Genetic correlation between temperature-induced plasticity of life-history traits in a soil arthropod

Temperature is considered one of the most important mediators of phenotypic plasticity in ectotherms. However, the costs and benefits shaping the evolution of different thermal responses are poorly elucidated. One of the possible constraints to phenotypic plasticity is its intrinsic genetic cost, such as genetic linkage or pleiotropy. Genetic coupling of the thermal response curves for different life history traits may significantly affect the evolution of thermal sensitivity in thermally fluctuating environments. We used the collembolan Orchesella cincta to study if there is genetic variation in temperature-induced phenotypic plasticity in life history traits, and if the degree of temperature-induced plasticity is correlated across traits. Egg development rate, juvenile growth rate and egg size of 19 inbred isofemale lines were measured at two temperatures. Our results show that temperature was a highly significant factor for all three traits. Egg development rate and juvenile growth rate increased with increasing temperature, while egg size decreased. Line by temperature interaction was significant for all traits tested; indicating that genetic variation for temperature-induced plasticity existed. The degree of plasticity was significantly positively correlated between egg development rate and growth rate, but plasticity in egg size was not correlated to the other two plasticity traits. The findings suggest that the thermal plasticities of egg development rate and growth rate are partly under the control of the same genes or genetic regions. Hence, evolution of the thermal plasticity of traits cannot be understood in isolation of the response of other traits. If traits have similar and additive effects on fitness, genetic coupling between these traits may well facilitate the evolution of optimal phenotypes. However, for this we need to know the selective forces under field conditions.     

Are latitudinal clines in body size adaptive?

Body size of animals often increases with increasing latitude. These latitudinal clines in body size have interested biologists for over 150 years. However, the mechanisms that generate these clines in size are still unclear, though latitudinal gradients in temperature appear to play an important role. More importantly, many studies that examine latitudinal clines in body size and the mechanisms responsible for these clines use phenotypic data, confounding genetic (adaptive) and non‐genetic (plasticity) sources of variation. Yet, most of these studies make adaptive conclusions based on phenotypic measures of size. Here I show the dangers of making adaptive inferences from phenotypic measures of size. In addition, I use a specific form of plasticity in body size of ectotherms, called the temperature–size rule, to illustrate how confusion about genetic and non‐genetic contributions to phenotypic variation has hampered progress in understanding the evolution of latitudinal clines in size. Field‐based measurements of body size can no doubt be influenced by plasticity, but demonstrating that latitudinal clines have a genetic basis is necessary to show that these patterns are adaptive.     

ADAPTIVE GENETIC VARIATION IN GROWTH AND DEVELOPMENT OF TADPOLES OF THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX

The distribution and proportion of the sexual species Rana lessonae to the hemiclonal hybrid R. esculenta among natural habitats suggests that these anurans may differ in adaptive abilities. I used a half-sib design to partition phenotypic and quantitative genetic variation in tadpole responses at two food levels into causal variance components. Rana lessonae displays strong phenotypic variation across food levels. Growth rate is strictly determined by environmental factors and includes weak maternal effects. Larval period and body size at metamorphosis both contain moderate levels of additive genetic variance. The sire x food interactions and the lack of environmental correlations indicate that adaptive phenotypic plasticity is present in both of these traits. In contrast, R. esculenta displays less phenotypic variation across food levels, especially for larval period. Variation in body size at metamorphosis is underlain by genetic variation as shown by high levels of additive genetic variance, yet growth rate and larval period are not. Significant environmental correlations between larval period at high food level and growth, larval period, and body size at low food, indicate phenotypic plasticity is absent. A positive phenotypic correlation between body size at metamorphosis and larval period for R. lessonae at both food levels suggests a trade-off between growing large and metamorphosing quickly to escape predation or pond drying. The lack of a similar correlation for R. esculenta at the high food level suggests it may be less constrained. Different levels of adaptive genetic variation among larval traits suggest that the sexual species and the hybridogenetic hemiclone differ in their abilities to cope with temporally and spatially heterogeneous environments.     

