The influence of context-dependent maternal effects on population dynamics: an experimental test

To sleep or not to sleep: Dormancy and life history traits in Eucypris virens (Crustacea, Ostracoda)

Dormancy represents an investment with its own costs and benefit. Besides the advantage obtained from the avoidance of harsh environments and from the synchronization of life cycles with seasonal changes, an organism could benefit from a temporary stop in growth and reproduction. To test this hypothesis a transgenerational experiment was carried out comparing the life history traits of clonal females of Eucypris virens from resting and non-resting eggs at two different photoperiods: short day length (6:18 L:D), proxy of favorable but unpredictable late winter-spring hydroperiod, and long day length (16:8 L:D) proxy of dry predictable unfavorable season, inducing resting egg production and within-generation plasticity (WGP). Clonal females that were dormancy deprived showed the highest age at first deposition and the lowest fecundity. Dormancy seems to work as a resetting mechanism of reproduction. Transgenerational plasticity (TGP) had a bounce back pattern: the phenotype of F1 generation was influenced by cues experienced in the F0 generation but the effects of F0 exposure were not evident in the F2. TGP might be adaptive when a mother experiences some kind of seasonality or stochasticity producing both resting and nonresting eggs. A positive relationship between the number of resting eggs and the total number of eggs per females suggested the absence of trade-off between dormancy and reproduction. Both WGP and TGP increase the mother long term fitness with important consequences on population dynamics, on the way a species spread throughout space and time and might respond to climate change.

Light plasticity of germination on the eastern Tibetan Plateau: Phylogeny, trait, and environmental correlates

Seeds often exhibit great plasticity of germination in response to environmental variability and uncertainty. The causes of this plasticity, however, remain poorly understood, and comparative phylogenic analyses of such plasticity are rare. Here, we analyzed a field germination dataset including 474 species exposed to three different levels of light availability, using comparative phylogenetic methods. We calculated the plasticity of germination in response to light availability (PG_GP) based on the maximum germination proportion (GP_max), PG_T50 based on the time required to reach 50% of GP_max, PG_RGV based on the relative germination velocity (RGV), and PG_Total based on all three of these germination traits. We found that closely related species shared similar light plasticity of germination behavior. Different aspects of germination plasticity in response to light availability were related to specific traits or local environment. PG_GP was associated with adult longevity and local water habitat, while PG_T50 was related to seed mass and local water habitat, and PG_RGV was marginally significantly related to plant height. PG_Total was significantly associated with adult longevity and water habitat. These results suggested that different aspects of germination plasticity were located at specific niche dimension, and local habitats with sufficient soil moisture induced great plasticity germination in response to light environment. As such, they can simplify our understanding of germination, promote the exploration of the general law of germination, and further increase our understanding of species diversity maintenance, adaptation, and evolution from the perspective of germination.

Sex ratio in the mother's environment affects offspring population dynamics: maternal effects on population regulation

Classic population regulation theories usually concern the influence of immediate factors on current populations, but studies investigating the effect of parental environment factors on their offspring populations are scarce. The maternal environments can affect offspring life-history traits across generations, which may affect population dynamics and be a mechanism of population regulation. In cyclical parthenogens, sexual reproduction is typically linked with dormancy, thereby providing a negative feedback to population growth. In this study, we manipulated population sex ratios in the mother's environment to investigate whether this factor affected future population dynamics by regulating offspring sexual reproduction in the rotifer Brachionus calyciflorus. Compared with females in male-biased environments, those in female-biased environments produced fewer mictic (sexual) offspring, and their amictic (asexual) offspring also produced a lower proportion of mictic females at a gradient of population densities. Moreover, populations that were manipulated under male-biased conditions showed significantly smaller population sizes than those under female-biased conditions. Our results indicated that in cyclical parthenogens, mothers could adjust the sexual reproduction of their offspring in response to the current population sex ratio, thus providing fine-scale regulation of population dynamics in addition to population density.

Effect of maternal ammonia stress on population dynamics of the rotifer Brachionus calyciflorus offspring

Maternal effects play important roles in phenotypic variations among individuals and are thus considered to regulate population performance in responses to environmental stress. High ammonia levels are known to suppress population growth of the rotifer Brachionus calyciflorus. However, it remains unclear whether maternal environmental ammonia stress influences the offspring phenotypic variation and, if so, how it affects the offspring population dynamics in the rotifer. The present work examined variations in life history, morphology, feeding and digestive activities of B. calyciflorus offspring affected by maternal ammonia stress and the effect of the above variations on offspring population dynamics. We observed increased fitness in the offspring population affected by the cumulative maternal effect. There was a trade-off between offspring (F₁) survival and reproductive investment under maternal (F₀) ammonia stress. Population growth of the offspring possibly increased via decreasing body size and posterolateral spine length while enhancing cellulase activity. Moreover, the absence of the posterolateral spine of the rotifer was a sensitive response to maternal ammonia stress. These findings underscore maternal environmental stress as an important source of phenotypic variations and highlight these multiple responses work together to affect population dynamics.

Plasticity is a locally adapted trait with consequences for ecological dynamics in novel environments

Phenotypic plasticity is predicted to evolve in more variable environments, conferring an advantage on individual lifetime fitness. It is less clear what the potential consequences of that plasticity will have on ecological population dynamics. Here, we use an invertebrate model system to examine the effects of environmental variation (resource availability) on the evolution of phenotypic plasticity in two life history traits-age and size at maturation-in long-running, experimental density-dependent environments. Specifically, we then explore the feedback from evolution of life history plasticity to subsequent ecological dynamics in novel conditions. Plasticity in both traits initially declined in all microcosm environments, but then evolved increased plasticity for age-at-maturation, significantly so in more environmentally variable environments. We also demonstrate how plasticity affects ecological dynamics by creating founder populations of different plastic phenotypes into new microcosms that had either familiar or novel environments. Populations originating from periodically variable environments that had evolved greatest plasticity had lowest variability in population size when introduced to novel environments than those from constant or random environments. This suggests that while plasticity may be costly it can confer benefits by reducing the likelihood that offspring will experience low survival through competitive bottlenecks in variable environments. In this study, we demonstrate how plasticity evolves in response to environmental variation and can alter population dynamics-demonstrating an eco-evolutionary feedback loop in a complex animal moderated by plasticity in growth.

