Biology subjects

Bergland, A. O.

Publications and source records attributed to Bergland, A. O..

At least 19 recordsLinked to original sources

bioRxiv2026

Intragenic epistasis in couch potato shapes life history traits and climatic adaptation in Drosophila melanogaster

Populations adapting to heterogeneous environments often harbor many alleles that covary with environmental gradients, yet how so many adaptive variants can be maintained and show signatures of local adaptation remains unclear. Epistatic non-additive interactions among alleles may concentrate fitness effects at specific allelic combinations to shape complex traits and facilitate adaptation. We evaluate compositional epistasis among three intronic SNPs in the couch potato (cpo) gene, each independently clinal along the North American latitudinal gradient, using recombinant outbred populations fixed for distinct three-locus combinations. Flies carrying all three temperate alleles (cpoTTA) or all three tropical alleles (cpoCGT) show non-additive increases in traits associated with fitness in their respective environments: dormancy, starvation resistance, and lipid storage in temperate flies and fecundity in tropical flies. Intermediate genotype combinations generally show weaker and negative deviations from additive predictions. Temperate genotypes show reduced cpo expression and independently create predicted transcription factor binding sites, suggesting a regulatory basis for the observed phenotypic effects. Consistent with selection shaping haplotype frequencies against a backdrop of ongoing recombination, the temperate and tropical genotypes are enriched above random expectation across the latitudinal cline, with directionally consistent but non-significant seasonal patterns. Our findings demonstrate that non-additive interactions among non-coding variants within a single gene can produce large, pleiotropic fitness effects and shape genotype distributions in natural populations, providing a mechanism by which a small number of regulatory variants can underlie complex climatic adaptation.

evolutionary biology↗

bioRxiv2026

Adaptation to Environmental Variability Shapes Dormancy in Daphnia

Dormancy is a widespread adaptive strategy that allows organisms to survive in temporally varying habitats by suspending development and reproduction. Although environmental variability is expected to shape dormancy strategies, it remains unclear how differences in environmental variability and predictability influence both the production of dormant embryos and the termination of dormancy. We addressed these questions by comparing D. pulex and D. obtusa, two closely related species that inhabit environments differing in variability and predictability. We hypothesized that D. obtusa, which inhabits ephemeral environments, would exhibit a greater propensity for sexual reproduction and dormancy and would require stronger cues to break dormancy than D. pulex, which occurs in more permanent, predictable habitats. Consistent with our hypothesis, D. obtusa lineages produced significantly more males and ephippia than D. pulex when reared under identical laboratory conditions, indicating greater investment in sexual reproduction and dormancy. Contrary to our hypothesis, we found no difference in responsiveness to cues between the two species. Across species, embryos broke dormancy and hatched most readily after experiencing changes in cold and light, even if not experienced at the same time. In contrast, desiccation reduced the propensity to break dormancy. Together, these results indicate that species occupying more ephemeral environments invest more heavily in the production of dormant offspring, but that the environmental cues regulating dormancy termination appear broadly similar between species. This pattern suggests that while investment in dormancy may evolve in response to environmental variability, the mechanisms controlling dormancy termination are more conserved.

evolutionary biology↗

bioRxiv2026

Phenotypic and Genomic Evidence of Adaptive Tracking in Thermal Tolerance of Wild Populations of an Invasive Drosophila.

Adaptive tracking is an evolutionary process in which allele frequencies and phenotypes shift in response to temporally fluctuating environments. Currently it is unclear whether adaptive tracking causes predictable evolution of complex traits such as thermal tolerance. We investigated seasonal adaptive tracking of critical thermal minimum (CTmin) and genome-wide allele frequencies over multiple years in the invasive fly Drosophila suzukii. CTmin increased throughout the growing season, showing a lag of several generations between increasing temperature and evolutionary change. Genetic analyses indicate CTmin is highly polygenic, with little overlap between alleles associated with CTmin and other seasonally fluctuating alleles. Thus, polygenic traits may track seasonal environments without leaving strong genomic signals. By contrast, there were strong seasonal genomic signatures for alleles associated with oligogenic traits as such pesticide resistance and olfactory behavior. These findings suggest that seasonal adaptive tracking shapes a broad suite of traits that contribute to D. suzukiis invasion success.