Effects of temperature on reproductive output, egg provisioning, juvenile hormone and vitellogenin titres in the butterfly Bicyclus anynana

Environmentally induced phenotypic plasticity is common in nature. Hormones, affecting multiple traits and signaling to a variety of distant target tissues, provide a mechanistic link between environments, genes and trait expression, and may therefore well be involved in the regulation phenotypic plasticity. Here, we investigate whether in the tropical butterfly Bicyclus anynana temperature-mediated plasticity in egg size and number, with fewer but larger eggs produced at lower temperatures and vice versa, is under control of juvenile hormone, and whether different temperatures cause differences in egg composition. Female B. anynana butterflies showed the expected response to temperature, however, we found no evidence for an involvement of juvenile hormone. Neither haemolymph JH II and JH III titres nor vitellogenin levels differed across temperatures. The smaller eggs produced at the higher temperature contained relatively higher amounts of water, free carbohydrates and proteins, but relatively lower amounts of lipids. While these smaller eggs had a lower absolute energy content, total reproductive investment was higher at the higher temperature (due to a higher fecundity). Overall, our study indicates that temperature-mediated plasticity in reproduction in B. anynana is mechanistically related to a biophysical model, with oocyte production (differentiation) and oocyte growth (vitellogenesis) having differential temperature sensitivities.     

Mapping phenotypic plasticity and genotype-environment interactions affecting life-history traits in Caenorhabditis elegans

Phenotypic plasticity and genotype-environment interactions (GEI) play an important role in the evolution of life histories. Knowledge of the molecular genetic basis of plasticity and GEI provides insight into the underlying mechanisms of life-history changes in different environments. We used a genomewide single-nucleotide polymorphism map in a recombinant N2 x CB4856 inbred panel of the nematode Caenorhabditis elegans to study the genetic control of phenotypic plasticity to temperature in four fitness-related traits, that is, age at maturity, fertility, egg size and growth rate. We mapped quantitative trait loci (QTL) for the respective traits at 12 and 24 degrees C, as well as their plasticities. We found genetic variation and GEI for age at maturity, fertility, egg size and growth rate. GEI in fertility and egg size was attributed to changes in rank order of reaction norms. In case of age at maturity and growth rate, GEI was caused mainly by differences in the among-line variance. In total, 11 QTLs were detected, five QTL at 12 degrees C and six QTL at 24 degrees C, which were associated with life-history traits. Five QTL associated with age at maturity, fertility and growth rate showed QTL x environment interaction. These colocalized with plasticity QTL for the respective traits suggesting allelic sensitivity to temperature. Further fine mapping, complementation analyses and gene silencing are planned to identify candidate genes underlying phenotypic plasticity for age at maturity, fertility and growth.     

Cooler butterflies lay larger eggs: developmental plasticity versus acclimation

We use a full factorial design to investigate the effects of maternal and paternal developmental temperature, as well as female oviposition temperature, on egg size in the butterfly Bicyclus anynana. Butterflies were raised at two different temperatures and mated in four possible sex-by-parental-temperature crosses. The mated females were randomly divided between high and low oviposition temperatures. On the first day after assigning the females to different temperatures, only female developmental temperature affected egg size. Females reared at the lower temperature laid larger eggs than those reared at a higher temperature. When eggs were measured again after an acclimation period of 10 days, egg size was principally determined by the prevailing temperature during oviposition, with females ovipositing at a lower temperature laying larger eggs. In contrast to widely used assumptions, the effects of developmental temperature were largely reversible. Male developmental temperature did not affect egg size in either of the measurements. Overall, developmental plasticity and acclimation in the adult stage resulted in very similar patterns of egg size plasticity. Consequently, we argue that the most important question when testing the significance of acclamatory changes is not at which stage a given plasticity is induced, but rather whether plastic responses to environmental change are adaptive or merely physiological constraints.     

