Explaining the sawtooth: latitudinal periodicity in a circadian gene correlates with shifts in generation number

Voltinism Shifts in Response to Climate Warming Generally Benefit Populations of Multivoltine Butterflies

Climate change is implicated as one contributor to insect declines. Insects may respond to warming by advancing phenology and increasing the number of generations each year (voltinism). However, if earlier phenology changes cue-response relationships, then late-season generations might lack time to complete diapause development before winter and result in doomed 'lost generations'. Using 27 years of monitoring of 30 multivoltine butterfly species, we find the opposite, as larger late-season generations (voltinism shifts) are associated with more positive overwinter population growth rates. The potential threat of lost generations is limited to late-season species at cooler sites in years with early frosts. Overall, long-term population trends are positively correlated with larger late-season generations, suggesting that they are an adaptive response to climate warming. Still, overwinter population growth rates and long-term population trends have declined over time as the benefits of voltinism shifts have been insufficient to reverse population declines.

Hybrids versus parental species: insights from wing phenotype similarities and differences in triatomine insects

Introduction

The genus Panstrongylus is one of the most important within the subfamily Triatominae, which includes vectors of Trypanosoma cruzi, the etiological agent of Chagas disease (CD). In particular, Panstrongylus chinai and P. howardi have drawn attention for their role in disease transmission. These species exhibit notable ecological and morphological differences. Previous studies have investigated aspects such as morphometry, cytogenetics, and ecological niches, including experimental crosses between these species that resulted in viable F1 hybrids. However, no F2 generation was produced, as the eggs laid were empty and failed to hatch, limiting the study to F1 hybrids.

Methods

We analyzed wing morphometric traits (size and shape) from 262 individuals, including P. chinai, P. howardi, and their hybrids, using geometric morphometry techniques. This study aimed to build upon previous findings by analyzing the wing morphometric and environmental adaptations of P. chinai, P. howardi, and their hybrids (♀P. howardi × ♂P. chinai) to determine whether the hybrids exhibited similarities in wing size and shape, regardless of maternal or paternal phenotype.

Results

Differences in centroid size were observed between the parental species, with P. howardi having a larger size, but no significant differences were found among the hybrids. Females showed greater shape similarity between P. howardi and the ♀P. chinai × ♂P. howardi hybrids, while males showed similarity among hybrids. Discriminant analysis was more effective for distinguishing parental groups than with hybrids. The K-means algorithm successfully classified the parental species and hybrid groups, although with low assignment percentages and a different number of groups than expected.

Discussion

The smaller wing size in hybrid offspring may indicate lower fitness, potentially due to genetic effects or reduced viability. Geometric morphometry effectively distinguishes parental species from hybrids, supporting previous research in Triatominae. The study suggests that environmental and reproductive pressures may influence these species and explores the dispersive capabilities of triatomines, contributing to the understanding of hybridization processes.

High voltinism, late-emerging butterflies are sensitive to interannual variation in spring temperature in North Carolina

Climate change has been repeatedly linked to phenological shifts in many taxa, but the factors that drive variation in phenological sensitivity remain unclear. For example, relatively little is known about phenological responses in areas that have not exhibited a consistent warming trend, making it difficult to project phenological responses in response to future climate scenarios for these regions. We used an extensive community science dataset to examine changes in the adult flight onset dates of 38 butterfly species with interannual variation in spring temperatures in the Piedmont region of North Carolina, a region that did not experience a significant overall warming trend in the second half of the 20th century. We also explored whether voltinism, overwintering stage, and mean adult flight onset dates explain interspecific variation in phenological sensitivity to spring temperature. We found that 12 out of 38 species exhibited a significant advance in adult flight onset dates with higher spring temperatures. In comparison, none of the 38 species exhibited a significant advance with year. There was a significant interaction between mean onset flight date and voltinism, such that late-emerging, multivoltine species tended to be the most sensitive to spring temperature changes. We did not observe a significant correlation between phenological sensitivity and the overwintering stage. These results suggest that butterfly arrival dates may shift as temperatures are projected to rise in the southeastern United States, with late-emerging, multivoltine species potentially exhibiting the greatest shifts in adult flight onset dates.

Efficient CRISPR/Cas9-mediated genome editing in the European corn borer, Ostrinia nubilalis

The European corn borer (Ostrinia nubilalis) is an agricultural pest and burgeoning model for research on speciation, seasonal adaptation and insect resistance management. Although previous work in O. nubilalis has identified genes associated with differences in life cycle, reproduction, and resistance to Bt toxins, the general lack of a robust gene-editing protocol for O. nubilalis has been a barrier to functional validation of candidate genes. Here, we demonstrate an efficient and practical methodology for heritable gene mutagenesis in O. nubilalis using the CRISPR/Cas9 genome editing system. Precise loss-of-function (LOF) mutations were generated at two circadian clock genes, period (per) and pigment-dispersing factor receptor (pdfr), and a developmental gene, prothoracicotropic hormone (ptth). Precluding the need for a visible genetic marker, gene-editing efficiency remained high across different single guide RNAs (sgRNA) and germline transmission of mutations to F1 offspring approached 100%. When single or dual sgRNAs were injected at a high concentration, gene-specific phenotypic differences in behaviour and development were identified in F0 mutants. Specifically, F0 gene mutants demonstrated that PER, but not PDFR, is essential for normal timing of eclosion. PTTH F0 mutants were significantly heavier and exhibited a higher incidence of diapause. This work will accelerate future studies of gene function in O. nubilalis and facilitate the development of similar screens in other Lepidopteran and non-model insects.