evolutionary biology↗

bioRxiv2025

Cosmopolitan inversions have a major impact on trait variation and the power of different GWAS approaches to identify associations

The ability of genomic inversions to reduce recombination and generate linkage can have a major impact on genetically based phenotypic variation in populations. However, the increase in linkage associated with inversions can create hurdles for identifying associations between loci within inversions and the traits they impact. As a consequence, the role of inversions in mediating genetic variation in complex traits remains to be fully understood. This study uses the fruit fly Drosophila melanogaster to investigate the impact of inversions on trait variation. We tested the effects of common inversions among a diverse assemblage of traits including aspects of behavior, morphology, and physiology, and identified that the cosmopolitan inversions In(2L)t and In(3R)Mo are associated with many traits. We compared the ability of different approaches of accounting for relatedness and inversion presence during genome-wide association to identify signals of association with SNPs. We report that commonly used association methods are underpowered within inverted regions, while alternative approaches such as leave-one-chromosome-out improve the ability to identify associations. In all, our research enhances our understanding of inversions as components of trait variation and provides insight into approaches for identifying genomic regions driving these associations. Authors SummaryGenomic inversions are large mutations that flip the orientation of sections of DNA, and the presence of inversions has the potential to impact many traits at once. Inversions exist in many organisms including humans and the fruit fly Drosophila melanogaster. We take existing knowledge on Drosophila traits and identify several inversions which impact many kinds of traits. Approaches such as GWAS can be used to identify the DNA mutations most associated with variation in a trait of interest, but many GWAS methods do not perform well when inversions contribute to variation in phenotype. We show that a common GWAS approach in Drosophila is not only unable to find association within inversions, but is overall underpowered. In contrast, we show a different approach is better able to identify mutations within inversions that are potentially associated with fruit fly traits. These findings help scientists studying a wide range of organisms a better understand the role of inversions among different kinds of traits, and supports the broad research of identifying gene associations within these regions of inversion.

genetics↗

bioRxiv2025

The inversion In(2L)t impacts complex, environmentally sensitive behaviors in Drosophila melanogaster

Genetic variation in behavioral traits allows organisms to respond and adapt to environmental challenges. Genetic variation in behavior is often affected by many genes and thus has a complex genetic basis. Inversions, the reorientation of genes along the chromosome, tightly link genetic variants together because they suppress recombination. Therefore, inversions are believed to have a major impact on phenotypic variation because they combine the effects of multiple genes, which can pleiotropically alter multiple aspects of behavior. This study investigates how the inversion In(2L)t, found in Drosophila melanogaster populations around the world, impacts different aspects of behavior in an environment-sensitive manner. We test the activity, foraging, and startle-induced behavior of flies with different In(2L)t genotypes across sex and temperatures. We observe that Drosophila homozygous for In(2L)t sleep less frequently, spend more time away from a food source, and have a longer duration of startle response. Additionally, the impacts of In(2L)t on aspects of behavior can be sex-specific and are largely consistent across temperatures. Taken together, our research demonstrates that inversions can regulate aspects of behavior, and suggests hypotheses explaining the distribution of In(2L)t across space and time.

genetics↗

bioRxiv2025

Patterns of gene family evolution and selection across Daphnia

Gene family expansion underlies a host of biological innovations across the tree of life. Understanding why specific gene families expand or contract requires comparative genomic investigations clarifying further how species adapt in the wild. This study investigates the gene family change dynamics within several species of Daphnia, a group of freshwater microcrustaceans that are useful model systems for evolutionary genetics. We employ comparative genomics approaches to understand the forces driving gene evolution and draw upon candidate gene families that change gene numbers across Daphnia. Our results suggest that genes related to stress responses and glycoproteins generally expand across taxa, and we investigate evolutionary hypotheses of adaptation that may underpin expansions. Through these analyses, we shed light on the interplay between gene expansions and selection within other ecologically relevant stress response gene families. While we show generalities in gene family turnover in genes related to stress response (i.e., DNA repair mechanisms), most gene family evolution is driven in a species-specific manner. Additionally, while we show general trends towards positive selection within some expanding gene families, many genes are not undergoing selection, highlighting the complex nature of diversification and evolution within Daphnia. Our research enhances the understanding of individual gene family evolution within Daphnia and provides a case study of ecologically relevant genes prone to change.