The role of maternal effects on offspring performance in familiar and novel environments

Maternal effects are an important evolutionary force that may either facilitate adaptation to a new environment or buffer against unfavourable conditions. The degree of variation in traits expressed by siblings from different mothers is often sensitive to environmental conditions. This could generate a Maternal-by-Environment interaction (M × E) that inflates estimates of Genotype-by-Environment effects (G × E). We aimed to test for environment-specific maternal effects (M × E) using a paternal full-sib/half-sib breeding design in the seed beetle Callosobruchus maculatus, where we split and reared offspring from the same mother on two different bean host types-original and novel. Our quantitative genetic analysis indicated that maternal effects were very small on both host types for all the measured life-history traits. There was also little evidence that maternal oviposition preference for a particular host type predicted her offspring's performance on that host. Further, additive genetic variance for most traits was relatively high on both hosts. While there was higher heritability for offspring reared in the novel host, there was no evidence for G × Es, and most cross-host genetic correlations were positive. This suggests that offspring from the same family ranked similarly for performance on both host types. Our results point to a genetic basis of host adaptation in the seed beetle, rather than maternal effects. Even so, we encourage researchers to test for potential M × Es because, due to a lack of testing, it remains unclear how often they arise.

Trans-generational viral transmission and immune priming are dose-dependent

It is becoming increasingly apparent that trans-generational immune priming (i.e. the transfer of the parental immunological experience to its progeny resulting in offspring protection from pathogens that persist across generations) is a common phenomenon not only in vertebrates, but also invertebrates. Likewise, it is known that covert pathogenic infections may become 'triggered' into an overt infection by various stimuli, including exposure to heterologous infections. Yet, rarely have both phenomena been explored in parallel. Using as a model system the African armyworm Spodoptera exempta, an eruptive agricultural pest and its endemic dsDNA virus (Spodoptera exempta nucleopolyhedrovirus, SpexNPV), the aim of this study was to explore the impact of parental inoculating-dose on trans-generational pathogen transmission and immune priming (in its broadest sense). Larvae were orally challenged with one of five doses of SpexNPV and survivors from these treatments were mated and their offspring monitored for viral mortality. Offspring from parents challenged with low viral doses showed evidence of 'immune priming' (i.e. enhanced survival following SpexNPV challenge); in contrast, offspring from parents challenged with higher viral doses exhibited greater susceptibility to viral challenge. Most offspring larvae died of the virus they were orally challenged with; in contrast, most offspring from parents that had been challenged with the highest doses were killed by the vertically transmitted virus (90%) and not the challenge virus. These results demonstrate that the outcome of a potentially lethal virus challenge is critically dependent on the level of exposure to virus in the parental generation-either increasing resistance at very low parental viral doses (consistent with trans-generational immune priming) or increasing susceptibility at higher parental doses (consistent with virus triggering). We discuss the implications of these findings for understanding both natural epizootics of baculoviruses and for using them as biological control agents.

Density-dependent natural selection mediates harvest-induced trait changes

Rapid life-history changes caused by size-selective harvesting are often interpreted as a response to direct harvest selection against a large body size. However, similar trait changes may result from a harvest-induced relaxation of natural selection for a large body size via density-dependent selection. Here, we show evidence of such density-dependent selection favouring large-bodied individuals at high population densities, in replicated pond populations of medaka fish. Harvesting, in contrast, selected medaka directly against a large body size and, in parallel, decreased medaka population densities. Five years of harvesting were enough for harvested and unharvested medaka populations to inherit the classically predicted trait differences, whereby harvested medaka grew slower and matured earlier than unharvested medaka. We show that this life-history divergence was not driven by direct harvest selection for a smaller body size in harvested populations, but by density-dependent natural selection for a larger body size in unharvested populations.

Parental habituation to human disturbance over time reduces fear of humans in coyote offspring

A fundamental tenet of maternal effects assumes that maternal variance over time should have discordant consequences for offspring traits across litters. Yet, seldom are parents observed across multiple reproductive bouts, with few studies considering anthropogenic disturbances as an ecological driver of maternal effects. We observed captive coyote (Canis latrans) pairs over two successive litters to determine whether among‐litter differences in behavior (i.e., risk‐taking) and hormones (i.e., cortisol and testosterone) corresponded with parental plasticity in habituation. Thus, we explicitly test the hypothesis that accumulating experiences of anthropogenic disturbance reduces parental fear across reproductive bouts, which should have disparate phenotypic consequences for first‐ and second‐litter offspring. To quantify risk‐taking behavior, we used foraging assays from 5–15 weeks of age with a human observer present as a proxy for human disturbance. At 5, 10, and 15 weeks of age, we collected shaved hair to quantify pup hormone levels. We then used a quantitative genetic approach to estimate heritability, repeatability, and between‐trait correlations. We found that parents were riskier (i.e., foraged more frequently) with their second versus first litters, supporting our prediction that parents become increasingly habituated over time. Second‐litter pups were also less risk‐averse than their first‐litter siblings. Heritability for all traits did not differ from zero (0.001–0.018); however, we found moderate support for repeatability in all observed traits (r = 0.085–0.421). Lastly, we found evidence of positive phenotypic and cohort correlations among pup traits, implying that cohort identity (i.e., common environment) contributes to the development of phenotypic syndromes in coyote pups. Our results suggest that parental habituation may be an ecological cue for offspring to reduce their fear response, thus emphasizing the role of parental plasticity in shaping their pups’ behavioral and hormonal responses toward humans.