A supergene controls facultative diapause in the crop pest Helicoverpa armigera

Many insect species, including the economically important pest Helicoverpa armigera, avoid unfavorable conditions by suspending development. This form of phenotypic plasticity-facultative diapause-is a complex trait, though its evolution and intricate genetic architecture remain poorly understood. To investigate how such a polygenic trait could be locally adapted, we explore its genetic architecture. We map a large-effect diapause-associated locus to the Z chromosome by crossing high- and low-latitude populations. By generating multiple chromosome-scale assemblies, we identify an ∼5.93-Mb chromosomal inversion that constitutes the locus. Within this inversion, 33 genes harbor divergent non-synonymous mutations, notably including three circadian rhythm genes: Period, Clock, and Cycle. CRISPR-Cas9 knockout experiments confirm that each gene is independently essential for pupal diapause. Thus, a diapause supergene arose within H. armigera via a Z chromosome inversion, enabling local climatic adaptation in this economically important crop pest.

Forecasting habitat suitability and niche shifts of two global maize pests: Ostrinia furnacalis and Ostrinia nubilalis (Lepidoptera: Crambidae)

BACKGROUND

Ostrinia furnacalis (ACB) and Ostrinia nubilalis (ECB) are devastating pests of the agricultural crop maize worldwide. However, little is known about their potential distribution and niche shifts during their global invasion. Since long-term selection to past climate variability has shaped their historical niche breadth, such niche shifts may provide an alternative basis for understanding their responses to present and future climate change. By integrating the niche unfilling, stability, and expansion situations into a single framework, our study quantifies the patterns of niche shift in the spatial distribution of these two pests during the different periods.

RESULTS

Our results show that the overall suitable habitats of ACB and ECB in the future decrease but highly and extremely suitable habitat will become more widespread, suggesting these two insects may occur more frequently in specific regions. Compared with Southeast Asia and Australia, the ACB niche in China exhibited expansion rather than unfilling. For ECB, initial niches have a tendency to be retained in Eurasia despite there also being potential for expansion in North America. The niche equivalency and similarity test results further indicate that niche shifts were common for both ACB and ECB in different survival regions during their colonization of new habitat and their suitable habitat changes during the paleoclimate were associated with climatic changes.

CONCLUSIONS

These findings improve our understanding of the ecological characteristics of ACB and ECB worldwide, and will be useful in the development of prevention and control strategies for two insect pests worldwide. © 2024 Society of Chemical Industry.

Testing for variation in photoperiodic plasticity in a butterfly: Inconsistent effects of circadian genes between geographic scales

The genetic components of the circadian clock have been implicated as involved in photoperiodic regulation of winter diapause across various insect groups, thereby contributing to adaptation to adverse seasonal conditions. So far, the effects of within-population variation in these genes have not been well explored. Here, we present an experimental test of the effects of within-population variation at two circadian genes, timeless and period, on photoperiodic responses in the butterfly Pararge aegeria. While nonsynonymous candidate SNPs in both of these genes have previously shown to be associated with diapause induction on a between-population level, in the present experiment no such effect was found on a within-population level. In trying to reconcile these results, we examine sequence data, revealing considerable, previously unknown protein-level variation at both timeless and period across Scandinavian populations, including variants unique to the population studied here. Hence, we hypothesize that these variants may counteract the previously observed diapause-averting effect of the candidate SNPs, possibly explaining the difference in results between the experiments. Whatever the cause, these results highlight how the effects of candidate SNPs may sometimes vary across genetic backgrounds, which complicates evolutionary interpretations of geographic patterns of genetic variation.

Aseasonal, undirected migration in insects: 'Invisible' but common

Many insect pests are long-distance migrants, moving from lower latitudes where they overwinter to higher latitudes in spring to exploit superabundant, but seasonally ephemeral, host crops. These seasonal long-distance migration events are relatively easy to recognize, and justifiably garner much research attention. Evidence indicates several pest species that overwinter in diapause, and thus inhabit a year-round range, also engage in migratory flight, which is somewhat "invisible" because displacement is nondirectional and terminates among conspecifics. Support for aseasonal, undirected migration is related to recognizing true migratory flight behavior, which differs fundamentally from most other kinds of flight in that it is nonappetitive. Migrating adults are not searching for resources and migratory flight is not arrested by encounters with potential resources. The population-level consequence of aseasonal, undirected migration is spatial mixing of individuals within the larger metapopulation, which has important implications for population dynamics, gene flow, pest management, and insect resistance management.

