Potential Fitness Costs Research Articles

Trends in insecticide resistance in natural populations of Culex quinquefasciatus and its impact on mosquito fitness in Dschang, West Cameroon

Specific information about the dispersion of Culex quinquefasciatus from Dschang in western Cameroon is scarce, and evidence-based interventions are needed. Common use of larvicides and adulticides conduct to the development of vectors resistance which can lead to deep biological changes, including fitness costs. We assessed the profile of insecticide resistance in field populations of Cx. quiquefasciatus and its potential fitness cost in a lineage selected for deltamethrin and permethrin resistance in the laboratory for two generations. The resistance intensity of the Cx. quinquefasciatus population was moderated when the population was exposed to deltamethrin at 10 × . Preexposure to PBO led to the restoration of full susceptibility to both deltamethrin and permethrin. Compared with that of the control group, female fecundity rates, egg hatchability, and pupation rates were significantly lower in the insecticide exposed groups. Larval development time and adults emergence rates were comparable between insecticide-exposed groups and the control. Insecticide-exposed adults lived longer than control adults did. Our findings suggest that the mechanisms selected for pyrethroid resistance are associated with negative impacts on different life-trait parameters and support the hypothesis that insecticide resistance is related to a high fitness cost.

Ontogeny of skeletogenesis in yellow perch and effects of early thermal environment on bone development.

Environmental temperature has a major impact on ontogeny of early development in teleost fish because of differential effects on rates of growth, cellular differentiation, and metabolism. Environmental temperature can thus lead to changes in relative rates of the development of physiological and anatomical components of the body including bone formation. Changes in ontogeny or rate of skeletogenesis can lead to variations in morphological or physiological phenotypes and may affect the relationship of form and function for foraging and predator avoidance. This study examined the normal ontogeny of skeleton in yellow perch, assessing development of the axial skeleton, the cranio-facial region, and the fins, in yellow perch larvae and pre-juveniles. The ontogeny of skeletogenesis in yellow perch was comparable to other related species. Fish were also reared at constant temperatures of 12, 15, or 18°C to examine the influence of developmental temperature; post-hatch fish were moved to a common 18°C. Warm incubation temperatures (15 and 18°C) increased the extent of ossification for some bones in all body regions, particularly in the exogenous feeding, and the pre-juvenile fish (30 and 40 days post hatch). This was evident in both the extent of ossification and counts of meristic characters of ossified bones. Significantly lower ossification and meristic counts in 40 dph fish reared at 12°C may limit jaw functionality and suggests an undeveloped vertebral column and fins. Future studies should investigate swim performance and foraging to determine if delayed bone development has potential fitness costs.

A plasmid-chromosome crosstalk in multidrug resistant enterobacteria

Conjugative plasmids promote the dissemination and evolution of antimicrobial resistance in bacterial pathogens. However, plasmid acquisition can produce physiological alterations in the bacterial host, leading to potential fitness costs that determine the clinical success of bacteria-plasmid associations. In this study, we use a transcriptomic approach to characterize the interactions between a globally disseminated carbapenem resistance plasmid, pOXA-48, and a diverse collection of multidrug resistant (MDR) enterobacteria. Although pOXA-48 produces mostly strain-specific transcriptional alterations, it also leads to the common overexpression of a small chromosomal operon present in Klebsiella spp. and Citrobacter freundii strains. This operon includes two genes coding for a pirin and an isochorismatase family proteins (pfp and ifp), and shows evidence of horizontal mobilization across Proteobacteria species. Combining genetic engineering, transcriptomics, and CRISPRi gene silencing, we show that a pOXA-48-encoded LysR regulator is responsible for the plasmid-chromosome crosstalk. Crucially, the operon overexpression produces a fitness benefit in a pOXA-48-carrying MDR K. pneumoniae strain, suggesting that this crosstalk promotes the dissemination of carbapenem resistance in clinical settings.

Pleiotropic effects of Ebony on pigmentation and development in the Asian multi-coloured ladybird beetle, Harmonia axyridis (Coleoptera: Coccinellidae).