Maternal Effects in a Wild Songbird Are Environmentally Plastic but Only Marginally Alter the Rate of Adaptation

Despite ample evidence for the presence of maternal effects (MEs) in a variety of traits and strong theoretical indications for their evolutionary consequences, empirical evidence to what extent MEs can influence evolutionary responses to selection remains ambiguous. We tested the degree to which MEs can alter the rate of adaptation of a key life-history trait, clutch size, using an individual-based model approach parameterized with experimental data from a long-term study of great tits (Parus major). We modeled two types of MEs: (i) an environmentally plastic ME, in which the relationship between maternal and offspring clutch size depended on the maternal environment via offspring condition, and (ii) a fixed ME, in which this relationship was constant. Although both types of ME affected the rate of adaptation following an abrupt environmental shift, the overall effects were small. We conclude that evolutionary consequences of MEs are modest at best in our study system, at least for the trait and the particular type of ME we considered here. A closer link between theoretical and empirical work on MEs would hence be useful to obtain accurate predictions about the evolutionary consequences of MEs more generally.

Parental and embryonic experiences with predation risk affect prey offspring behaviour and performance

Because phenotypic plasticity can operate both within and between generations, phenotypic outcomes are often shaped by a complex history of environmental signals. For example, parental and embryonic experiences with predation risk can both independently and interactively influence prey offspring traits early in their life. Parental and embryonic risk experiences can also independently shape offspring phenotypes throughout an offspring's ontogeny, but the persistence of their interactive effects throughout offspring ontogeny is unknown. We examined the effects of parental and embryonic experiences with predation risk on the response of 1-year-old prey (the carnivorous snail, Nucella lapillus) offspring to current predation risk. We found that parental and embryonic risk experiences had largely independent effects on offspring performance and that these effects were context dependent. Parental experience with risk had strong impacts on multiple offspring traits in the presence of current risk that generally improved offspring performance under risk, but embryonic risk experience had relatively weaker effects and only operated in the absence of current risk to reduce offspring growth. These results illustrate that past environmental experiences can dynamically shape organism phenotypes across ontogeny and that attention to these effects is key to a better understanding of predator/prey dynamics in natural systems.

Maternal Effects Via Resting eggs in Predator Defense of the Rotifer Brachionus calyciflorus

Maternal effects play important evolutionary and ecological roles. Amictic female mothers of monogonont rotifer Brachionus calyciflorus can transmit predatory information of Asplanchna brightwellii in their environment to their offspring by changing the offspring's defensive morphology to increase their fitness. However, it remains unclear whether such maternal effects also exist during sexual reproduction of a mictic mother. This study explored the maternal effect in mictic mothers using the B. calyciflorus and A. brightwellii as a prey-predator model. We collected resting eggs from two groups of mictic mothers that previously experienced environments with (P environment) or without (NP environment) Asplanchna kairomones. Stem females from the resting eggs of each maternal group were also hatched and reared in P and NP environments. The population growth rate of offspring who experienced the same environment as their mictic mothers was significantly higher than those that experienced a different environment. When exposed to a gradient of predator kairomone levels, the posterolateral spine of the offspring elongated with increasing kairomone concentration. Offspring from the P mictic mother showed significantly shorter posterolateral spines than those from the NP mictic mother at each predator kairomone level. Offspring originating from NP mictic mothers clearly elongated their posterolateral spines at low concentrations of predator kairomones, while those from P mothers elongated their posterolateral spines only at the highest levels of predator kairomone. Our findings highlight the existence of anticipatory maternal effects during the sexual process via resting eggs of B. calyciflorus in response to predator kairomone.

The role of maternal age and context-dependent maternal effects in the offspring provisioning of a long-lived marine teleost

Despite evidence of maternal age effects in a number of teleost species, there have been challenges to the assertion that maternal age intrinsically influences offspring quality. From an evolutionary perspective, maternal age effects result in young females paradoxically investing in less fit offspring despite a greater potential fitness benefit that might be gained by allocating this energy to individual somatic growth. Although a narrow range of conditions could lead to a maternal fitness benefit via the production of lower quality offspring, evolutionary theorists suggest these conditions are seldom met and that the reported maternal age effects are more likely products of the environmental context. Our goal was to determine if maternal effects operated on offspring provisioning in a long-lived rockfish (genus Sebastes), and to evaluate any such effects as an intrinsic function of maternal age or a context-dependent effect of the offspring release environment. We found that offspring provisioning is a function of both maternal age and the timing of offspring release; older females exhibit increased provisioning over younger females throughout the spawning season despite a decrease in provisioning across all maternal ages as the season progresses. These findings suggest a role for both maternal age effects and a potential context-dependent maternal effect in population productivity, carrying important implications when modelling population persistence and resilience.

Context-Dependent Parental Effects on Clonal Offspring Performance

Parental environments may potentially affect offspring fitness, and the expression of such parental effects may depend on offspring environments and on whether one considers an individual offspring or all offspring of a parent. Using a well-studied clonal herb, Alternanthera philoxeroides, we first grew parent plants in high and low soil-nutrient conditions and obtained 1st generation clonal offspring from these two environments. Then we grew offspring of these two types of 1st generation clonal offspring also in high and low nutrient conditions. We measured and analyzed mean performance and summed performance of the four types of 2nd generation clonal offspring. High nutrient availability of parental environments markedly increased both mean performance (i.e., the average fitness measure across all individual offspring produced by a parent) and summed performance (i.e., the sum of the fitness measure of all offspring produced by a parent) of the 2nd generation clonal offspring. The positive parental effects on summed performance of the 2nd generation clonal offspring were stronger when the 1st generation clonal offspring grew in the high instead of the low nutrient conditions, but the positive parental effects on their mean performance did not depend on the nutrient environments of the 1st generation clonal offspring. The results provide novel evidence that parental environmental effects persist across vegetative generations and strongly depend on offspring environments and levels of plants.