evolutionary biology↗

bioRxiv2024

Pervasive fitness trade-offs revealed by rapid adaption in large experimental populations of Drosophila melanogaster

Trade-offs are an inherent feature of organismal biology that are expected play a fundamental role in the evolution of natural populations. Efforts to quantify trade-offs are largely confined to phenotypic measurements and the identification of negative genetic-correlations among fitness-relevant traits. Here, we use time-series genomic data collected during experimental evolution in large, genetically diverse populations of Drosophila melanogaster to directly measure the manifestation of trade-offs in response to fluctuating selection on ecological timescales. Specifically, we first conducted a lab-based selection experiment to quantify a genome-wide signal of antagonistic pleiotropy elicited in response to shifting population densities and associated with reproduction and stress tolerance selection. In doing so, we identified a putative role of two cosmopolitan inversions in these trade-offs. We then conducted an independent experiment to show that a simple manipulation of increasing population density under controlled lab-based conditions identified loci that are relevant to selection during population expansion and collapse in a complex, semi-natural setting. In concert, our results reveal how adaptation in complex, natural environments can be coarse-grained in such a manner to drive repeatable and predictable patterns of genomic variation, and further add credence to models positing a role of generic fitness trade-offs in the maintenance of variation in natural populations.

evolutionary biology↗

bioRxiv2024

Limited population structure but signals of recent selection in introduced African Fig Fly (Zaprionus indianus) in North America

Invasive species have devastating consequences for human health, food security, and the environment. Many invasive species adapt to new ecological niches following invasion, but little is known about the early steps of adaptation. Here we examine population genomics of a recently introduced drosophilid in North America, the African Fig Fly, Zaprionus indianus. This species is likely intolerant of subfreezing temperatures and recolonizes temperate environments yearly. We generated a new chromosome-level genome assembly for Z. indianus. Using resequencing of over 200 North American individuals collected over four years in temperate Virginia, plus a single collection from subtropical Florida, we tested for signatures of population structure and adaptation within invasive populations. We show founding populations are sometimes small and contain close genetic relatives, yet temporal population structure and differentiation of populations is mostly absent across North America. However, we identify two haplotypes that are differentiated between African and invasive populations and show signatures of selective sweeps. Both haplotypes contain genes in the cytochrome P450 pathway, indicating these sweeps may be related to pesticide resistance. X chromosome evolution in invasive populations is strikingly different from the autosomes, and a haplotype on the X chromosome that is highly differentiated between Virginia and Florida populations is a candidate for temperate adaptation. These results show that despite limited population structure, populations may rapidly evolve genetic differences early in an invasion. Further uncovering how these genomic regions influence invasive potential and success in new environments will advance our understanding of how organisms evolve in changing environments. Article SummaryInvasive species (organisms that have been moved outside their natural range by human activities) can cause problems for both humans and the environment. We studied the genomes of over 200 individuals of a newly invasive fruit fly in North America, the African Fig Fly. We found genetic evidence that these recently introduced flies may be evolving in their new environments, which could make them stronger competitors and more likely to become pests.

evolutionary biology↗

bioRxiv2024

Drosophila melanogaster pigmentation demonstrates adaptive phenotypic parallelism but genomic unpredictability over multiple timescales