Critical Facets of European Corn Borer Adult Movement Ecology Relevant to Mitigating Field Resistance to Bt-Corn

The European corn borer (Ostrinia nubilalis, Hübner) has been managed successfully in North America since 1996 with transgenic Bt-corn. However, field-evolved resistance to all four available insecticidal Bt proteins has been detected in four provinces of Canada since 2018. Evidence suggests resistance may be spreading and evolving independently in scattered hotspots. Evolution and spread of resistance are functions of gene flow, and therefore dispersal, so design of effective resistance management and mitigation plans must take insect movement into account. Recent advances in characterizing European corn borer movement ecology have revealed a number of surprises, chief among them that a large percentage of adults disperse from the natal field via true migratory behavior, most before mating. This undermines a number of common key assumptions about adult behavior, patterns of movement, and gene flow, and stresses the need to reassess how ecological data are interpreted and how movement in models should be parameterized. While many questions remain concerning adult European corn borer movement ecology, the information currently available is coherent enough to construct a generalized framework useful for estimating the spatial scale required to implement possible Bt-resistance prevention, remediation, and mitigation strategies, and to assess their realistic chances of success.

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.

Circadian and Neuroendocrine Basis of Photoperiodism Controlling Diapause in Insects and Mites: A Review

The photoperiodic system is concealed in the highly complex black-box, comprising four functional subunits: 1) a photo/thermo-sensitive input unit, 2) a photoperiodic clock based on a circadian system, 3) a condenser unit counting the number of inductive signals, and 4) a neuroendocrine switch that triggers a phenotypic shift. This review aims to summarize the research history and current reach of our understanding on this subject to connect it with the molecular mechanism of the circadian clock rapidly being unveiled. The review also focuses on the mode of intersubunit information transduction. It will scan the recent advancement in research on each functional subunit, but special attention will be given to the circadian clock-endocrine conjunct and the role of melatonin signaling in the regulation of insect photoperiodism. Prothoracicotropic hormone (PTTH) probably plays the most crucial role in the regulation of pupal diapause, which is the simplest model system of diapause regulation by hormones investigated so far, particularly in the Chinese oak silkmoth (Antheraea pernyi). A search for the trigger to release the PTTH found some candidates, that is, indoleamines. Indolamine metabolism is controlled by arylalkylamine N-acetyltransferase (aaNAT). Indolamine dynamics and aaNAT enzymatic activity changed according to photoperiods. aaNAT activity and melatonin content in the brain showed not only a photoperiodic response but also a circadian fluctuation. aaNAT had multiple E-boxes, suggesting that it is a clock-controlled gene (ccg), which implies that cycle (cyc, or brain-muscle Arnt-like 1 = Bmal1)/Clock (Clk) heterodimer binds to E-box and stimulates the transcription of aaNAT, which causes the synthesis of melatonin. RNAi against transcription modulators, cyc, or Clk downregulated aaNAT transcription, while RNAi against repressor of cyc/Clk, per upregulated aaNAT transcription. Immunohistochemical localization showed that the circadian neurons carry epitopes of melatonin-producing elements such as aaNAT, the precursor serotonin, HIOMT, and melatonin as well as clock gene products such as cyc-ir, Per-ir, and dbt-ir, while PTTH-producing neurons juxtaposed against the clock neurons showed hMT2-ir in A. pernyi brain. Melatonin probably binds to the putative melatonin receptor (MT) that stimulates Ca2+ influx, which in turn activates PKC. This induces Rab 8 phosphorylation and exocytosis of PTTH, leading to termination of diapause. All the PTTH-expressing neurons have PKC-ir, and Rab8-ir. When diapause is induced and maintained under short days, serotonin binding to 5HTR1B suppresses PTTH release in a yet unknown way. RNAi against this receptor knocked out photoperiodism; short day response is blocked and diapause was terminated even under the short day condition. The result showed that a relatively simple system controls both induction and termination in pupal diapause of A. pernyi: the circadian system regulates the transcription of aaNAT as a binary switch, the enzyme produces a melatonin rhythm that gates PTTH release, and 5HTR1B and MT are probably also under photoperiodic regulation. Finally, we listed the remaining riddles which need to be resolved, to fully understand this highly complex system in future studies.