Environmental and genetic determinants of transcriptional plasticity in Chinook salmon

Variation in gene transcription is widely believed to be the mechanistic basis of phenotypically plastic traits; however, comparatively little is known about the inheritance patterns of transcriptional variation that would allow us to predict its response to selection. In addition, acclimation to different environmental conditions influences acute transcriptional responses to stress and it is unclear if these effects are heritable. To address these gaps in knowledge, we assayed levels of messenger RNA for 14 candidate genes at rest and in response to a 24-h confinement stress for 72 half-sib families of Chinook salmon reared in two different environments (hatchery and semi-natural stream channel). We observed extensive plasticity for mRNA levels of metabolic and stress response genes and demonstrated that mRNA level plasticity due to rearing environment affects mRNA level plasticity in response to stress. These effects have important implications for natural populations experiencing multiple stressors. We identified genotype-by-environment interactions for mRNA levels that were dominated by maternal effects; however, mRNA level response to challenge also exhibited a non-additive genetic basis. Our results indicate that while plasticity for mRNA levels can evolve, predicting the outcome of selection will be difficult. The inconsistency in genetic architecture among treatment groups suggests there is considerable cryptic genetic variation for gene expression.

Breeding matters: Natal experience influences population state-dependent host acceptance by an eruptive insect herbivore

Eruptive forest insects are highly influential agents of change in forest ecosystems, and their effects have increased with recent climate change. State-dependent life histories contribute significantly to the population dynamics of eruptive forest insect herbivores; however, the proximate mechanisms by which these species shift between states is poorly understood. Laboratory bioassays were conducted using the mountain pine beetle (Dendroctonus ponderosae) to determine the effect of maternal host selection on offspring host preferences, as they apply to population state-dependent behaviors. Female mountain pine beetles exhibited state-dependent preference for artificial host material amended with monoterpenes in the absence of other cues, such that individuals reared in high-density epidemic-state simulations rejected low monoterpene conditions, while low-density endemic-state beetles accepted low monoterpene conditions. State-specific behavior in offspring was dependent on rearing conditions, as a function of maternal host selection, and these effects were observed within one generation. Density-dependent host selection behaviors exhibited by female mountain pine beetle offspring is reinforced by context-dependent maternal effects arising from parental host selection, and in situ exposure to conspecifics. These results demonstrate potential proximate mechanisms that control population dynamics in eruptive forest insects, and will allow for more accurate predictions of continued impact and spread of these species.

A fitness trade-off between seasons causes multigenerational cycles in phenotype and population size

Although seasonality is widespread and can cause fluctuations in the intensity and direction of natural selection, we have little information about the consequences of seasonal fitness trade-offs for population dynamics. Here we exposed populations of Drosophila melanogaster to repeated seasonal changes in resources across 58 generations and used experimental and mathematical approaches to investigate how viability selection on body size in the non-breeding season could affect demography. We show that opposing seasonal episodes of natural selection on body size interacted with both direct and delayed density dependence to cause populations to undergo predictable multigenerational density cycles. Our results provide evidence that seasonality can set the conditions for life-history trade-offs and density dependence, which can, in turn, interact to cause multigenerational population cycles.

Transgenerational plasticity following a dual pathogen and stress challenge in fruit flies

BACKGROUND

Phenotypic plasticity operates across generations, when the parental environment affects phenotypic expression in the offspring. Recent studies in invertebrates have reported transgenerational plasticity in phenotypic responses of offspring when the mothers had been previously exposed to either live or heat-killed pathogens. Understanding whether this plasticity is adaptive requires a factorial design in which both mothers and their offspring are subjected to either the pathogen challenge or a control, in experimentally matched and mismatched combinations. Most prior studies exploring the capacity for pathogen-mediated transgenerational plasticity have, however, failed to adopt such a design. Furthermore, it is currently poorly understood whether the magnitude or direction of pathogen-mediated transgenerational responses will be sensitive to environmental heterogeneity. Here, we explored the transgenerational consequences of a dual pathogen and stress challenge administered in the maternal generation in the fruit fly, Drosophila melanogaster. Prospective mothers were assigned to a non-infectious pathogen treatment consisting of an injection with heat-killed bacteria or a procedural control, and a stress treatment consisting of sleep deprivation or control. Their daughters and sons were similarly assigned to the same pathogen treatment, prior to measurement of their reproductive success.

RESULTS

We observed transgenerational interactions involving pathogen treatments of mothers and their offspring, on the reproductive success of daughters but not sons. These interactions were unaffected by sleep deprivation.

CONCLUSIONS

The direction of the transgenerational effects was not consistent with that predicted under a scenario of adaptive transgenerational plasticity. Instead, they were indicative of expectations based on terminal investment.

Harvested populations are more variable only in more variable environments

The interaction between environmental variation and population dynamics is of major importance, particularly for managed and economically important species, and especially given contemporary changes in climate variability. Recent analyses of exploited animal populations contested whether exploitation or environmental variation has the greatest influence on the stability of population dynamics, with consequences for variation in yield and extinction risk. Theoretical studies however have shown that harvesting can increase or decrease population variability depending on environmental variation, and requested controlled empirical studies to test predictions. Here, we use an invertebrate model species in experimental microcosms to explore the interaction between selective harvesting and environmental variation in food availability in affecting the variability of stage‐structured animal populations over 20 generations. In a constant food environment, harvesting adults had negligible impact on population variability or population size, but in the variable food environments, harvesting adults increased population variability and reduced its size. The impact of harvesting on population variability differed between proportional and threshold harvesting, between randomly and periodically varying environments, and at different points of the time series. Our study suggests that predicting the responses to selective harvesting is sensitive to the demographic structures and processes that emerge in environments with different patterns of environmental variation.