Melanin plays a pivotal role in insect body pigmentation, significantly contributing to their adaptation to diverse biotic and abiotic environmental challenges. Several genes involved in insect melanin synthesis showed pleiotropic effects on insect development and reproduction. Among these, the N-β-alanyl dopamine synthetase gene (Ebony) is integral to the pigmentation process. However, the full spectrum of its pleiotropic impacts is not yet thoroughly understood. In this study, we identified and characterised the HaEbony gene in the Asian multi-coloured ladybird beetle (Harmonia axyridis) and found that HaEbony gene is a conserved gene within the Coleoptera order. We aimed to further explore the multiple roles of HaEbony in the physiology and behaviour in H. axyridis. The CRISPR/Cas9 system was applied to generate multiple HaEbony knockout allele (HaEbony+/-), showing nucleotide deletion in the G0 and G1 generations. Remarkably, the resultant HaEbony+/- mutants consistently displayed darker pigmentation than their wild-type counterparts across larval, pupal and adult stages. Furthermore, these HaEbony+/- individuals (G0) demonstrated an enhanced predatory efficiency, evidenced by a higher number of aphids consumed compared to the wild type. A significant finding was the reduced egg hatchability in both G0 and G1 generations of the HaEbony+/- group, highlighting a potential reproductive fitness cost associated with HaEbony deficiency. In conclusion, our study not only sheds light on the multifaceted roles of HaEbony in H. axyridis but also highlights the potential of employing CRISPR/Cas9-targeted modifications of the Ebony gene. Such genetic interventions could enhance the environmental adaptability and predatory efficacy of ladybirds, presenting a novel strategy in biological control application.

In vivo fitness of sul gene-dependent sulfonamide-resistant Escherichia coli in the mammalian gut.

The widespread sulfonamide resistance genes sul1, sul2, and sul3 in food and gut bacteria have attracted considerable attention. In this study, we assessed the in vivo fitness of sul gene-dependent sulfonamide-resistant Escherichia coli, using a murine model. High fitness costs were incurred for sul1 and sul3 gene-dependent E. coli strains in vivo. A fitness advantage was found in three of the eight mice after intragastric administration of sul2 gene-dependent E. coli strains. We isolated three compensatory mutant strains (CMSs) independently from three mice that outcompeted the parent strain P2 in vivo. Whole-genome sequencing revealed seven identical single nucleotide polymorphism (SNP) mutations in the three CMSs compared with strain P2, an additional SNP mutation in strain S2-2, and two additional SNP mutations in strain S2-3. Furthermore, tandem mass tag-based quantitative proteomic analysis revealed abundant differentially expressed proteins (DEPs) in the CMSs compared with P2. Of these, seven key fitness-related DEPs distributed in two-component systems, galactose and tryptophan metabolism pathways, were verified using parallel reaction monitoring analysis. The DEPs in the CMSs influenced bacterial motility, environmental stress tolerance, colonization ability, carbohydrate utilization, cell morphology maintenance, and chemotaxis to restore fitness costs and adapt to the mammalian gut environment.IMPORTANCESulfonamides are traditional synthetic antimicrobial agents used in clinical and veterinary medical settings. Their long-term excessive overuse has resulted in widespread microbial resistance, limiting their application for medical interventions. Resistance to sulfonamides is primarily conferred by the alternative genes sul1, sul2, and sul3 encoding dihydropteroate synthase in bacteria. Studying the potential fitness cost of these sul genes is crucial for understanding the evolution and transmission of sulfonamide-resistant bacteria. In vitro studies have been conducted on the fitness cost of sul genes in bacteria. In this study, we provide critical insights into bacterial adaptation and transmission using an in vivo approach.

Overriding Mendelian inheritance in Arabidopsis with a CRISPR toxin-antidote gene drive that impairs pollen germination.

Synthetic gene drives, inspired by natural selfish genetic elements and transmitted to progeny at super-Mendelian (>50%) frequencies, present transformative potential for disseminating traits that benefit humans throughout wild populations, even facing potential fitness costs. Here we constructed a gene drive system in plants called CRISPR-Assisted Inheritance utilizing NPG1 (CAIN), which uses a toxin-antidote mechanism in the male germline to override Mendelian inheritance. Specifically, a guide RNA-Cas9 cassette targets the essential No Pollen Germination 1 (NPG1) gene, serving as the toxin to block pollen germination. A recoded, CRISPR-resistant copy of NPG1 serves as the antidote, providing rescue only in pollen cells that carry the drive. To limit potential consequences of inadvertent release, we used self-pollinating Arabidopsis thaliana as a model. The drive demonstrated a robust 88-99% transmission rate over two successive generations, producing minimal resistance alleles that are unlikely to inhibit drive spread. Our study provides a strong basis for rapid genetic modification or suppression of outcrossing plant populations.

