Testing specialization hypothesis on a stress gradient

Specialization can allow plants to perform well in their home environments at the expense of poor performance in other habitats. A great difference in performance across habitats is observed as high phenotypic plasticity in performance traits and a by‐product of selection. However, phenotypic plasticity (particularly adaptive plasticity) can be an active response to the selection by allowing the maintenance of performance. Therefore, specialization and adaptive plasticity delineate two opposing strategies. The specialization hypothesis presents a non‐adaptive interpretation of plasticity and predicts that phenotypic plasticity of performance traits is greater in specialization to good habitats, whereas bad habitat specialists express low plasticity in performance traits. We tested the specialization hypothesis using plants adapted to extremely stressful mine‐site habitats (sites with highly acidic soil and heavy metal contamination). Seeds of five herbaceous species were collected from high stress (mine site) and low stress habitats. We established a glasshouse experiment where seedlings from high and low stress habitats were grown under near neutral pH and acid soil treatments. We compared performance trait plasticity (e.g. biomass) from high stress and low stress populations and found that there was no significant difference in performance traits between high and low stress populations across treatments. The overall result did not support the specialization hypothesis. Moreover, our results suggest that the species invaded the mine sites are either extreme generalists or the surrounding populations retain some stress tolerant genotypes that are capable of invading the mine sites.     

Phenotypic plasticity, heterochrony and ontogenetic repatterning during juvenile development of divergent Arctic charr (Salvelinus alpinus)

Phenotypic plasticity is a developmental process that plays a role as a source of variation for evolution. Models of adaptive divergence make the prediction that increasing ecological specialization should be associated with lower levels of plasticity. We tested for differences in the magnitude, rate and trajectory of morphological plasticity in two lake populations of Arctic charr (Salvelinus alpinus) that exhibited variation in the degree of resource polymorphism. We reared offspring on diet treatments that mimicked benthic and pelagic prey. Offspring from the more divergent population had lower levels of morphological plasticity. Allometry influenced the rate of shape change over ontogeny, with differences in rate among ecomorphs being minimal when allometric variation was removed. However, plasticity in the spatial trajectory of development was extensive across ecomorphs, both with and without the inclusion of allometric variation, suggesting that different aspects of shape development can evolve independently.     

Shade avoidance syndrome in Picea omorika seedlings: a growth‐room experiment

The adaptiveness of shade avoidance responses to density was studied in Picea omorika seedlings raised in a growth‐room. Siblings of a synthetic population comprising 117 families from six natural populations were exposed to contrasting density conditions in order to score variation in phenotypic expression of several epicotyl and bud traits included in the shade avoidance syndrome. As predicted for the adaptive plasticity to foliage shade, epicotyl elongation traits tended toward higher, while axillary bud traits toward lower values in high‐density vs. low‐density conditions. Phenotypic selection analysis revealed that the elongated plants had greater relative fitness than the suppressed ones in both density treatments which could be ascribed to the effect of direct selection on epicotyl length. There was no evidence for plasticity costs associated with the expression of the shade avoidance phenotype either under low or under high density, with only a single exception. Estimates of variance component genetic correlations across densities were significantly different from unity for the majority of the seedling traits studied, indicating the existence of heritable variation within reaction norms of these traits. However, since all these correlations were positive in sign and large in magnitude, this conclusively means that the level of the additive genetic variation for plasticity in the shade‐avoidance traits of P. omorika is rather low.     

Phenotypic plasticity and specialization in clonal versus non-clonal plants: A data synthesis

Reproductive strategies can be associated with ecological specialization and generalization. Clonal plants produce lineages adapted to the maternal habitat that can lead to specialization. However, clonal plants frequently display high phenotypic plasticity (e.g. clonal foraging for resources), factors linked to ecological generalization. Alternately, sexual reproduction can be associated with generalization via increasing genetic variation or specialization through rapid adaptive evolution. Moreover, specializing to high or low quality habitats can determine how phenotypic plasticity is expressed in plants. The specialization hypothesis predicts that specialization to good environments results in high performance trait plasticity and specialization to bad environments results in low performance trait plasticity. The interplay between reproductive strategies, phenotypic plasticity, and ecological specialization is important for understanding how plants adapt to variable environments. However, we currently have a poor understanding of these relationships. In this study, we addressed following questions: 1) Is there a relationship between phenotypic plasticity, specialization, and reproductive strategies in plants? 2) Do good habitat specialists express greater performance trait plasticity than bad habitat specialists? We searched the literature for studies examining plasticity for performance traits and functional traits in clonal and non-clonal plant species from different habitat types. We found that non-clonal (obligate sexual) plants expressed greater performance trait plasticity and functional trait plasticity than clonal plants. That is, non-clonal plants exhibited a specialist strategy where they perform well only in a limited range of habitats. Clonal plants expressed less performance loss across habitats and a more generalist strategy. In addition, specialization to good habitats did not result in greater performance trait plasticity. This result was contrary to the predictions of the specialization hypothesis. Overall, reproductive strategies are associated with ecological specialization or generalization through phenotypic plasticity. While specialization is common in plant populations, the evolution of specialization does not control the nature of phenotypic plasticity as predicted under the specialization hypothesis.     