Consequences of coupled barriers to gene flow for the build-up of genomic differentiation

Theory predicts that when different barriers to gene flow become coincident, their joint effects enhance reproductive isolation and genomic divergence beyond their individual effects, but empirical tests of this "coupling" hypothesis are rare. Here, we analyze patterns of gene exchange among populations of European corn borer moths that vary in the number of acting barriers, allowing for comparisons of genomic variation when barrier traits or loci are in coincident or independent states. We find that divergence is mainly restricted to barrier loci when populations differ by a single barrier, whereas the coincidence of temporal and behavioral barriers is associated with divergence of two chromosomes harboring barrier loci. Furthermore, differentiation at temporal barrier loci increases in the presence of behavioral divergence and differentiation at behavioral barrier loci increases in the presence of temporal divergence. Our results demonstrate how the joint action of coincident barrier effects leads to levels of genomic differentiation that far exceed those of single barriers acting alone, consistent with theory arguing that coupling allows indirect selection to combine with direct selection and thereby lead to a stronger overall barrier to gene flow. Thus, the state of barriers-independent or coupled-strongly influences the accumulation of genomic differentiation.

Local adaptation of life cycles in a butterfly is associated with variation in several circadian clock genes

Many insects exhibit geographical variation in voltinism, the number of generations produced per year. This includes high-latitude species in previously glaciated areas, meaning that divergent selection on life cycle traits has taken place during or shortly after recent colonization. Here, we use a population genomics approach to compare a set of nine Scandinavian populations of the butterfly Pararge aegeria that differ in life cycle traits (diapause thresholds and voltinism) along both north-south and east-west clines. Using a de novo-assembled genome, we reconstruct colonization histories and demographic relationships. Based on the inferred population structure, we then scan the genome for candidate loci showing signs of divergent selection potentially associated with population differences in life cycle traits. The identified candidate genes include a number of components of the insect circadian clock (timeless, timeless2, period, cryptochrome and clockwork orange). Most notably, the gene timeless, which has previously been experimentally linked to life cycle regulation in P. aegeria, is here found to contain a novel 97-amino acid deletion unique to, and fixed in, a single population. These results add to a growing body of research framing circadian gene variation as a potential mechanism for generating local adaptation of life cycles.

Influence of voltine ecotype and geographic distance on genetic and haplotype variation in the Asian corn borer

Diapause is an adaptive dormancy strategy by which arthropods endure extended periods of adverse climatic conditions. Seasonal variation in larval diapause initiation and duration in Ostrinia furnacalis may influence adult mating generation number (voltinism) across different local environments. The degree to which voltine ecotype, geographic distance, or other ecological factors influence O. furnacalis population genetic structure remains uncertain. Genetic differentiation was estimated between voltine ecotypes collected from 8 locations. Mitochondrial haplotypes were significantly different between historically allopatric univoltine and bivoltine locations, but confounded by a strong correlation with geographic distance. In contrast, single nucleotide polymorphism (SNP) genotypes show low but significant levels of variation and a lack of influence of geographic distance between allopatric voltine locations. Regardless, 11 of 257 SNP loci were predicted to be under selection, suggesting population genetic homogenization except at loci proximal to factors putatively under selection. These findings provide evidence of haplotype divergent voltine ecotypes that may be maintained in allopatric and sympatric areas despite relatively high rates of nuclear gene flow, yet influence of voltinism on maintenance of observed haplotype divergence remains unresolved.

The role of climate change in pollinator decline across the Northern Hemisphere is underestimated

Pollinator biodiversity loss occurs at unprecedented rates globally, with particularly sharp declines documented in the North Temperate Zone. There is currently no consensus on the main drivers of the decline. Although climate change is expected to drive biodiversity loss in the future, current warming is often suggested to have positive impacts on pollinator assemblages in higher latitudes. Consequently, pollinator conservation initiatives in Europe and the USA tend to lack climate adaptation initiatives, an omission of which may be risky if climate change has significant negative impacts on pollinators. To gain an understanding of the impacts of climate change on pollinator biodiversity in the Northern Hemisphere, we conducted a literature review on genetic, species and community level diversity. Our findings suggest that global heating most likely causes homogenization of pollinator assemblages at all levels of pollinator biodiversity, making them less resilient to future stochasticity. Aspects of biodiversity that are rarely measured (e.g. genetic diversity, β-diversity, species evenness) tend to be most affected, while some dimensions of climate change, such as fluctuations in winter weather conditions, changes in the length of the vegetational season and increased frequency of extreme weather events, that seldom receive attention in empirical studies, tend to be particularly detrimental to pollinators. Negative effects of global heating on pollinator biodiversity are most likely exacerbated by homogenous and fragmented landscapes, widespread across Europe and the US, which limit opportunities for range-shifts and reduce micro-climatic buffering. This suggests the need for conservation initiatives to focus on increasing landscape connectivity and heterogeneity at multiple spatial scales.