High resistance risk of Phytophthora colocasiae to azoxystrobin in southeastern of China

Quinone outside inhibitor (QoI) has been applied to manage taro leaf blight caused by Phytophthora colocasiae in southeastern of China for many years. The risk of P. colocasiae to QoI and the potential resistant mechanism remain unknown. In this study, the 74 P. colocasiae strains were sampled from southeastern of China. Sequence analysis of the QoI target Cytb showed one nucleotide variant in the fragment of this gene in this population, producing two haplotypes. The nucleotide variant leads to codon change at 142 (GGT to GCT) producing A142 (alanine) and G142 (glycine) in Hap_1 and Hap_2 strains, respectively. The sensitivity differentiation to azoxystrobin of two haplotypes were observed in vitro. The Hap_1 and Hap_2 strains were confirmed resistant and sensitive by control efficacy of label rate fungicide application, which was 3.0% and 88.8% treated with 500μg/mL azoxystrobin, respectively. In addition, 10.0μg/mL azoxystrobin plus 50μg/mL salicylhydroxamic acid (SHAM) supplemented in PDA medium was identified as a discriminatory dose for differentiation of these two phenotype strains. The azoxystrobin resistant frequency reached 86.5%, indicating prevalence of QoI resistance in the field. Further fitness related features showed that no significant difference in temperature sensitivity, mycelial growth rate, sporangia production, zoospore release and aggressiveness between azoxystrobin-resistant and sensitive strains indicating no potential fitness cost for azoxystrobin resistance. Taken together, azoxystrobin resistance need to be taken into consideration to manage taro leaf blight in southeastern of China.

Infection‐related sexual conflict in mating behaviors of wolf spiders

AbstractResistance to mating by females has been hypothesized as a tactic to assess potential mates and avoid undesirable ones. Previous studies show infection with the pathogenic bacteria, Pseudomonas aeruginosa, triggers a costly immune response and negatively impacts fitness in the wolf spider Schizocosa ocreata (Hentz 1844). Studies also show that infection can be transmitted during copulation, suggesting that there are potential fitness costs associated with mating with an infected individual. We examined impacts of infection on mating interactions of male and female S. ocreata. As studies show infection can be detected through chemical cues, we tested whether males or females avoid mating with infected individuals when given the opportunity. We paired infected and uninfected (control) males and females and recorded their behavior. We found that mating outcome was independent of infection for males and females. While higher male courtship rates and more frequent female receptivity displays were associated with successful mating, we saw no direct effect of infection status on mating outcome. However, among spider pairs that did not mate, females were found to be significantly more resistant and aggressive toward infected males than control males. Male aggression, on the other hand, was greater in pairs that did result in mating. These results suggest that sexual conflict in mating can be related to infection status, and that females can recognize infected males and alter their behavior in response.

CRISPR-enabled investigation of fitness costs associated with the E198A mutation in β-tubulin of Colletotrichum siamense.

Understanding fitness costs associated with fungicide resistance is critical to improve resistance management strategies. E198A in b-tubulin confers resistance to the fungicide thiophanate-methyl and has been widely reported in several plant pathogens including Colletotrichum siamense. To better understand potential fitness costs associated with the resistance, a ribonucleoprotein (RNP) complex mediated CRISPR/Cas9 system was used to create a point mutation (E198A) through homology directed repair (HDR) in each of the sensitive (E198) C. siamense isolates collected from strawberries, raspberries, and peaches. The RNP complex was delivered into fungal protoplasts using polyethylene glycol-mediated (PEG) transfection. The transformation efficiency, the proportion of transformants of sensitive parental isolates containing the E198A mutation, averaged 72%. No off-target mutations were observed when sequences similar to the b-tubulin target region with a maximum of four mismatch sites were analyzed, suggesting that the CRISPR/Cas9 system used in this study was highly specific for genome editing in C. siamense. Of the 41 comparisons of fitness between mutant and wild type isolates through in vitro and detached fruit assays, mutant isolates appeared to be as fit (24 of 41 comparisons), if not more fit than wild-type isolates (10 of 41 comparisons). The use of CRISPR/Cas9 to evaluate fitness costs associated with point mutations in this study represents a novel and useful method, since wild-type and mutant isolates were genetically identical except for the target mutation.

Ornaments indicate parasite load only if they are dynamic or parasites are contagious.