Size‐induced phenotypic reaction norms in a parasitoid wasp: an examination of life‐history and behavioural traits

The amount of resources available during development often affects body size, causing phenotypic variation in life‐history traits and reproductive behaviours. However, past studies have seldom examined the reaction norms of both life‐history and behavioural traits versus body size. We measured the phenotypic plasticity of several life‐history (age‐specific egg load, egg size, longevity) and behavioural (oviposition rate, host marking rate, walking speed) traits of the egg parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) in response to body size variation. We predicted that life‐history traits would show more evidence of size compensation than behavioural traits, resulting in fewer positively‐sloped size versus trait reaction norms among the former. As predicted by life‐history models, smaller wasps appear to shift resource allocation towards early‐life reproduction, having a similar egg load to large individuals 9 days after emergence. Surprisingly, longevity was unaffected by body size. However, egg size, the number of offspring produced during oviposition bouts, and the rate of subsequent egg synthesis were greater for larger individuals. In addition, as predicted, the reaction norms of behavioural traits versus body size were all positively sloped. Thus, despite possible adaptive compensatory plasticity of life‐history traits by small individuals, behavioural constraints directly related to body size would contribute to maintaining a positive size–fitness relationship.     

Environmental effects on sexual size dimorphism of a seed-feeding beetle

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.     

THE EFFECT OF TEMPERATURE ON BODY SIZE AND FECUNDITY IN FEMALE DROSOPHILA MELANOGASTER: EVIDENCE FOR ADAPTIVE PLASTICITY

The reaction norm linking rearing temperature and size in Drosophila melanogaster results in progressively larger flies as the temperature is lowered from 30°C to 18°C, but it has remained unclear whether this phenotypic plasticity is part of an adaptive response to temperature. We found that female D. melanogaster reared to adulthood at 18°C versus 25°C showed a 12% increase in dry weight. Measurements of the fecundity of these two types of fly showed that the size change had no effect on lifetime fecundity, regardless of the adult test temperature. Thus the phenotypic plasticity breaks the usual positive correlation between body size and fecundity. However, at a given temperature, early fecundity (defined as productivity for days 5 through 12 after eclosion at 25°C and days 7 through 17 at 18°C) was highest when the rearing and test temperatures were the same. The early fecundity advantage due to rearing at the test temperature was 25% at 18°C and 16% at 25°C, a result consistent with the overall phenotypic response to temperature being adaptive. This conclusion is further supported by the finding that the temperature treatments resulted in a trade-off between early fecundity and longevity, a trade-off that parallels the known genetic correlation. Another parallel is that both the temperature-induced and genetic effects are independent of total fecundity. By contrast, within the temperature treatments, the phenotypic correlation between early fecundity and longevity was positive, illustrating the danger of assuming that phenotypic and genetic correlations are similar, or even of the same sign.     

Maternal effects on phenotypic plasticity in larvae of the salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis>

Maternal effects are widespread and influence a variety of traits, for example, life history strategies, mate choice, and capacity to avoid predation. Therefore, maternal effects may also influence phenotypic plasticity of offspring, but few studies have addressed the relationship between maternal effects and phenotypic plasticity of offspring. We examined the relationship between a maternally influenced trait (egg size) and the phenotypic plasticity of the induction rate of the broad-headed morph in the salamander Hynobius retardatus. The relationship between egg size and the induction of the broad-headed morph was tested across experimental crowding conditions (densities of low conspecifics, high conspecifics, and high heterospecific anuran), using eggs and larvae from eight natural populations with different larval densities of conspecifics and heterospecifics. The broad-headed morph has a large mouth that enables it to consume either conspecifics or heterospecifics, and this ability gives survival advantages over the normal morph. We have determined that there is phenotypic plasticity in development, as shown by an increase in the frequency of broad-headed morph in response to an increase in the density of conspecifics and heterospecifics. This reaction norm differed between populations. We also determined that the frequency of the broad-headed morph is affected by egg size in which larger egg size resulted in expression of the broad-headed morph. Furthermore, we determined that selection acting on the propensity to develop the broad-headed morph has produced a change in egg size. Lastly, we found that an increase in egg size alters the reaction norm to favor development of the broad-headed morph. For example, an equal change in experimental density produces a greater change in the frequency of the broad-headed morph in larvae developing from large eggs than it does in larvae developing from small eggs. Population differences in plasticity might be the results of differences in egg size between populations, which is caused by the adaptive integration of the plasticity and egg size. Phenotypic plasticity can not evolve independently of maternal effects.     