Induction of diapause by clock proteins period and timeless via changes in PTTH and ecdysteroid titer in the sugar beet moth, Scrobipalpa ocellatella (Lepidoptera: Gelechiidae)

The sugar beet moth, Scrobipalpa ocellatella (Boyd), one of the most severe sugar beet pests, causes quantitative and qualitative yield losses late in the autumn. Previously, it was shown that low temperature and short-day photoperiod together cause diapause induction in pupae. Here, the interaction of the critical elements of the diapause induction, including the period (PER), timeless (TIM), prothoracicotropic hormone (PTTH), and ecdysteroid titer, were investigated. Immunohistochemistry results showed that the number of period immunoreactivity (PER-ir) and TIM-ir cells in nondiapause pupae (NDP) was lower than in the brain of the diapause pupae (DP). Moreover, the number of PER-ir and TIM-ir cells in the protocerebrum and optic lobe (OL) of NDP was lower than DP. Moreover, lower PTTH content in the brain and hemolymph of DP was confirmed by competitive enzyme-linked immunosorbent assay. Enzyme immunoassay showed a lower 20-hydroxyecdysone (20E) titer in the hemolymph of the DP compared with the NDP. Within a short-day condition, PER and TIM titers increased in the brain leading to decreasing PTTH titers in the brain and hemolymph that caused decreasing 20E titer in the hemolymph, leading to the induction of diapause. This study suggests that PER and TIM could be one of the brain factors that play an essential role in regulating diapause in S. ocellatella.

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.

Shrinking body sizes in response to warming: explanations for the temperature-size rule with special emphasis on the role of oxygen

Body size is central to ecology at levels ranging from organismal fecundity to the functioning of communities and ecosystems. Understanding temperature-induced variations in body size is therefore of fundamental and applied interest, yet thermal responses of body size remain poorly understood. Temperature-size (T-S) responses tend to be negative (e.g. smaller body size at maturity when reared under warmer conditions), which has been termed the temperature-size rule (TSR). Explanations emphasize either physiological mechanisms (e.g. limitation of oxygen or other resources and temperature-dependent resource allocation) or the adaptive value of either a large body size (e.g. to increase fecundity) or a short development time (e.g. in response to increased mortality in warm conditions). Oxygen limitation could act as a proximate factor, but we suggest it more likely constitutes a selective pressure to reduce body size in the warm: risks of oxygen limitation will be reduced as a consequence of evolution eliminating genotypes more prone to oxygen limitation. Thus, T-S responses can be explained by the 'Ghost of Oxygen-limitation Past', whereby the resulting (evolved) T-S responses safeguard sufficient oxygen provisioning under warmer conditions, reflecting the balance between oxygen supply and demands experienced by ancestors. T-S responses vary considerably across species, but some of this variation is predictable. Body-size reductions with warming are stronger in aquatic taxa than in terrestrial taxa. We discuss whether larger aquatic taxa may especially face greater risks of oxygen limitation as they grow, which may be manifested at the cellular level, the level of the gills and the whole-organism level. In contrast to aquatic species, terrestrial ectotherms may be less prone to oxygen limitation and prioritize early maturity over large size, likely because overwintering is more challenging, with concomitant stronger end-of season time constraints. Mechanisms related to time constraints and oxygen limitation are not mutually exclusive explanations for the TSR. Rather, these and other mechanisms may operate in tandem. But their relative importance may vary depending on the ecology and physiology of the species in question, explaining not only the general tendency of negative T-S responses but also variation in T-S responses among animals differing in mode of respiration (e.g. water breathers versus air breathers), genome size, voltinism and thermally associated behaviour (e.g. heliotherms).

Changes in flight period predict trends in abundance of Massachusetts butterflies

Phenological shifts are well-documented in the ecological literature. However, their significance for changes in demography and abundance is less clear. We used 27 years of citizen science monitoring to quantify trends in phenology and relative abundance across 89 butterfly species. We calculated shifts in phenology using quantile regression and shifts in relative abundance using list length analysis and counts from field trips. Elongated activity periods within a year were the strongest predictor of increases in relative abundance. These changes may be driven in part by changes in voltinism, as this association was stronger in multivoltine species. Some species appear to be adding a late-season generation, whereas other species appear to be adding a spring generation, revealing a possible shift from vagrant to resident. Our results emphasise the importance of evaluating phenological changes throughout species' flight period and understanding the consequences for such climate-related changes on viability or population dynamics.

Assessing ecological and physiological costs of melanism in North American Papilio glaucus females: two decades of dark morph frequency declines

Polymorphisms for melanic form of insects may provide various selective advantages. However, melanic alleles may have significant/subtle pleiotrophic "costs." Several potential pleiotrophic effects of the W (=Y)-linked melanism gene in Papilio glaucus L. (Lepidoptera) showed no costs for melanic versus yellow in adult size, oviposition preferences, fecundity, egg viability, larval survival/growth rates, cold stress tolerance, or postdiapause emergence times. Sexual selection (males choosing yellow rather than mimetic dark females) had been suggested to provide a balanced polymorphism in P. glaucus, but spermatophore counts in wild females and direct field tethering studies of size-matched pairs of virgin females (dark and yellow), show that male preferences are random or frequency-dependent from Florida to Michigan, providing no yellow counter-advantages. Recent frequency declines of dark (melanic/mimetic) females in P. glaucus populations are shown in several major populations from Florida (27.3°N latitude) to Ohio (38.5° N). Summer temperatures have increased significantly at all these locations during this time (1999-2018), but whether dark morphs may be more vulnerable (in any stage) to such climate warming remains to be determined. Additional potential reasons for the frequency declines in mimetic females are discussed: (i) genetic introgression of Z-linked melanism suppressor genes from P. canadensis (R & J) and the hybrid species, P. appalachiensis (Pavulaan & Wright), (ii) differential developmental incompatibilities, or Haldane effects, known to occur in hybrids, (iii) selection against intermediately melanic ("dusty") females (with the W-linked melanic gene, b+) which higher temperatures can cause.