Phenotypic plasticity in growth and fecundity induced by strong population fluctuations affects reproductive traits of female fish

Fish are known for their high phenotypic plasticity in life-history traits in relation to environmental variability, and this is particularly pronounced among salmonids in the Northern Hemisphere. Resource limitation leads to trade-offs in phenotypic plasticity between life-history traits related to the reproduction, growth, and survival of individual fish, which have consequences for the age and size distributions of populations, as well as their dynamics and productivity. We studied the effect of plasticity in growth and fecundity of vendace females on their reproductive traits using a series of long-term incubation experiments. The wild parental fish originated from four separate populations with markedly different densities, and hence naturally induced differences in their growth and fecundity. The energy allocation to somatic tissues and eggs prior to spawning served as a proxy for total resource availability to individual females, and its effects on offspring survival and growth were analyzed. Vendace females allocated a rather constant proportion of available energy to eggs (per body mass) despite different growth patterns depending on the total resources in the different lakes; investment into eggs thus dictated the share remaining for growth. The energy allocation to eggs per mass was higher in young than in old spawners and the egg size and the relative fecundity differed between them: Young females produced more and smaller eggs and larvae than old spawners. In contrast to earlier observations of salmonids, a shortage of maternal food resources did not increase offspring size and survival. Vendace females in sparse populations with ample resources and high growth produced larger eggs and larvae. Vendace accommodate strong population fluctuations by their high plasticity in growth and fecundity, which affect their offspring size and consequently their recruitment and productivity, and account for their persistence and resilience in the face of high fishing mortality.

How well can body size represent effects of the environment on demographic rates? Disentangling correlated explanatory variables

Demographic rates are shaped by the interaction of past and current environments that individuals in a population experience. Past environments shape individual states via selection and plasticity, and fitness-related traits (e.g. individual size) are commonly used in demographic analyses to represent the effect of past environments on demographic rates. We quantified how well the size of individuals captures the effects of a population's past and current environments on demographic rates in a well-studied experimental system of soil mites. We decomposed these interrelated sources of variation with a novel method of multiple regression that is useful for understanding nonlinear relationships between responses and multicollinear explanatory variables. We graphically present the results using area-proportional Venn diagrams. Our novel method was developed by combining existing methods and expanding upon them. We showed that the strength of size as a proxy for the past environment varied widely among vital rates. For instance, in this organism with an income breeding life history, the environment had more effect on reproduction than individual size, but with substantial overlap indicating that size encompassed some of the effects of the past environment on fecundity. This demonstrates that the strength of size as a proxy for the past environment can vary widely among life-history processes within a species, and this variation should be taken into consideration in trait-based demographic or individual-based approaches that focus on phenotypic traits as state variables. Furthermore, the strength of a proxy will depend on what state variable(s) and what demographic rate is being examined; that is, different measures of body size (e.g. length, volume, mass, fat stores) will be better or worse proxies for various life-history processes.

Early and late maternal effects on hatching phenology of Heterocypris incongruens (Crustacea: Ostracoda)

In ephemeral ponds, the hatching asynchrony of resting eggs may be adaptive and the result of a maternal bet-hedging strategy. A mother can influence the progeny phenology through conditions experienced during life cycle even in early development stages. We investigated the consequences of a hatching delay for offspring and compared early and late maternal effects in a clonal lineage of Heterocypris incongruens. We used females from genetically identical, 40 months old, resting eggs that hatched, asynchronically, after a first (FI) or a second (SI) inundation event. Maternal origin (FI or SI) was considered an early effect involving the maternal response to hatching stimuli during the embryological dormant stage. Maternal age at deposition and egg size were considered late effects that account for maternal conditions during active stage. We compared size and development time of eggs produced by FI and SI females under laboratory condition (24°C 12:12 L:D photoperiod). Maternal origin affected development time to adulthood which was later in FI than in SI females, and fecundity that was higher in FI than in SI females. SI eggs were smaller than FI eggs: size was affected by maternal age at deposition and was directly related to the egg development time. Development time varied from 1 to 117 days and was shorter in SI eggs than in FI eggs. Our results showed that maternal response during embryological stage affects the performance in successive active stages and suggested that hatching asynchrony may be considered a risk spread strategy.

Genetic variation of transgenerational plasticity of offspring germination in response to salinity stress and the seed transcriptome of Medicago truncatula

BACKGROUND

Transgenerational plasticity provides phenotypic variation that contributes to adaptation. For plants, the timing of seed germination is critical for offspring survival in stressful environments, as germination timing can alter the environmental conditions a seedling experiences. Stored seed transcripts are important determinants of seed germination, but have not previously been linked with transgenerational plasticity of germination behavior. In this study we used RNAseq and growth chamber experiments of the model legume M. trucantula to test whether parental exposure to salinity stress influences the expression of stored seed transcripts and early offspring traits and test for genetic variation.

RESULTS

We detected genotype-dependent parental environmental effects (transgenerational plasticity) on the expression levels of stored seed transcripts, seed size, and germination behavior of four M. truncatula genotypes. More than 50% of the transcripts detected in the mature, ungerminated seed transcriptome were annotated as regulating seed germination, some of which are involved in abiotic stress response and post-embryonic development. Some genotypes showed increased seed size in response to parental exposure to salinity stress, but no parental environmental influence on germination timing. In contrast, other genotypes showed no seed size differences across contrasting parental conditions but displayed transgenerational plasticity for germimation timing, with significantly delayed germination in saline conditions when parental plants were exposed to salinity. In genotypes that show significant transgenerational plastic germination response, we found significant coexpression networks derived from salt responsive transcripts involved in post-transcriptional regulation of the germination pathway. Consistent with the delayed germination response to saline conditions in these genotypes, we found genes associated with dormancy and up-regulation of abscisic acid (ABA).

CONCLUSIONS

Our results demonstrate genetic variation in transgenerational plasticity within M. truncatula and show that parental exposure to salinity stress influences the expression of stored seed transcripts, seed weight, and germination behavior. Furthermore, we show that the parental environment influences gene expression to modulate biological pathways that are likely responsible for offspring germination responses to salinity stress.