Comparing indigenous and introduced populations of Melaleuca quinquenervia (Cav.) Blake: response of seedlings to water and pH levels

Melaleuca quinquenervia is a wetland tree species indigenous to eastern Australia. It was separately introduced to east and west Florida as an ornamental, but has since become invasive, dominating several habitat types. We tested the predictions that (1) Australian populations would exhibit more genetic variation than Florida populations, due to founder effect, and (2) high phenotypic plasticity would be found in all populations, due to the wide range of habitats occupied. We compared the phenotypic plasticity and familial variation among three Australian populations, two east Florida, and two west Florida populations in a greenhouse experiment. We grew seedlings collected from different maternal trees in each population under two water levels and three pH levels, reflecting the natural range of water levels and soil pH in Florida and Australian Melaleuca stands. We measured leaf size and shape, growth rate and above-ground biomass of seedlings and determined the components of phenotypic variance (familial, environmental, and their interaction) using univariate and multivariate analysis of variance. All traits showed significant among-population and among-family variation, as well as significant phenotypic plasticity, in response to both water level and pH level changes. Sensitivity to pH was particularly high, presumably because plants were grown under pHs ranging from 4.7 to 7.4, and because pH can influence nutrient availability. Familial variation contains genetic variation, but it may also be confounded with maternal environmental effects. Comparing Australian to Floridian Melaleuca, amounts of familial variation and phenotypic plasticity varied by trait. Overall, Australian Melaleuca had more among-population variation than Floridian Melaleuca, presumably reflecting the wider latitudinal range and longer time for evolutionary change in Australia, but had similar amounts of among-family variation, within any one population. If maternal effects are strong, among-population differences may merely reflect greater environmental differences among Australian sites than Florida sites. Australian Melaleuca had less phenotypic plasticity, possibly due to founder effects in Florida or to subsequent adaptive evolution of phenotypic plasticity in Floridian populations. Floridian Melaleuca shows little loss of familial variation, compared to indigenous Australian populations, and that, in combination with its high phenotypic plasticity, should allow it to continue to colonize new areas successfully.     

Temperature-mediated plasticity in egg and body size in egg size-selected lines of a butterfly

We examine genotype–environment interactions by using lines of the butterfly Bicyclus anynana artificially selected for differences in egg size in a full factorial design with two developmental and two oviposition temperatures. In accordance with the temperature–size rule, egg size and pupal mass increased by 4–5 and 8%, respectively, at lower temperatures. Genotype–environment interactions for both traits suggest that plasticity is largely independent of the trait value, and that there is potential for evolutionary change. These findings cast further doubt on the notion that temperature-mediated plasticity might be purely a physiological constraint.     

Quantitative genetic analysis of the body size and shape of Drosophila buzzatii

Summary Body size in Drosophila is known to be closely related to a number of traits with important life history consequences, such as fecundity, dispersal ability and mating success. We examine the quantitative genetic basis of body size in three populations of the cactophilic species Drosophila buzzatii, which inhabit climatically different areas of Australia. Flies were reared individually to eliminate any common environmental component in a full-sib design with families split between two temperatures (18° and 25 °C). The means of several size measures differ significantly among populations while the genetic correlations among these traits generally do not differ, either among populations from different natural environments or between the different laboratory temperatures. This stability of correlation structure is necessary if laboratory estimates of genetic correlations are to have any connection with the expression of genetic variation in the field. The amount of variance due to genotype-by-environment interactions (family x temperature of development) varied among populations, apparently in parallel with the magnitudes of seasonal and diurnal variation in temperature experienced by the different populations. A coastal population, inhabiting a relatively thermally benign environment, showed no interaction, while two inland populations, inhabiting thermally more extreme areas, showed interaction. This interaction term is a measure of the amount of genetic variation in the degree of phenotypic plasticity of body size in response to temperature of development. Thus the inland flies vary in their ability to attain a given body size at a particular temperature while the coastal flies do not. This phenotypic plasticity is shown to be due primarily to differences among genotypes in the amount of response to the change in temperature. A possible selective basis for the maintenance of genetic variation for the levels of phenotypic plasticity is proposed.     