Developmental trap or demographic bonanza? Opposing consequences of earlier phenology in a changing climate for a multivoltine butterfly

A rapidly changing climate has the potential to interfere with the timing of environmental cues that ectothermic organisms rely on to initiate and regulate life history events. Short-lived ectotherms that exhibit plasticity in their life history could increase the number of generations per year under warming climate. If many individuals successfully complete an additional generation, the population experiences an additional opportunity to grow, and a warming climate could lead to a demographic bonanza. However, these plastic responses could become maladaptive in temperate regions, where a warmer climate could trigger a developmental pathway that cannot be completed within the growing season, referred to as a developmental trap. Here we incorporated detailed demography into commonly used photothermal models to evaluate these demographic consequences of phenological shifts due to a warming climate on the formerly widespread, multivoltine butterfly (Pieris oleracea). Using species-specific temperature- and photoperiod-sensitive vital rates, we estimated the number of generations per year and population growth rate over the set of climate conditions experienced during the past 38 years. We predicted that populations in the southern portion of its range have added a fourth generation in recent years, resulting in higher annual population growth rates (demographic bonanzas). We predicted that populations in the Northeast United States have experienced developmental traps, where increases in the thermal window initially caused mortality of the final generation and reduced growth rates. These populations may recover if more growing degree days are added to the year. Our framework for incorporating detailed demography into commonly used photothermal models demonstrates the importance of using both demography and phenology to predict consequences of phenological shifts.

Evolutionary and functional genetics of insect diapause: a call for greater integration

Diapause in response to seasonality is an important model for rapid evolutionary adaptation that is highly genetically variable, and experiences strong natural selection. Forward genetic methods using various genomic and transcriptomic approaches have begun to characterize the genetic architecture and candidate genes underlying diapause evolution. Largely in parallel, reverse genetic studies have identified functional roles for candidate genes that may or may not be genetically variable. We illustrate the disconnect between the evolutionary and physiological literature using a suite of studies of the role of the circadian clock in diapause regulation. These extensive studies in two different disciplines provide excellent opportunities for integration, which should facilitate rapid progress in understanding both the regulation and evolution of diapause.

Influence of host plant, geography and pheromone strain on genomic differentiation in sympatric populations of Ostrinia nubilalis

Patterns of mating for the European corn borer (Ostrinia nubilalis) moth depend in part on variation in sex-pheromone blend. The ratio of (E)-11- and (Z)-11-tetradecenyl acetate (E11- and Z11-14:OAc) in the pheromone blend that females produce and males respond to differs between strains of O. nubilalis. Populations also vary in female oviposition preference for and larval performance on maize (C4) and nonmaize (C3) host plants. The relative contributions of sexual and ecological trait variation to the genetic structure of O. nubilalis remains unknown. Host-plant use (¹³ C/¹⁴ C ratios) and genetic differentiation were estimated among sympatric E and Z pheromone strain O. nubilalis males collected in sex-pheromone baited traps at 12 locations in Pennsylvania and New York between 2007 and 2010. Among genotypes at 65 single nucleotide polymorphism marker loci, variance at a position in the pheromone gland fatty acyl-reductase (pgfar) gene at the locus responsible for determining female pheromone ratio (Pher) explained 64% of the total genetic differentiation between males attracted to different pheromones (male response, Resp), providing evidence of sexual inter-selection at these unlinked loci. Principal coordinate, Bayesian clustering, and distance-based redundancy analysis (dbRDA) demonstrate that host plant history or geography does not significantly contribute to population variation or differentiation among males. In contrast, these analyses indicate that pheromone response and pgfar-defined strain contribute significantly to population genetic differentiation. This study suggests that behavioural divergence probably plays a larger role in driving genetic variation compared to host plant-defined ecological adaptation.