Phenotypic plasticity of reproductive effort in a colonial ascidian,Botryllus schlosseri

Phenotypic plasticity is the capability of a genotype to produce different phenotypes in different environments. Previous studies have indicated phenotypic variability in asexual, male, and female reproduction in Botryllus schlosseri, a hermaphroditic, colonial ascidian, but not explicitly tested for genotype by environment interactions that indicate genetic variation in plastic responses. Consequently, clones derived from an estuarine population were deployed at their native site and a warmer, higher productivity site 10 km up-river. Male reproduction was assayed by testis size, female reproduction by the number of eggs produced, and asexual reproduction by colony growth rate. To test for ontogenetic effects, data were collected from two different generations of zooids born in the field. Analyses of variance indicated plasticity in asexual and female reproduction during the first zooid generation and plasticity in all three traits during the third zooid generation. Reaction norms varied significantly among genotypes in direction and magnitude for asexual reproduction at both times, implying that selection on asexual reproduction is weak. Sperm production during the third zooid generation was significantly lower at the nonnative site, but there was no genotype by environment interaction. The reaction norms for female reproduction varied significantly among genotypes in direction and magnitude during the first zooid generation, but only varied in magnitude during the third generation, with egg production being higher in all genotypes at the nonnative site. Comparisons of weighted frequency distributions between sites demonstrated that differences in egg production in the third generation were due to increases in the proportion of reproductive zooids within a colony. The greater emphasis on female reproduction at a site associated with higher food availability and temperature, and the greater emphasis on male reproduction at a colder, food-limited site, supports predictions from sex allocation theory.     

Phenotypic integration may constrain phenotypic plasticity in plants

Phenotypic plasticity is essential for plant adaptation to changing environments but some factors limit its expression, causing plants to fail in producing the best phenotype for a given environment. Phenotypic integration refers to the pattern and magnitude of character correlations and it might play a role as an internal constraint to phenotypic plasticity. We tested the hypothesis that phenotypic integration – estimated as the number of significant phenotypic correlations between traits – constrains phenotypic plasticity of plants. The rationale is that, for any phenotypic trait, the more linked with other traits it is, the more limited is its range of variation. In the perennial species Convolvulus chilensis (Convolvulaceae) and Lippia alba (Verbenaceae) we determined the relationship between phenotypic plasticity to relevant environmental factors – shading for C. chilensis and drought for L. alba– and the magnitude of phenotypic integration of morphological and biomass allocation traits. In C. chilensis plants, plasticity to shading of a given trait decreased with the number of significant correlations that it had with the other traits. Likewise, the characters that showed greater plasticity to experimental drought in L. alba plants had fewer significant phenotypic correlations with other characters. We report a novel limit to phenotypic plasticity of plants by showing that the phenotypic trait architecture may constrain their plastic, functional responses to the environment.     

Genetic and phenotypic variation in diet breadth in a generalist herbivore

Summary The genetic and plastic components of polyphagy were investigated in a population ofLymantria dispar, the gypsy moth. A simple genetic experiment assessed the expression of (1) genetic variability in life history traits within each of four environments, (2) genetic variability in diet breadth, expressed as a change in the ranks of family performance across hosts, and (3) homeostasis (equivalent performance by a family across hosts) versus phenotypic plasticity (variable performance by a family across hosts). Sibs from each of 14 families, randomly selected from a single population, were reared on four diets: two natural hosts — chestnut and red oak, and two synthetic hosts — a standard laboratory diet and a low-protein version of this diet. Average population performance, measured in terms of development time and pupal weight, was better on standard laboratory diet than on low-protein diet, and was equal on chestnut and red oak for pupal weight, but better on chestnut oak for development time. Average population performance provided no information about the genetic component of host use ability. The gypsy moth expressed genetic variation in development time within each host environment and in pupal weight within natural host environments. Phenotypic plasticity was expressed by a significant number of families in development time and pupal weight across synthetic hosts and, to a lesser extent, across natural hosts. It was only across natural hosts that genetic variation in diet breadth was expressed, and this was confined to females. Genetic variability in diet breadth may be maintained in this species as a consequence of the unpredictability of its food sources.     