THE EVOLUTIONARY GENETICS OF AN ADAPTIVE MATERNAL EFFECT: EGG SIZE PLASTICITY IN A SEED BEETLE

In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for transgenerational phenotypic plasticity whereby, in response to a predictive environmental cue, a mother can change the type of eggs that she makes or can program a developmental switch in her offspring, which produces offspring prepared for the environmental conditions predicted by the cue. One potentially common mechanism by which females manipulate the phenotype of their progeny is egg size plasticity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid on seeds of two host plants (Acacia greggii and Cercidium floridum; h² ranged between 0.217 and 0.908), and that the heritability of egg size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation across host plants (r_G > 0.6). However, the cross-environment genetic correlation is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size plasticity is thus capable of evolving in response to natural selection.     

Adaptation to marginal habitats by evolution of increased phenotypic plasticity

In an island population receiving immigrants from a larger continental population, gene flow causes maladaptation, decreasing mean fitness and producing continued directional selection to restore the local mean phenotype to its optimum. We show that this causes higher plasticity to evolve on the island than on the continent at migration-selection equilibrium, assuming genetic variation of reaction norms is such that phenotypic variance is higher on the island, where phenotypes are not canalized. For a species distributed continuously in space along an environmental gradient, higher plasticity evolves at the edges of the geographic range, and in environments where phenotypes are not canalized. Constant or evolving partially adaptive plasticity also alleviates maladaptation owing to gene flow in a heterogeneous environment and produces higher mean fitness and larger population size in marginal populations, preventing them from becoming sinks and facilitating invasion of new habitats. Our results shed light on the widely observed involvement of partially adaptive plasticity in phenotypic clines, and on the mechanisms causing geographic variation in plasticity.     

Heritability of gonad size varies across season in a wild songbird

Many organisms advance their seasonal reproduction in response to global warming. In birds, which regress their gonads to a nonfunctional state each winter, these shifts are ultimately constrained by the time required for gonadal development in spring. Gonadal development is photoperiodically controlled and shows limited phenotypic plasticity in relation to environmental factors, such as temperature. Heritable variation in the time required for full gonadal maturation to be completed, based on both onset and speed of development and resulting in seasonally different gonad sizes among individuals, is thus a crucial prerequisite for an adaptive advancement of seasonal reproduction in response to changing temperatures. We measured seasonal gonadal development in climate‐controlled aviaries for 144 great tit (Parus major) pairs, which consisted of siblings obtained as whole broods from the wild. We show that the extent of ovarian follicle development (follicle size) in early spring is highly heritable (h² = 0.73) in females, but found no heritability of the extent of testis development in males. However, heritability in females decreased as spring advanced, caused by an increase in environmental variance and a decrease in additive genetic variation. This low heritability of the variation in a physiological mechanism underlying reproductive timing at the time of selection may hamper genetic adaptation to climate change, a key insight as this great tit population is currently under directional selection for advanced egg‐laying.     

ADAPTIVE PLASTICITY IN DEVELOPMENT OF SCAPHIOPUS COUCHII TADPOLES IN DESERT PONDS

Couch's spadefoot toads (Scaphiopus couchii) breed in ephemeral desert ponds that are highly variable in duration. Rapid development is expected to be advantageous in short-duration ponds, but slower development, allowing more time for growth, may be advantageous in ponds of longer duration. Previous experiments have revealed both genetic variation in development time and phenotypic plasticity in response to pond drying. In this paper, I examine the norms of reaction of five sibships of tadpoles to see whether there is genetic variation in the effect of pond duration, i.e., in phenotypic plasticity. Several important results emerged. 1) Differences among sibships in development time that were seen in the lab were also seen in the field. 2) There was no evidence for genetic variation in plasticity of development; all sibships exhibited faster development and decreased larval period in ponds of short duration. Plasticity in development appears to be adaptive, as size at metamorphosis was correlated with duration of larval period. The slowest developing sibship, however, suffered higher mortality compared to other sibships in short duration ponds. 3) Sibships did not differ in growth or size at metamorphosis in short-duration ponds, but the slowest developing sibship metamorphosed at the largest size in long duration ponds, resulting in a significant genotype x environment interaction for size at metamorphosis. Thus, although only one of the five sibships responded differently, there appears to be genetic variation for plasticity in growth, and a genetically determined trade-off between fitness in short-duration ponds (via rapid development) and fitness in long duration ponds (via large size at metamorphosis). This may explain the existence of both phenotypic plasticity and genetic variation in development. A single genotype, although capable of adaptive plasticity, is not sufficiently flexible to have equally high fitness in both long- and short-duration ponds.     

