similis as a model insect to determine the impacts of urbanization on the incidence and timing of diapause (dormancy), not only in the laboratory but also in rural and urban conditions. In the laboratory, diapause entry was affected by night-time light levels as low as 0.01 lux. We placed fly cages on outdoor shelves in urban and rural areas to determine the timing of diapause entry; it was retarded by approximately four weeks in urban areas relative to that in rural areas. Moreover, almost all flies in the site facing an urban residential area failed to enter diapause, even by late autumn. Although an autumnal low temperature in the urban area would mitigate the negative effect of artificial light at night, strong light pollution seriously disrupts the flesh fly seasonal adaptation.

Diapause and bet‐hedging strategies in insects: a meta‐analysis of reaction norm shapes

Many organisms escape from lethal climatological conditions by entering a resistant resting stage called diapause, which needs to be optimally timed with seasonal change. As climate change exerts selection pressure on phenology, the evolution of mean diapause timing, but also of phenotypic plasticity and bet‐hedging strategies is expected. The potential of the latter strategy as a means of coping with environmental unpredictability has received little attention in the climate change literature.Populations should be adapted to spatial variation in local conditions; contemporary patterns of phenological strategies across a geographic range may hence provide information about their evolvability. We thus extracted 458 diapause reaction norms from 60 studies. First, we correlated mean diapause timing with mean winter onset. Then we partitioned the reaction norm variance into a temporal component (phenotypic plasticity) and among‐offspring variance (diversified bet‐hedging) and correlated this variance composition with variability of winter onset. Mean diapause timing correlated reasonably well with mean winter onset, except for populations at high latitudes, which apparently failed to track early onsets. Variance among offspring was, however, limited and correlated only weakly with environmental variability, indicating little scope for bet‐hedging. The apparent lack of phenological bet‐hedging strategies may pose a risk in a less predictable climate, but we also highlight the need for more data on alternative strategies.

Divergence in Photoperiod Responses of a Classical Biological Control Agent, Galerucella calmariensis (Coleoptera: Chrysomelidae), Across a Climatic and Latitudinal Gradient.

A key knowledge gap in classical biological control is to what extent insect agents evolve to novel environments. The introduction of biological control agents to new photoperiod regimes and climates may disrupt the coordination of diapause timing that evolved to the growing season length in the native range. We tested whether populations of Galerucella calmariensis L. have evolved in response to the potential mismatch of their diapause timing since their intentional introduction to the United States from Germany in the 1990s. Populations collected from 39.4° to 48.8° latitude in the western United States were reared in growth chambers to isolate the effects of photoperiod on diapause induction and development time. For all populations, shorter day lengths increased the proportion of beetles that entered diapause instead of reproducing. The critical photoperiods, or the day length at which half of a population diapauses, differed significantly among the sampled populations, generally decreasing at lower latitudes. The latitudinal trend reflects changes in growing season length, which determines the number of generations possible, and in local day lengths, at the time when beetles are sensitive to this cue. Development times were similar across populations, with one exception, and did not vary with photoperiod. These results show that there was sufficient genetic variation from the two German source populations to evolve different photoperiod responses across a range of environmental conditions. This study adds to the examples of rapid evolution of seasonal adaptations in introduced insects.

Lipid metabolism in Calanus finmarchicus is sensitive to variations in predation risk and food availability

Late developmental stages of the marine copepods in the genus Calanus can spend extended periods in a dormant stage (diapause) that is preceded by the accumulation of large lipid stores. We assessed how lipid metabolism during development from the C4 stage to adult is altered in response to predation risk and varying food availability, to ultimately understand more of the metabolic processes during development in Calanus copepods. We used RNA sequencing to assess if perceived predation risk in combination with varied food availability affects expression of genes associated with lipid metabolism and diapause preparation in C. finmarchicus. The lipid metabolism response to predation risk differed depending on food availability, time and life stage. Predation risk caused upregulation of lipid catabolism with high food, and downregulation with low food. Under low food conditions, predation risk disrupted lipid accumulation. The copepods showed no clear signs of diapause preparation, supporting earlier observations of the importance of multiple environmental cues in inducing diapause in C. finmarchicus. This study demonstrates that lipid metabolism is a sensitive endpoint for the interacting environmental effects of predation pressure and food availability. As diapause may be controlled by lipid accumulation, our findings may contribute towards understanding processes that can ultimately influence diapause timing.

Unique genetic signatures of local adaptation over space and time for diapause, an ecologically relevant complex trait, in Drosophila melanogaster.