Choosing to mate with an infected partner has several potential fitness costs, including disease transmission and infection-induced reductions in fecundity and parental care. By instead choosing a mate with no, or few, parasites, animals avoid these costs and may also obtain resistance genes for offspring. Within a population, then, the quality of sexually selected ornaments on which mate choice is based should correlate negatively with the number of parasites with which a host is infected ("parasite load"). However, the hundreds of tests of this prediction yield positive, negative, or no correlation between parasite load and ornament quality. Here, we use phylogenetically controlled meta-analysis of 424 correlations from 142 studies on a wide range of host and parasite taxa to evaluate explanations for this ambiguity. We found that ornament quality is weakly negatively correlated with parasite load overall, but the relationship is more strongly negative among ornaments that can dynamically change in quality, such as behavioral displays and skin pigmentation, and thus can accurately reflect current parasite load. The relationship was also more strongly negative among parasites that can transmit during sex. Thus, the direct benefit of avoiding parasite transmission may be a key driver of parasite-mediated sexual selection. No other moderators, including methodological details and whether males exhibit parental care, explained the substantial heterogeneity in our data set. We hope to stimulate research that more inclusively considers the many and varied ways in which parasites, sexual selection, and epidemiology intersect.

Phenotypic and transcriptomic acclimation of the green microalga Raphidocelis subcapitata to high environmental levels of the herbicide diflufenican

Herbicide pollution poses a worldwide threat to plants and freshwater ecosystems. However, the understanding of how organisms develop tolerance to these chemicals and the associated trade-off expenses are largely unknown. This study aims to investigate the physiological and transcriptional mechanisms underlying the acclimation of the green microalgal model species Raphidocelis subcapitata (Selenastraceae) towards the herbicide diflufenican, and the fitness costs associated with tolerance development. Algae were exposed for 12weeks (corresponding to 100 generations) to diflufenican at the two environmental concentrations 10 and 310ng/L. The monitoring of growth, pigment composition, and photosynthetic performance throughout the experiment revealed an initial dose-dependent stress phase (week 1) with an EC50 of 397ng/L, followed by a time-dependent recovery phase during weeks 2 to 4. After week 4, R. subcapitata was acclimated to diflufenican exposure with a similar growth rate, content of carotenoids, and photosynthetic performance as the unexposed control algae. This acclimation state of the algae was explored in terms of tolerance acquisition, changes in the fatty acids composition, diflufenican removal rate, cell size, and changes in mRNA gene expression profile, revealing potential fitness costs associated with acclimation, such as up-regulation of genes related to cell division, structure, morphology, and reduction of cell size. Overall, this study demonstrates that R. subcapitata can quickly acclimate to environmental but toxic levels of diflufenican; however, the acclimation is associated with trade-off expenses that result in smaller cell size.

Interchromosomal linkage disequilibrium and linked fitness cost loci associated with selection for herbicide resistance.

The adaptation of weeds to herbicide is both a significant problem in agriculture and a model of rapid adaptation. However, significant gaps remain in our knowledge of resistance controlled by many loci and the evolutionary factors that influence the maintenance of resistance. Here, using herbicide-resistant populations of the common morning glory (Ipomoea purpurea), we perform a multilevel analysis of the genome and transcriptome to uncover putative loci involved in nontarget-site herbicide resistance (NTSR) and to examine evolutionary forces underlying the maintenance of resistance in natural populations. We found loci involved in herbicide detoxification and stress sensing to be under selection and confirmed that detoxification is responsible for glyphosate (RoundUp) resistance using a functional assay. We identified interchromosomal linkage disequilibrium (ILD) among loci under selection reflecting either historical processes or additive effects leading to the resistance phenotype. We further identified potential fitness cost loci that were strongly linked to resistance alleles, indicating the role of genetic hitchhiking in maintaining the cost. Overall, our work suggests that NTSR glyphosate resistance in I. purpurea is conferred by multiple genes which are potentially maintained through generations via ILD, and that the fitness cost associated with resistance in this species is likely a by-product of genetic hitchhiking.

A Globally Distributed Insecticide Resistance Allele Confers a Fitness Cost in the Absence of Insecticide in Aedes aegypti (Diptera: Culicidae), the Yellow Fever Mosquito.