Fitness consequences of maternal and grandmaternal effects

Transgenerational effects are broader than only parental relationships. Despite mounting evidence that multigenerational effects alter phenotypic and life-history traits, our understanding of how they combine to determine fitness is not well developed because of the added complexity necessary to study them. Here, we derive a quantitative genetic model of adaptation to an extraordinary new environment by an additive genetic component, phenotypic plasticity, maternal and grandmaternal effects. We show how, at equilibrium, negative maternal and negative grandmaternal effects maximize expected population mean fitness. We define negative transgenerational effects as those that have a negative effect on trait expression in the subsequent generation, that is, they slow, or potentially reverse, the expected evolutionary dynamic. When maternal effects are positive, negative grandmaternal effects are preferred. As expected under Mendelian inheritance, the grandmaternal effects have a lower impact on fitness than the maternal effects, but this dual inheritance model predicts a more complex relationship between maternal and grandmaternal effects to constrain phenotypic variance and so maximize expected population mean fitness in the offspring.

Trans-generational plasticity in physiological thermal tolerance is modulated by maternal pre-reproductive environment in the polychaete Ophryotrocha labronica

Maternal temperature is known to affect many aspects of offspring phenotype, but its effect on offspring physiological thermal tolerance has received less attention, despite the importance of physiological traits in defining organismal ability to cope with temperature changes. To fill this gap, we used the marine polychaete, Ophryotrocha labronica, to investigate the influence of maternal temperature on offspring upper and lower thermal tolerance limits, and assess whether maternal influence changed according to the stage of offspring pre-zygotic development at which a thermal cue was provided. Measurements were taken on adult offspring acclimated to 18°C or 30°C, produced by mothers previously reared at 24°C and then exposed to 18°C or 30°C at an early and late stage of oogenesis. When the shift from 24°C was provided early during oogenesis, mothers produced offspring with greater cold and heat tolerance whenever mother-offspring temperatures did not match, respect to when they matched, suggesting the presence of an anticipatory maternal effect triggered by the thermal variation. Conversely, when the cue was provided later during oogenesis, more tolerant offspring were observed when temperatures persisted across generations. In this case, maternal exposure to 18°C or 30°C may have benefited offspring performance, while limitations in the transmission of the thermal cue may account for the lack of correlation between maternal experiences and offspring performance when mother-offspring environments did not match. Our results provided evidence for a trans-generational effect of temperature on physiological performance characterised by a high context-dependency, and were discussed in the light of maternal pre-reproductive experiences.

Geographical differences in maternal basking behaviour and offspring growth rate in a climatically widespread viviparous reptile

In reptiles, the thermal environment during embryonic development affects offspring phenotypic traits and potentially offspring fitness. In viviparous species, mothers can potentially manipulate the embryonic thermal environment through their basking behaviour and, thus, may be able to manipulate offspring phenotype and increase offspring fitness. One way in which mothers can maximise offspring phenotype (and thus potentially affect offspring fitness) is by fine-tuning their basking behaviour to the environment in order to buffer the embryo from deleterious developmental temperatures. In widespread species, it is unclear whether populations that have evolved under different climatic conditions will exhibit different maternal behaviours and/or thermal effects on offspring phenotype. To test this, we provided extended or reduced basking opportunity to gravid spotted skinks (Niveoscincus ocellatus) and their offspring from two populations at the climatic extremes of the species' distribution. Gravid females fine-tuned their basking behaviour to the basking opportunity, which allowed them to buffer their embryos from potentially negative thermal effects. This fine-tuning of female basking behaviour appears to have led to the expression of geographical differences in basking behaviour, with females from the cold alpine regions being more opportunistic in their basking behaviour than females from the warmer regions. However, those differences in maternal behaviour did not preclude the evolution of geographic differences in thermal effects: offspring growth varied between populations, potentially suggesting local adaptation to basking conditions. Our results demonstrate that maternal effects and phenotypic plasticity can play a significant role in allowing species to cope in changing environmental conditions, which is particularly relevant in the context of climate change.

An automated image analysis system to measure and count organisms in laboratory microcosms

1. Because of recent technological improvements in the way computer and digital camera perform, the potential use of imaging for contributing to the study of communities, populations or individuals in laboratory microcosms has risen enormously. However its limited use is due to difficulties in the automation of image analysis. 2. We present an accurate and flexible method of image analysis for detecting, counting and measuring moving particles on a fixed but heterogeneous substrate. This method has been specifically designed to follow individuals, or entire populations, in experimental laboratory microcosms. It can be used in other applications. 3. The method consists in comparing multiple pictures of the same experimental microcosm in order to generate an image of the fixed background. This background is then used to extract, measure and count the moving organisms, leaving out the fixed background and the motionless or dead individuals. 4. We provide different examples (springtails, ants, nematodes, daphnia) to show that this non intrusive method is efficient at detecting organisms under a wide variety of conditions even on faintly contrasted and heterogeneous substrates. 5. The repeatability and reliability of this method has been assessed using experimental populations of the Collembola Folsomia candida. 6. We present an ImageJ plugin to automate the analysis of digital pictures of laboratory microcosms. The plugin automates the successive steps of the analysis and recursively analyses multiple sets of images, rapidly producing measurements from a large number of replicated microcosms.

Eco-evolutionary dynamics in response to selection on life-history

Understanding the consequences of environmental change on ecological and evolutionary dynamics is inherently problematic because of the complex interplay between them. Using invertebrates in microcosms, we characterise phenotypic, population and evolutionary dynamics before, during and after exposure to a novel environment and harvesting over 20 generations. We demonstrate an evolved change in life-history traits (the age- and size-at-maturity, and survival to maturity) in response to selection caused by environmental change (wild to laboratory) and to harvesting (juvenile or adult). Life-history evolution, which drives changes in population growth rate and thus population dynamics, includes an increase in age-to-maturity of 76% (from 12.5 to 22 days) in the unharvested populations as they adapt to the new environment. Evolutionary responses to harvesting are outweighed by the response to environmental change (∼ 1.4 vs. 4% change in age-at-maturity per generation). The adaptive response to environmental change converts a negative population growth trajectory into a positive one: an example of evolutionary rescue.