Organisms living in seasonally variable environments utilize cues such as light and temperature to induce plastic responses, enabling them to exploit favorable seasons and avoid unfavorable ones. Local adapation can result in variation in seasonal responses, but the genetic basis and evolutionary history of this variation remains elusive. Many insects, including Drosophila melanogaster, are able to undergo an arrest of reproductive development (diapause) in response to unfavorable conditions. In D. melanogaster, the ability to diapause is more common in high latitude populations, where flies endure harsher winters, and in the spring, reflecting differential survivorship of overwintering populations. Using a novel hybrid swarm-based genome wide association study, we examined the genetic basis and evolutionary history of ovarian diapause. We exposed outbred females to different temperatures and day lengths, characterized ovarian development for over 2800 flies, and reconstructed their complete, phased genomes. We found that diapause, scored at two different developmental cutoffs, has modest heritability, and we identified hundreds of SNPs associated with each of the two phenotypes. Alleles associated with one of the diapause phenotypes tend to be more common at higher latitudes, but these alleles do not show predictable seasonal variation. The collective signal of many small-effect, clinally varying SNPs can plausibly explain latitudinal variation in diapause seen in North America. Alleles associated with diapause are segregating in Zambia, suggesting that variation in diapause relies on ancestral polymorphisms, and both pro- and anti-diapause alleles have experienced selection in North America. Finally, we utilized outdoor mesocosms to track diapause under natural conditions. We found that hybrid swarms reared outdoors evolved increased propensity for diapause in late fall, whereas indoor control populations experienced no such change. Our results indicate that diapause is a complex, quantitative trait with different evolutionary patterns across time and space.

Eat or Sleep: Availability of Winter Prey Explains Mid-Winter and Spring Activity in an Arctic Calanus Population

Copepods of the genus Calanus have adapted to high levels of seasonality in prey availability by entering a period of hibernation during winter known as diapause, but repeated observations of active Calanus spp. have been made in January in high latitude fjords which suggests plasticity in over-wintering strategies. During the last decade, the period of Polar Night has been studied intensively in the Arctic. A continuous presence of an active microbial food web suggests the prevalence of low-level alternative copepod prey (such as microzooplankton) throughout this period of darkness. Here we provide further evidence of mid-winter zooplankton activity using a decadal record of moored acoustics from Kongsfjorden, Svalbard. We apply an individual based life-history model to investigate the fitness consequences of a range of over-wintering strategies (in terms of diapause timing and duration) under a variety of prey availability scenarios. In scenarios of no winter prey availability (Pwin = 0 g C L􀀀1), the optimal time to exit diapause is in March. However, as Pwin increases (up to 40 g C L􀀀1), there is little fitness difference in copepods exiting diapause in January compared to March. From this, we suggest that Calanus are able (in energetic terms) to either i) exit diapause early to deal with uncertainty in spring bloom timing, or ii) remain active throughout winter if diapause is not possible (i.e. environment not deep enough, or not enough lipid reserves built up over the previous summer). The range of viable overwintering strategies increases with increasing Pwin, suggesting that there is more flexibility for Calanus spp. in a scenario of non-zero Pwin.

Can the genomics of ecological speciation be predicted across the divergence continuum from host races to species? A case study in Rhagoletis.

Studies assessing the predictability of evolution typically focus on short-term adaptation within populations or the repeatability of change among lineages. A missing consideration in speciation research is to determine whether natural selection predictably transforms standing genetic variation within populations into differences between species. Here, we test whether and how host-related selection on diapause timing associates with genome-wide differentiation during ecological speciation by comparing ancestral hawthorn and newly formed apple-infesting host races of Rhagoletis pomonella to their sibling species Rhagoletis mendax that attacks blueberries. The associations of 57 857 single nucleotide polymorphisms in a diapause genome-wide-association study (GWAS) on the hawthorn race strongly predicted the direction and magnitude of genomic divergence among the three fly populations at a field site in Fennville, MI, USA. The apple race and R. mendax show parallel changes in the frequencies of putative inversions on three chromosomes associated with the earlier fruiting times of apples and blueberries compared to hawthorns. A diapause GWAS on R. mendax revealed compensatory changes throughout the genome accounting for the earlier eclosion of blueberry, but not apple flies. Thus, a degree of predictability, although not complete, exists in the genomics of diapause across the ecological speciation continuum in Rhagoletis. The generality of this result is placed in the context of other similar systems. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

Latitudinal clines in the timing and temperature‐sensitivity of photoperiodic reproductive diapause in Drosophila montana

Reproductive diapause is a primary mechanism used by arthropods to synchronize their life cycle with seasonal changes in temperate regions. Our study species, Drosophila montana, represents the northern insect species where flies enter reproductive diapause under short day conditions and where the precise timing of diapause is crucial for both survival and offspring production. We have studied clinal variation in the critical day length for female diapause induction (CDL) and their overall susceptibility to enter diapause (diapause incidence), as well as the temperature sensitivity of these traits. The study was performed using multiple strains from four latitudinal clines of the species – short clines in Finland and Alaska and long clines in the Rocky Mountains and the western coast of North America – and from one population in Kamchatka, Russia. CDL showed strong latitudinal clines on both continents, decreasing by one hour per five degrees decline in latitude, on average. CDL also decreased in all populations along with an increase in fly rearing temperature postponing the diapause to later calendar time, the effects of temperature being stronger in southern than in northern population. Female diapause incidence was close to 100% under short day/low temperature conditions in all populations, but decreased below 50% even under short days in 19°C in the southern North American western coast populations and in 22°C in most populations. Comparing a diversity of climatic data for the studied populations showed that while CDL is under a tight photoperiodic regulation linked with latitude, its length depends also on climatic factors determining the growing season length. Overall, the study deepens our understanding of how spatial and environmental parameters affect the seasonal timing of an important biological event, reproductive diapause and helps to estimate the evolutionary potential of insect populations to survive in changing climatic conditions.