The cosmopolitan mosquito Aedes aegypti is a vector of harmful arboviruses. Pyrethroid insecticides are used to reduce adult populations and prevent the spread of disease. Pyrethroids target the insect voltage-gated sodium channel (VGSC). Collectively, mutations in Vgsc that confer resistance are referred to as knock-down resistance or kdr. There are numerous kdr mutations found in A. aegypti Vgsc, and there is co-occurrence of some mutations. Full-length cDNA sequences have identified nine known kdr (e.g., 1534C) alleles. The 1534C allele is among the most common kdr alleles, but allele frequencies between populations vary considerably. We used the 1534C:RK strain, which has the 1534C (kdr) allele in the genetic background of the insecticide susceptible Rockefeller (ROCK) strain, and conducted population cage experiments to assess the potential intrinsic fitness cost of the 1534C allele relative to the susceptible allele (F1534) in the ROCK strain. Individuals were genotyped across generations using allele specific PCR. A fitness cost of the 1534C allele was detected across seven generations of mosquitos reared in the absence of insecticide selection pressure. The decrease in allele frequency was not due to drift. Comparison of our results to previous studies suggests that the magnitude of the fitness cost of kdr alleles in the absence of insecticide is disconnected from the level of resistance they confer, and that the fitness costs of different kdr alleles can be variable.

Impact of direct physical association and motility on fitness of a synthetic interkingdom microbial community

Mutualistic exchange of metabolites can play an important role in microbial communities. Under natural environmental conditions, such exchange may be compromised by the dispersal of metabolites and by the presence of non-cooperating microorganisms. Spatial proximity between members during sessile growth on solid surfaces has been shown to promote stabilization of cross-feeding communities against these challenges. Nonetheless, many natural cross-feeding communities are not sessile but rather pelagic and exist in turbulent aquatic environments, where partner proximity is often achieved via direct cell-cell adhesion, and cooperation occurs between physically associated cells. Partner association in aquatic environments could be further enhanced by motility of individual planktonic microorganisms. In this work, we establish a model bipartite cross-feeding community between bacteria and yeast auxotrophs to investigate the impact of direct adhesion between prokaryotic and eukaryotic partners and of bacterial motility in a stirred mutualistic co-culture. We demonstrate that adhesion can provide fitness benefit to the bacterial partner, likely by enabling local metabolite exchange within co-aggregates, and that it counteracts invasion of the community by a non-cooperating cheater strain. In a turbulent environment and at low cell densities, fitness of the bacterial partner and its competitiveness against a non-cooperating strain are further increased by motility that likely facilitates partner encounters and adhesion. These results suggest that, despite their potential fitness costs, direct adhesion between partners and its enhancement by motility may play key roles as stabilization factors for metabolic communities in turbulent aquatic environments.

Optimized prime editing efficiently generates glyphosate-resistant rice plants carrying homozygous TAP-IVS mutation in EPSPS

Optimized prime editing efficiently generates glyphosate-resistant rice plants carrying homozygous TAP-IVS mutation in EPSPS

Fitness costs of CRISPR-Cas systems in bacteria.

CRISPR-Cas systems provide bacteria with both specificity and adaptability in defence against invading genetic elements. From a theoretical perspective, CRISPR-Cas systems confer many benefits. However, they are observed at an unexpectedly low prevalence across the bacterial domain. While these defence systems can be gained horizontally, fitness costs may lead to selection against their carriage. Understanding the source of CRISPR-related fitness costs will help us to understand the evolutionary dynamics of CRISPR-Cas systems and their role in shaping bacterial genome evolution. Here, we review our current understanding of the potential fitness costs associated with CRISPR-Cas systems. In addition to potentially restricting the acquisition of genetic material that could confer fitness benefits, we explore five alternative biological factors that from a theoretical perspective may influence the fitness costs associated with CRISPR-Cas system carriage: (1) the repertoire of defence mechanisms a bacterium has available to it, (2) the potential for a metabolic burden, (3) larger-scale population and environmental factors, (4) the phenomenon of self-targeting spacers, and (5) alternative non-defence roles for CRISPR-Cas.

Salted roads lead to oedema and reduced locomotor function in amphibian populations

Abstract Human activities have caused massive losses of natural populations across the globe. Like many groups, amphibians have experienced substantial declines worldwide, driven by environmental changes such as habitat conversion, pollution, and disease emergence. Each of these drivers is often found in close association with the presence of roads. Here we report a novel consequence of roads affecting an amphibian native to much of North America, the wood frog (Rana sylvatica). Across 38 populations distributed from southern to central New England, we found that adult wood frogs living adjacent to roads had higher incidence and severity of oedema (indicated by obvious bloating caused by subcutaneous fluid accumulation) during the breeding season than frogs living away from the influence of roads. This effect was best explained by increased conductivity of breeding ponds, probably caused by runoff pollution from road salt used for de‐icing. Oedema severity was negatively correlated with locomotor performance in more northerly populations. Interestingly, northern populations experience more intense winters, which tends to result in more de‐icing salt runoff and increased energetic demands associated with overwintering cryoprotection needs. Thus, this emerging consequence of roads appears to impose potential fitness costs associated with locomotion, and these effects might be most impactful on populations living in regions where de‐icing is most intense. Together, our findings reveal a novel set of impacts of roads and runoff pollution on wood frog physiology and performance, which seem likely to contribute to population decline. Given the global prevalence of roads and increasing salinisation of freshwater habitats, oedema and related impacts could be widespread consequences faced by amphibian populations across much of the planet's temperate zones.