Ghosts of habitats past: environmental carry-over effects drive population dynamics in novel habitat

The phenotype of adults can be strongly influenced by the environmental conditions experienced during development. Consequently, variation in habitat quality across space and through time also leads to differences in the phenotypes of adults. This could create carry-over effects where differences in the natal habitat quality of colonizers influence population dynamics in new habitats. We tested this hypothesis experimentally by simulating dispersal of Tribolium castaneum from low- or high-quality natal habitat into new patches of low- or high-quality habitat. Differences in the natal habitat quality of colonizers altered population growth trajectories and led to carrying capacities that differed by up to 63% within a habitat type, indicating that patch dynamics are determined by the interaction of past and current habitat quality. Interestingly, even after multiple generations, the natal habitat of colonizers determined differences in adult traits that were related to density-dependent population regulation. These changes in adult phenotype could at least partially explain why carry-over effects continued to alter population dynamics for multiple generations until the end of the experiment. These results highlight the importance of variable habitat quality and carry-over effects for population dynamics.

Not all offspring are created equal: variation in larval characteristics in a serially spawning damselfish

The way organisms allocate their resources to growth and reproduction are key attributes differentiating life histories. Many organisms spawn multiple times in a breeding season, but few studies have investigated the impact of serial spawning on reproductive allocation. This study investigated whether resource allocation was influenced by parental characteristics and prior spawning history in a serial spawning tropical damselfish (Pomacentrus amboinensis). The offspring attributes of isolated parents of known characteristics were monitored over a 6-week breeding period in the field. Smaller females produced larvae of longer length and larger energy reserves at hatching. This finding is contrary to several other studies that found larger females produce offspring of greater quality. We found that resource allocation in the form of reproductive output was not influenced by the number of spawning events within the breeding season, with larger females producing the greatest number of offspring. Larval characteristics changed as spawning progressed. There was a general decline in length of larvae produced, with an increase in the size of the larval yolk-sac, for all females regardless of size as spawning progressed. This trend was accentuated by the smallest females. This change in larval characteristics may reflect a parental ability to forecast unfavourable conditions as the season progresses or a mechanism to ensure that some will survive no matter what conditions they encounter. This study highlights the importance of accounting for temporal changes in reproductive allocation in studies of reproductive trade-offs and investigations into the importance of parental effects.

Hormonally mediated maternal effects, individual strategy and global change

A challenge to ecologists and evolutionary biologists is predicting organismal responses to the anticipated changes to global ecosystems through climate change. Most evidence suggests that short-term global change may involve increasing occurrences of extreme events, therefore the immediate response of individuals will be determined by physiological capacities and life-history adaptations to cope with extreme environmental conditions. Here, we consider the role of hormones and maternal effects in determining the persistence of species in altered environments. Hormones, specifically steroids, are critical for patterning the behaviour and morphology of parents and their offspring. Hence, steroids have a pervasive influence on multiple aspects of the offspring phenotype over its lifespan. Stress hormones, e.g. glucocorticoids, modulate and perturb phenotypes both early in development and later into adulthood. Females exposed to abiotic stressors during reproduction may alter the phenotypes by manipulation of hormones to the embryos. Thus, hormone-mediated maternal effects, which generate phenotypic plasticity, may be one avenue for coping with global change. Variation in exposure to hormones during development influences both the propensity to disperse, which alters metapopulation dynamics, and population dynamics, by affecting either recruitment to the population or subsequent life-history characteristics of the offspring. We suggest that hormones may be an informative index to the potential for populations to adapt to changing environments.

Complex interactions between paternal and maternal effects: parental experience and age at reproduction affect fecundity and offspring performance in a butterfly

Parental effects can greatly affect offspring performance and are thus expected to impact population dynamics and evolutionary trajectories. Most studies have focused on maternal effects, whereas fathers are also likely to influence offspring phenotype, for instance when males transfer nutrients to females during mating. Moreover, although the separate effects of maternal age and the environment have been documented as a source of parental effects in many species, their combined effects have not been investigated. In the present study, we analyzed the combined effects of maternal and paternal age at reproduction and a mobility treatment in stressful conditions on offspring performance in the butterfly Pieris brassicae. Both paternal and maternal effects affected progeny traits but always via interactions between age and mobility treatment. Moreover, parental effects shifted from male effects expressed at the larval stage to maternal effects at the adult stage. Indeed, egg survival until adult emergence significantly decreased with father age at mating only for fathers having experienced the mobility treatment, whereas offspring adult life span decreased with increasing mother age at laying only for females that did not experience the mobility treatment. Overall, our results demonstrate that both parents' phenotypes influence offspring performance through nongenetic effects, their relative contribution varying over the course of progeny's life.

The benefits of maternal effects in novel and in stable environments

Natural selection favours phenotypes that match prevailing ecological conditions. A rapid process of adaptation is therefore required in changing environments. Maternal effects can facilitate such responses, but it is currently poorly understood under which circumstances maternal effects may accelerate or slow down the rate of phenotypic evolution. Here, we use a quantitative genetic model, including phenotypic plasticity and maternal effects, to suggest that the relationship between fitness and phenotypic variance plays an important role. Intuitive expectations that positive maternal effects are beneficial are supported following an extreme environmental shift, but, if too strong, that shift can also generate oscillatory dynamics that overshoot the optimal phenotype. In a stable environment, negative maternal effects that slow phenotypic evolution actually minimize variance around the optimum phenotype and thus maximize population mean fitness.