Populations are capable of responding to environmental change over ecological timescales via adaptive tracking. However, the translation from patterns of allele frequency change to rapid adaptation of complex traits remains unresolved. We used abdominal pigmentation in Drosophila melanogaster as a model phenotype to address the nature, genetic architecture, and repeatability of rapid adaptation in the field. We show that D. melanogaster pigmentation evolves as a highly parallel and deterministic response to shared environmental gradients across latitude and season in natural North American populations. We then experimentally evolved replicate, genetically diverse fly populations in field mesocosms to remove any confounding effects of demography and/or cryptic structure that may drive patterns in wild populations; we show that pigmentation rapidly responds, in parallel, in fewer than fifteen generations. Thus, pigmentation evolves concordantly in response to spatial and temporal climatic gradients. We next examined whether phenotypic differentiation was associated with allele frequency change at loci with established links to genetic variance in pigmentation in natural populations. We found that across all spatial and temporal scales, phenotypic patterns were associated with variation at pigmentation-related loci, and the sets of genes we identified in each context were largely nonoverlapping. Therefore, our findings suggest that parallel phenotypic evolution is associated with distinct components of the polygenic architecture shifting across each environmental gradient to produce redundant adaptive patterns.

evolutionary biology↗

bioRxiv2024

Balancing selection and the functional effects of shared polymorphism in cryptic Daphnia species

The patterns of genetic variation within and between related taxa represent the genetic history of a species. Shared polymorphisms, loci with identical alleles across species, are of unique interest as they may represent cases of ancient selection maintaining functional variation post-speciation. In this study, we investigate the abundance of shared polymorphism in the Daphnia pulex species complex. We test whether shared mutations are consistent with the action of balancing selection or alternative hypotheses such as hybridization, incomplete lineage sorting, or convergent evolution. We analyzed over 2,000 genomes from North American and European D. pulex and several outgroup species to examine the prevalence and distribution of shared alleles between the focal species pair, North American and European D. pulex. We show that while North American and European D. pulex diverged over ten million years ago, they retained tens of thousands of shared alleles. We found that the number of shared polymorphisms between North American and European D. pulex cannot be explained by hybridization or incomplete lineage sorting alone. Instead, we show that most shared polymorphisms could be the product of convergent evolution, that a limited number appear to be old trans-specific polymorphisms, and that balancing selection is affecting young and ancient mutations alike. Finally, we provide evidence that a blue wavelength opsin gene with trans-specific polymorphisms has functional effects on behavior and fitness in the wild. Ultimately, our findings provide insights into the genetic basis of adaptation and the maintenance of genetic diversity between species.

evolutionary biology↗

bioRxiv2023

Spatial and temporal variation in abundance of introduced African fig fly (Zaprionus indianus) (Diptera: Drosophilidae) in the eastern United States

The African fig fly, Zaprionus indianus (Gupta), has spread globally from its native range in tropical Africa, becoming an invasive crop pest in select areas such as Brazil. Z. indianus was first reported in the United States in 2005 and has since been documented as far north as Canada. As a tropical species, Z. indianus is expected to have low cold tolerance, likely limiting its ability to persist at northern latitudes. In North America, the geographic regions where Z. indianus can thrive and seasonal fluctuations in its abundance are not well understood. The purpose of this study was to characterize the temporal and spatial variation in Z. indianus abundance to better understand its invasion of the eastern United States. We sampled drosophilid communities over the growing season at two orchards in Virginia from 2020-2022 and several locations along the East Coast during the fall of 2022. Virginia abundance curves showed similar seasonal dynamics across years with individuals first detected around July and becoming absent around December. Massachusetts was the northernmost population and no Z. indianus were detected in Maine. Variation in Z. indianus relative abundance was high between nearby orchards and across different fruits within orchards but was not correlated with latitude. Fitness of wild-caught females decreased later in the season and at higher latitudes. The patterns of Z. indianus abundance shown here demonstrate an apparent susceptibility to cold and highlight a need for systematic sampling to accurately characterize the range and spread of Z. indianus.