Mark-release-recapture experiment in Burkina Faso demonstrates reduced fitness and dispersal of genetically-modified sterile malaria mosquitoes

Every year, malaria kills approximately 405,000 people in Sub-Saharan Africa, most of them children under the age of five years. In many countries, progress in malaria control has been threatened by the rapid spread of resistance to antimalarial drugs and insecticides. Novel genetic mosquito control approaches could play an important role in future integrated malaria control strategies. In July 2019, the Target Malaria consortium proceeded with the first release of hemizygous genetically-modified (GM) sterile and non-transgenic sibling males of the malaria mosquito Anopheles coluzzii in Burkina Faso. This study aimed to determine the potential fitness cost associated to the transgene and gather important information related to the dynamic of transgene-carrying mosquitoes, crucial for next development steps. Bayesian estimations confirmed that GM males had lower survival and were less mobile than their wild type (WT) siblings. The estimated male population size in Bana village, at the time of the release was 28,000 - 37,000. These results provide unique information about the fitness and behaviour of released GM males that will inform future releases of more effective strains of the A. gambiae complex.

Molecular and kinetic characterization of two acetylcholinesterases with particular focus on the roles of two amino acid substitutions (Y390N and F392W) in Bemisia tabaci

Two acetylcholinesterases (AChEs) are present in Bemisia tabaci (BtAChE1 and BtAChE2). A conserved AChE mutation conferring organophosphate (OP) resistance (F392W in BtAChE1) is saturated in field populations despite its potential fitness cost, and a highly conserved amino acid residue forming the backdoor of AChE is substituted with a unique amino acid (Y390N in BtAChE1) in B. tabaci. Thus, the roles and relationships of the two amino acid substitutions in the evolutionary adaptation of B. tabaci remain to be elucidated, and little information is available on the catalytic and molecular properties of BtAChE1 and BtAChE2. To determine which AChE is a more relevant target of OPs and carbamates, the molecular and kinetic properties of BtAChE1 and BtAChE2 were investigated. Both BtAChE1 and BtAChE2 were exclusively expressed in head and thorax but not in abdomen, bound to the membrane via GPI anchoring, and present as dimeric forms. Soluble monomeric form was detected only in BtAChE2. The catalytic activity of baculovirus-expressed BtAChE1 was 19.5-fold higher than that of BtAChE2. The inhibition assay revealed that the F392W mutation in BtAChE1 enhanced resistance to OPs. The artificial substitution of N390 (wild form) to Y (putative ancient form) led to reduced catalytic efficiency and increased inhibition by glycoalkaloids, suggesting that the Y390N substitution in BtAChE1 may have been required for Solanaceae host adaptation. BtAChE1 was proven to function as a main catalytic enzyme for ACh hydrolysis, thus being the main target of OPs and carbamates.

Fitness cost from fluctuating ultraviolet radiation in Daphnia magna.

Solar ultraviolet radiation (UVR) is an important environmental threat for organisms in aquatic systems, but its temporally variable nature makes the understanding of its effects ambiguous. The aim of our study was to assess potential fitness costs associated with fluctuating UVR in the aquatic zooplankter Daphnia magna. We investigated individual survival, reproduction and behaviour when exposed to different UVR treatments. Individuals exposed to fluctuating UVR, resembling natural variations in cloud cover, had the lowest fitness (measured as the number of offspring produced during their lifespan). By contrast, individuals exposed to the same, but constant UVR dose had similar fitness to control individuals (not exposed to UVR), but they showed a significant reduction in daily movement. The re-occurring threat response to the fluctuating UVR treatment thus had strong fitness costs for D. magna, and we found no evidence for plastic behavioural responses when continually being exposed to UVR, despite the regular, predictable exposure schedule. In a broader context, our results imply that depending on how variable a stressor is in nature, populations may respond with alternative strategies, a framework that could promote rapid population differentiation and local adaptation.