Population responses to perturbations: the importance of trait-based analysis illustrated through a microcosm experiment

Environmental change continually perturbs populations from a stable state, leading to transient dynamics that can last multiple generations. Several long-term studies have reported changes in trait distributions along with demographic response to environmental change. Here we conducted an experimental study on soil mites and investigated the interaction between demography and an individual trait over a period of nonstationary dynamics. By following individual fates and body sizes at each life-history stage, we investigated how body size and population density influenced demographic rates. By comparing the ability of two alternative approaches, a matrix projection model and an integral projection model, we investigated whether consideration of trait-based demography enhances our ability to predict transient dynamics. By utilizing a prospective perturbation analysis, we addressed which stage-specific demographic or trait-transition rate had the greatest influence on population dynamics. Both body size and population density had important effects on most rates; however, these effects differed substantially among life-history stages. Considering the observed trait-demography relationships resulted in better predictions of a population's response to perturbations, which highlights the role of phenotypic plasticity in transient dynamics. Although the perturbation analyses provided comparable predictions of stage-specific elasticities between the matrix and integral projection models, the order of importance of the life-history stages differed between the two analyses. In conclusion, we demonstrate how a trait-based demographic approach provides further insight into transient population dynamics.

Individual variation and population dynamics: lessons from a simple system

The mapping of environment, through variation in individuals' life histories, to dynamics can be complex and often poorly known. Consequently, it is not clear how important it is dynamically. To explore this, I incorporated lessons from an empirical system, a soil mite, into an individual-based model. Individuals compete for resource and allocate this according to eight 'genetic' rules that specify investment in growth or reserves (which influences survival or fecundity), size at maturation and reproductive allocation. Density dependence, therefore, emerges from competition for food, limiting individual's growth and fecundity. We use this model to examine the role that genetic and phenotypically plastic variation plays in dynamics, by fixing phenotypes, by allowing phenotypes to vary plastically and by creating genetic variation between individuals. Variation, and how it arises, influences short- and long-run dynamics in a way comparable in magnitude with halving food supply. In particular, by switching variation on and off, it is possible to identify a range of processes necessary to capture the dynamics of the 'full model'. Exercises like this can help identify key processes and parameters, but a concerted effort is needed across many different systems to search for shared understanding of both process and modelling.

Effects of paternal phenotype and environmental variability on age and size at maturity in a male dimorphic mite

Investigating how the environment affects age and size at maturity of individuals is crucial to understanding how changes in the environment affect population dynamics through the biology of a species. Paternal phenotype, maternal, and offspring environment may crucially influence these traits, but to my knowledge, their combined effects have not yet been tested. Here, I found that in bulb mites (Rhizoglyphus robini), maternal nutrition, offspring nutrition, and paternal phenotype (males are fighters, able to kill other mites, or benign scramblers) interactively affected offspring age and size at maturity. The largest effect occurred when both maternal and offspring nutrition was poor: in that case offspring from fighter sires required a significantly longer development time than offspring from scrambler sires. Investigating parental effects on the relationship between age and size at maturity revealed no paternal effects, and only for females was its shape influenced by maternal nutrition. Overall, this reaction norm was nonlinear. These non-genetic intergenerational effects may play a complex, yet unexplored role in influencing population fluctuations-possibly explaining why results from field studies often do not match theoretical predictions on maternal effects on population dynamics.

Phenotypic plasticity and population viability: the importance of environmental predictability

Phenotypic plasticity plays a key role in modulating how environmental variation influences population dynamics, but we have only rudimentary understanding of how plasticity interacts with the magnitude and predictability of environmental variation to affect population dynamics and persistence. We developed a stochastic individual-based model, in which phenotypes could respond to a temporally fluctuating environmental cue and fitness depended on the match between the phenotype and a randomly fluctuating trait optimum, to assess the absolute fitness and population dynamic consequences of plasticity under different levels of environmental stochasticity and cue reliability. When cue and optimum were tightly correlated, plasticity buffered absolute fitness from environmental variability, and population size remained high and relatively invariant. In contrast, when this correlation weakened and environmental variability was high, strong plasticity reduced population size, and populations with excessively strong plasticity had substantially greater extinction probability. Given that environments might become more variable and unpredictable in the future owing to anthropogenic influences, reaction norms that evolved under historic selective regimes could imperil populations in novel or changing environmental contexts. We suggest that demographic models (e.g. population viability analyses) would benefit from a more explicit consideration of how phenotypic plasticity influences population responses to environmental change.

Variation within and between closely related species uncovers high intra-specific variability in dispersal

Mounting evidence shows that contrasting selection pressures generate variability in dispersal patterns among individuals or populations of the same species, with potential impacts on both species dynamics and evolution. However, this variability is hardly considered in empirical works, where a single dispersal function is considered to adequately reflect the species-specific dispersal ability, suggesting thereby that within-species variation is negligible as regard to inter-specific differences in dispersal abilities. We propose here an original method to make the comparison of intra- and inter-specific variability in dispersal, by decomposing the diversity of that trait along a phylogeny of closely related species. We used as test group European butterflies that are classic study organisms in spatial ecology. We apply the analysis separately to eight metrics that reflect the dispersal propensity, the dispersal ability or the dispersal efficiency of populations and species. At the inter-specific level, only the dispersal ability showed the signature of a phylogenetic signal while neither the dispersal propensity nor the dispersal efficiency did. At the within-species level, the partitioning of dispersal diversity showed that dispersal was variable or highly variable among populations: intra-specific variability represented from 11% to 133% of inter-specific variability in dispersal metrics. This finding shows that dispersal variation is far from negligible in the wild. Understanding the processes behind this high within-species variation should allow us to properly account for dispersal in demographic models. Accordingly, to encompass the within species variability in life histories the use of more than one value per trait per species should be encouraged in the construction of databases aiming at being sources for modelling purposes.