Linking the spatial scale of environmental variation and the evolution of phenotypic plasticity: selection favors adaptive plasticity in fine-grained environments

Adaptive phenotypic plasticity and adaptive genetic differentiation enable plant lineages to maximize their fitness in response to environmental heterogeneity. The spatial scale of environmental variation relative to the average dispersal distance of a species determines whether selection will favor plasticity, local adaptation, or an intermediate strategy. Habitats where the spatial scale of environmental variation is less than the dispersal distance of a species are fine grained and should favor the expression of adaptive plasticity, while coarse-grained habitats, where environmental variation occurs on spatial scales greater than dispersal, should favor adaptive genetic differentiation. However, there is relatively little information available characterizing the link between the spatial scale of environmental variation and patterns of selection on plasticity measured in the field. I examined patterns of spatial environmental variation within a serpentine mosaic grassland and selection on an annual plant (Erodium cicutarium) within that landscape. Results indicate that serpentine soil patches are a significantly finer-grained habitat than non-serpentine patches. Additionally, selection generally favored increased plasticity on serpentine soils and diminished plasticity on non-serpentine soils. This is the first empirical example of differential selection for phenotypic plasticity in the field as a result of strong differences in the grain of environmental heterogeneity within habitats.     

Temperature effects on egg size and their fitness consequences in the yellow dung fly Scathophaga stercoraria

Organisms and parts of an organism like eggs or individual cells developing in colder environments tend to grow bigger. A unifying explanation for this Bergmann's rule extended to ectotherms has not been found, and whether this is an adaptive response or a physiological constraint is debated. The dependence of egg and clutch size on the mother's temperature environment were investigated in the yellow dung fly Scathophaga stercoraria. Smaller eggs were laid at warmer temperatures in the field and the laboratory, where possible confounding variables were controlled for. As clutch size at the same time was unaffected by temperature, this effect was not due to a trade-off between egg size and number. Temperature-dependent egg sizes even persisted within individuals: when females were transferred to a cooler (warmer) environment, they laid third-clutch eggs that were larger (smaller) than their first-clutch eggs. The fitness consequences of these temperature-mediated egg sizes were further investigated in two laboratory experiments. Neither egg and pre-adult survivorship nor larval growth rate were maximized, nor was development time minimized, at the ambient temperature corresponding to the mother's temperature environment. This does not support the beneficial acclimation hypothesis. Instead, this study yielded some, but by no means conclusive indications of best performance by offspring from eggs laid at intermediate temperatures, weakly supporting the optimal temperature hypothesis. In one experiment the smaller eggs laid at 24 °C had reduced survivorship at all ambient temperatures tested. Smaller eggs thus generally performed poorly. The most parsimonious interpretation of these results is that temperature-mediated variation in egg size is a maternal physiological response (perhaps even a constraint) of unclear adaptive value. This revised version was published online in November 2006 with corrections to the Cover Date.     

Plasticity to wind is modular and genetically variable in Arabidopsis thaliana

Thigmomorphogenesis, the characteristic phenotypic changes by which plants react to mechanical stress, is a widespread and probably adaptive type of phenotypic plasticity. However, little is known about its genetic basis and population variation. Here, we examine genetic variation for thigmomorphogenesis within and among natural populations of the model system Arabidopsis thaliana. Offspring from 17 field-collected European populations was subjected to three levels of mechanical stress exerted by wind. Overall, plants were remarkably tolerant to mechanical stress. Even high wind speed did not significantly alter the correlation structure among phenotypic traits. However, wind significantly affected plant growth and phenology, and there was genetic variation for some aspects of plasticity to wind among A. thaliana populations. Our most interesting finding was that phenotypic traits were organized into three distinct and to a large degree statistically independent covariance modules associated with plant size, phenology, and growth form, respectively. These phenotypic modules differed in their responsiveness to wind, in the degree of genetic variability for plasticity, and in the extent to which plasticity affected fitness. It is likely, therefore, that thigmomorphogenesis in this species evolves quasi-independently in different phenotypic modules.