Standing geographic variation in eclosion time and the genomics of host race formation in Rhagoletis pomonella fruit flies

Taxa harboring high levels of standing variation may be more likely to adapt to rapid environmental shifts and experience ecological speciation. Here, we characterize geographic and host-related differentiation for 10,241 single nucleotide polymorphisms in Rhagoletis pomonella fruit flies to infer whether standing genetic variation in adult eclosion time in the ancestral hawthorn (Crataegus spp.)-infesting host race, as opposed to new mutations, contributed substantially to its recent shift to earlier fruiting apple (Malus domestica). Allele frequency differences associated with early vs. late eclosion time within each host race were significantly related to geographic genetic variation and host race differentiation across four sites, arrayed from north to south along a 430-km transect, where the host races co-occur in sympatry in the Midwest United States. Host fruiting phenology is clinal, with both apple and hawthorn trees fruiting earlier in the North and later in the South. Thus, we expected alleles associated with earlier eclosion to be at higher frequencies in northern populations. This pattern was observed in the hawthorn race across all four populations; however, allele frequency patterns in the apple race were more complex. Despite the generally earlier eclosion timing of apple flies and corresponding apple fruiting phenology, alleles on chromosomes 2 and 3 associated with earlier emergence were paradoxically at lower frequency in the apple than hawthorn host race across all four sympatric sites. However, loci on chromosome 1 did show higher frequencies of early eclosion-associated alleles in the apple than hawthorn host race at the two southern sites, potentially accounting for their earlier eclosion phenotype. Thus, although extensive clinal genetic variation in the ancestral hawthorn race exists and contributed to the host shift to apple, further study is needed to resolve details of how this standing variation was selected to generate earlier eclosing apple fly populations in the North.

Local adaptation of photoperiodic plasticity maintains life cycle variation within latitudes in a butterfly

The seasonal cycle varies geographically and organisms are under selection to express life cycles that optimally exploit their spatiotemporal habitats. In insects, this often means producing an annual number of generations (voltinism) appropriate to the local season length. Variation in voltinism may arise from variation in environmental factors (e.g., temperature or photoperiod) acting on a single reaction norm shared across populations, but it may also result from local adaptation of reaction norms. However, such local adaptation is poorly explored at short geographic distances, especially within latitudes. Using a combination of common-garden rearing and life cycle modeling, we have investigated the causal factors behind voltinism variation in Swedish populations of the butterfly Pararge aegeria, focusing on a set of populations that lie within a single degree of latitude but nonetheless differ in season length and voltinism. Despite considerable differences in ambient temperature between populations, modeling suggested that the key determinant of local voltinism was in fact interpopulation differences in photoperiodic response. These include differences in the induction thresholds for winter diapause, as well as differences in photoperiodic regulation of larval development, a widespread but poorly studied phenomenon. Our results demonstrate previously neglected ways that photoperiodism may mediate insect phenological responses to temperature, and emphasize the importance of local adaptation in shaping phenological patterns in general, as well as for predicting the responses of populations to changes in climate.

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.

Keeping time without a spine: what can the insect clock teach us about seasonal adaptation?

Seasonal change in daylength (photoperiod) is widely used by insects to regulate temporal patterns of development and behaviour, including the timing of diapause (dormancy) and migration. Flexibility of the photoperiodic response is critical for rapid shifts to new hosts, survival in the face of global climate change and to reproductive isolation. At the same time, the daily circadian clock is also essential for development, diapause and multiple behaviours, including correct flight orientation during long-distance migration. Although studied for decades, how these two critical biological timing mechanisms are integrated is poorly understood, in part because the core circadian clock genes are all transcription factors or regulators that are able to exert multiple effects throughout the genome. In this chapter, we discuss clocks in the wild from the perspective of diverse insect groups across eco-geographic contexts from the Antarctic to the tropical regions of Earth. Application of the expanding tool box of molecular techniques will lead us to distinguish universal from unique mechanisms underlying the evolution of circadian and photoperiodic timing, and their interaction across taxonomic and ecological contexts represented by insects.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Genomic mechanisms of sympatric ecological and sexual divergence in a model agricultural pest, the European corn borer

The European corn borer, Ostrinia nubilalis, is a model species for elucidating mechanisms underlying adaptively differentiated subpopulations in the face of reciprocal gene flow, and is a major pest of cultivated maize in North America and Eurasia. Strains are characterized by different pheromone communication systems in combination with voltinism strains that are adapted to distinct local climate and photoperiod through adjustments in diapause traits. However, only partial barriers to inter-strain hybridization exist in areas of sympatry. Recent research shows that genes governing important strain-specific isolating traits are disproportionately located on the Z-chromosome. Furthermore, co-adapted combinations of some of these genes are non-recombining due to location within a large chromosomal inversion, and assist in maintaining strain integrity despite hybridization.