ecology↗

bioRxiv2022

Maternal control of spontaneous dormancy termination in Daphnia pulex

This study examined maternal influence and life-history consequences of diapause termination timing in Daphnia pulex. We raised clonal isolates of D. pulex in mesocosms and observed hatching rates prior to and after exposing embryos to a cold shock. A substantial proportion of individuals hatched early, prior to the cold shock. We found that siblings from the same ephippium were more likely than expected by chance to emerge at the same time, even after dissection and separation, suggesting the presence of a maternal effect that influences diapause duration. We also found that for individuals who emerged early, the time to first reproduction was significantly delayed, and individuals produced fewer resting embryos in subsequent generations. We suggest that early diapause termination may be driven by maternal effects to generate offspring that emerge from dormancy at different times.

evolutionary biology↗

bioRxiv2022

A cosmopolitan inversion drives seasonal adaptation in overwintering Drosophila

Drosophila melanogaster living in temperate regions evolve as they track seasonal fluctuations. Yet, we lack an understanding of the genetic architecture of seasonal adaptive tracking. By sequencing orchard populations collected across multiple years, we characterized the genomic signal of seasonal demography and identified that the cosmopolitan inversion In(2L)t drives seasonal adaptation. In(2L)t shows footprints of selection that are inconsistent with simple explanations of genetic drift, as well as signatures of partial selective sweeps. A meta-analysis of phenotypic studies shows that seasonal loci within In(2L)t are associated with behavior, life-history, physiology, and morphology traits. Our results identify candidate regions that underlie seasonal adaptive tracking and link them to phenotype. This work supports the general hypothesis that inversions are important drivers of rapid adaptation. One-Sentence SummaryA chromosomal inversion drives adaptive evolution between seasons in wild fruit flies.

evolutionary biology↗

bioRxiv2021

Unique signals of clinal and seasonal allele frequency change at eQTLs in Drosophila melanogaster

Populations of short-lived organisms can respond to spatial and temporal environmental heterogeneity through local adaptation. Local adaptation can be reflected on both phenotypic and genetic levels, and it has been documented in many organisms. Although some complex fitness-related phenotypes have been shown to vary across latitudinal clines and seasons in similar ways in Drosophila melanogaster populations, we lack a general understanding of the genetic architecture of local adaptation across space and time. To address this problem, we examined patterns of allele frequency change across latitudinal clines and between seasons at previously reported expression quantitative trait loci (eQTLs). We divided eQTLs into groups by utilizing differential expression profiles of fly populations collected across a latitudinal cline or exposed to different environmental conditions. We also examined clinal and seasonal patterns of allele frequency change at eQTLs grouped by tissues. In general, we find that clinally varying polymorphisms are enriched for eQTLs, and that these eQTLs change in frequency in predictable ways across the cline and in response to starvation tolerance. The enrichment of eQTL among seasonally varying polymorphisms is more subtle, and the direction of allele frequency change at eQTL appears to be somewhat idiosyncratic. Taken together, we suggest that clinal adaptation at eQTLs is distinct than that of seasonal adaptation.

evolutionary biology↗

bioRxiv2021

Stabilizing selection shapes variation in phenotypic plasticity

The adaptive nature of phenotypic plasticity is widely documented in natural populations. However, little is known about the evolutionary forces that shape genetic variation in plasticity within populations. Here we empirically address this issue by testing the hypothesis that stabilizing selection shapes genetic variation in the anti-predator developmental plasticity of Daphnia pulex. The anti-predator morphological defense is characterized by pedestal and spikes that grow on the back of the Daphnia neck following exposure to predator cure. We characterized variation in this plasticity using a novel, high-throughput phenotyping method that describes the entire dorsal shape amongst >100 D. pulex strains originating from a natural population in the UK. We found low genetic diversity for morphological defenses among genetically diverse clones upon predation risk exposure. The strongest reduction in genetic variation was observed in areas of greatest phenotypic plasticity, which we interpret as evidence of stabilizing selection. By assessing among-clone variance in clonally related, field derived strains, we contrasted mutational variation (Vm) to standing variation (Vg). We found that Vg/Vm is lowest in areas of greatest plasticity. These data strongly suggest that stabilizing selection operates directly on phenotypic plasticity, providing a rare glimpse into the evolution of fitness related traits in natural populations.

evolutionary biology↗

bioRxiv2021

Polygenic variation in sexual investment across an ephemerality gradient in Daphnia pulex