Non-Pleiotropic Coupling of Daily and Seasonal Temporal Isolation in the European Corn Borer

Speciation often involves the coupling of multiple isolating barriers to produce reproductive isolation, but how coupling is generated among different premating barriers is unknown. We measure the degree of coupling between the daily mating time and seasonal mating time between strains of European corn borer (Ostrinia nubilalis) and evaluate the hypothesis that the coupling of different forms of allochrony is due to a shared genetic architecture, involving genes with pleiotropic effects on both timing phenotypes. We measure differences in gene expression at peak mating times and compare these genes to previously identified candidates that are associated with changes in seasonal mating time between the corn borer strains. We find that the E strain, which mates earlier in the season, also mates 2.7 h earlier in the night than the Z strain. Earlier daily mating is correlated with the differences in expression of the circadian clock genes cycle, slimb, and vrille. However, different circadian clock genes associate with daily and seasonal timing, suggesting that the coupling of timing traits is maintained by natural selection rather than pleiotropy. Juvenile hormone gene expression was associated with both types of timing, suggesting that circadian genes activate common downstream modules that may impose constraint on future evolution of these traits.

A combination of sexual and ecological divergence contributes to rearrangement spread during initial stages of speciation

Chromosomal rearrangements between sympatric species often contain multiple loci contributing to assortative mating, local adaptation and hybrid sterility. When and how these associations arise during the process of speciation remains a subject of debate. Here, we address the relative roles of local adaptation and assortative mating on the dynamics of rearrangement evolution by studying how a rearrangement covaries with sexual and ecological trait divergence within a species. Previously, a chromosomal rearrangement that suppresses recombination on the Z (sex) chromosome was identified in European corn borer moths (Ostrinia nubilalis). We further characterize this recombination suppressor and explore its association with variation in sex pheromone communication and seasonal ecological adaptation in pairs of populations that are divergent in one or both of these characteristics. Direct estimates of recombination suppression in pedigree mapping families indicated that more than 39% of the Z chromosome (encompassing up to ~10 megabases and ~300 genes) resides within a nonrecombining unit, including pheromone olfactory receptor genes and a major quantitative trait locus that contributes to ecotype differences (Pdd). Combining direct and indirect estimates of recombination suppression, we found that the rearrangement was occasionally present between sexually isolated strains (E vs. Z) and between divergent ecotypes (univoltine vs. bivoltine). However, it was only consistently present when populations differed in both sexual and ecological traits. Our results suggest that independent of the forces that drove the initial establishment of the rearrangement, a combination of sexual and ecological divergence is required for rearrangement spread during speciation.

Genomic Evidence for Adaptive Inversion Clines in Drosophila melanogaster

Clines in chromosomal inversion polymorphisms-presumably driven by climatic gradients-are common but there is surprisingly little evidence for selection acting on them. Here we address this long-standing issue in Drosophila melanogaster by using diagnostic single nucleotide polymorphism (SNP) markers to estimate inversion frequencies from 28 whole-genome Pool-seq samples collected from 10 populations along the North American east coast. Inversions In(3L)P, In(3R)Mo, and In(3R)Payne showed clear latitudinal clines, and for In(2L)t, In(2R)NS, and In(3R)Payne the steepness of the clinal slopes changed between summer and fall. Consistent with an effect of seasonality on inversion frequencies, we detected small but stable seasonal fluctuations of In(2R)NS and In(3R)Payne in a temperate Pennsylvanian population over 4 years. In support of spatially varying selection, we observed that the cline in In(3R)Payne has remained stable for >40 years and that the frequencies of In(2L)t and In(3R)Payne are strongly correlated with climatic factors that vary latitudinally, independent of population structure. To test whether these patterns are adaptive, we compared the amount of genetic differentiation of inversions versus neutral SNPs and found that the clines in In(2L)t and In(3R)Payne are maintained nonneutrally and independent of admixture. We also identified numerous clinal inversion-associated SNPs, many of which exhibit parallel differentiation along the Australian cline and reside in genes known to affect fitness-related traits. Together, our results provide strong evidence that inversion clines are maintained by spatially-and perhaps also temporally-varying selection. We interpret our data in light of current hypotheses about how inversions are established and maintained.

Transcriptome profiling reveals mechanisms for the evolution of insect seasonality

Rapid evolutionary change in seasonal timing can facilitate ecological speciation and resilience to climate warming. However, the molecular mechanisms behind shifts in animal seasonality are still unclear. Evolved differences in seasonality occur in the European corn borer moth (Ostrinia nubilalis), in which early summer emergence in E-strain adults and later summer emergence in Z-strain adults is explained by a shift in the length of the termination phase of larval diapause. Here, we sample from the developmental time course of diapause in both strains and use transcriptome sequencing to profile regulatory and amino acid changes associated with timing divergence. Within a previously defined QTL, we nominate 48 candidate genes including several in the insulin signaling and circadian rhythm pathways. Genome-wide transcriptional activity is negligible during the extended Z-strain termination, whereas shorter E-strain termination is characterized by a rapid burst of regulatory changes involved in resumption of the cell cycle, hormone production, and stress response. Although gene expression during diapause termination in Ostrinia is similar to that found previously in flies, nominated genes for shifts in timing are species-specific. Hence, across distant relatives the evolution of insect seasonality appears to involve unique genetic switches that direct organisms into distinct phases of the diapause pathway through wholesale restructuring of conserved gene regulatory networks