Species across the tree of life can switch between asexual and sexual reproduction. In facultatively sexual species, the ability to switch between reproductive modes is often environmentally dependent and subject to local adaptation. However, the ecological and evolutionary factors that influence the maintenance and turnover of polymorphism associated with facultative sex remain unclear. To address this basic question, we studied the ecological and evolutionary dynamics of polymorphism in reproductive strategy in a metapopulation of the model facultative sexual, Daphnia pulex, located in the southern United Kingdom. We found that patterns of clonal diversity, but not genetic diversity varied with ephemerality. Reconstruction of a multi-year pedigree demonstrated the co-existence of clones that were found to differ in their investment into male production. Mapping of quantitative variation in male production using lab-generated and field-collected individuals identified multiple putative QTL underlying this trait, and we identified a plausible candidate gene. The evolutionary history of these QTL suggests that they are relatively young, and male limitation in this system is a rapidly evolving trait. Our work highlights the dynamic nature of the genetic structure and composition of facultative sex across space and time and suggests that quantitative genetic variation in reproductive strategy can undergo rapid evolutionary turnover.

evolutionary biology↗

bioRxiv2020

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 at relatively high frequencies 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. Author SummaryAnimals exhibit diverse strategies to cope with unfavorable conditions in temperate, seasonally varying environments. The model fly, Drosophila melanogaster, can enter a physiological state known as diapause under winter-like conditions. Diapause is characterized by an absence of egg maturation in females and is thought to conserve energy for survival during stressful times. The ability to diapause is more common in flies from higher latitudes and in offspring from flies that have recently overwintered. Therefore, diapause has been thought to be a recent adaptation to temperate climates. We identified hundreds of genetic variants that affect diapause and found that some vary predictably across latitudes in North America. We found little signal of repeated seasonality in diapause-associated genetic variants, but our populations evolved an increased ability to diapause in the winter when they were exposed to natural conditions. Combined, our results suggest that diapause-associated variants evolve differently across space and time. We find little evidence that diapause evolved recently in temperate environments; rather, SNPs associated with diapause tend to be quite common in Zambia, suggesting that diapause may promote survival under stresses other than cold. Our results provide future targets for research into the genetic underpinnings of this complex, ecologically relevant trait.

genetics↗

bioRxiv2019

Phenotypic plasticity, but not genetic adaptation, underlies seasonal variation in the cold hardening response of Drosophila melanogaster

In temperate regions, an organisms ability to rapidly adapt to seasonally varying environments is essential for its survival. In response to seasonal changes in selection pressure caused by variation in temperature, humidity, and food availability, some organisms exhibit plastic changes in phenotype. In other cases, seasonal variation in selection pressure can rapidly increase the frequency of genotypes that offer survival or reproductive advantages under the current conditions. Little is known about the relative influences of plastic and genetic changes in short lived organisms experiencing seasonal environmental fluctuations. Cold hardening is a seasonally relevant plastic response in which exposure to cool, but nonlethal, temperatures significantly increases the organisms ability to later survive at freezing temperatures. In the present study, we demonstrate seasonal variation in cold hardening in Drosophila melanogaster and test the extent to which plasticity and adaptive tracking underlie that seasonal variation. We measured the cold hardening response of flies from outdoor mesocosms over the summer, fall, and winter. We bred outdoor mesocosm-caught flies for two generations in the lab and matched each outdoor cohort to an indoor control cohort of similar genetic background. We measured the cold hardening response of indoor and field-caught flies and their laboratory-reared F1 and F2 progeny to determine the roles of seasonal environmental plasticity, parental effects, and genetic changes on cold hardening. We also tested the relationship between cold hardening and other factors, including age, developmental density, food substrate, presence of antimicrobials, and supplementation with live yeast. We found strong plastic responses to a variety of field- and lab-based environmental effects, but no evidence of seasonally varying parental or genetic effects on cold hardening. We therefore conclude that seasonal variation in the cold hardening response results from environmental influences and not genetic changes.

evolutionary biology↗