Small scale genotypic richness stabilizes plot biomass and increases phenotypic variance in the invasive grass Phalaris arundinacea

Aims We aim to understand how small-scale genotypic richness and genotypic interactions influence the biomass and potential invasiveness of the invasive grass, Phalaris arundinacea under two different disturbance treatments: intact plots and disturbed plots, where all the native vegetation has been removed. Specifically, we address the following questions (i) Does genotypic richness increase biomass production? (ii) Do genotypic interactions promote or reduce biomass production? (iii) Does the effect of genotypic richness and genotypic interactions differ in different disturbance treatments? Finally (iv) Is phenotypic variation greater as genotypic richness increases?Methods We conducted a 2-year common garden experiment in which we manipulated genotype richness using eight genotypes planted under both intact and disturbed conditions in a wetland in Burlington, Vermont (44°27′23″N, 73°11′29″W). The experiment consisted of a randomized complete block design of three blocks, each containing 20 plots (0.5 m 2) per disturbed treatment. We calculated total plot biomass and partitioned the net biodiversity effect into three components: dominance effect, trait-dependent complementarity and trait-independent complementarity. We calculated the phenotypic variance for each different genotype richness treatment under the two disturbance treatments.Important findings Our results indicate that local genotypic richness does not increase total biomass production of the invasive grass P. arundinacea in either intact or disturbed treatments. However, genotypic interactions underlying the responses showed very different patterns in response to increasing genotypic richness. In the intact treatment, genotypic interactions resulted in the observed biomass being greater than the predicted biomass from monoculture plots (e.g., overyielding) and this was driven by facilitation. However, facilitation was reduced as genotypic richness increased. In the disturbed treatment, genotypic interactions resulted in underyielding with observed biomass being slightly less than expected from the performance of genotypes in monocultures; however, underyielding was reduced as genotypic richness increased. Thus, in both treatments, higher genotypic richness resulted in plot biomass nearing the additive biomass from individual monocultures. In general, higher genotypic richness buffered populations against interactions that would have reduced biomass and potentially spread. Phenotypic variance also had contrasting patterns in intact and disturbed treatments. In the intact treatment, phenotypic variance was low across all genotypic richness levels, while in the disturbed treatment, phenotypic variance estimates increased as genotypic richness increased. Thus, under the disturbed treatment, plots with higher genotypic richness had a greater potential response to selection. Therefore, limiting the introduction of new genotypes, even if existing genotypes of the invasive species are already present, should be considered a desirable management strategy to limit the invasive behavior of alien species.     

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.     

Patterns of genotypic variation and phenotypic plasticity of light response in two tropical Piper (Piperaceae) species

Patterns of phenotypic plasticity and genotypic variation in light response of growth and photosynthesis were examined in two species of rain forest shrub that differ in ecological distribution within the forest. We further examined correlations among photosynthetic and growth traits. We hypothesized that the pioneer species, Piper sancti-felicis, would display greater phenotypic plasticity than the shade-tolerant species, Piper arieianum. We further proposed that, in both species, genotypic effects would be more apparent in growth-related traits than photosynthetic traits due to more concentrated selection pressure on gas-exchange traits. P. sancti-felicis did not demonstrate greater phenotypic plasticity of light response. Although many of the traits measured had significant genotype effects, neither species showed any significant effects of genotype on light response of photosynthesis, suggesting little genetic variation for this trait within populations. A principal components analysis clearly illustrated both species and light effects, with the treatments dividing neatly along the axis of the first principal component and the species separating along the second principal component axis. Results indicated general similarities between the species in their trait correlation structure and level of integration among traits, but characteristic differences were observed in the patterns of change between low and high light. Both species had more correlations than expected within groups of growth-related or photosynthetic traits; strong correlations of traits between these two groups were underrepresented. The similar pattern of genetic variation and phenotypic integration observed in these two congeners may be due more to their close phylogenetic relation than to their ecological distributions.     

Life history responses to irradiance at the early seedling stage of Picea omorika (Pančić) Purkyňe: adaptiveness and evolutionary limits

A multivariate selection analysis has been implemented for testing the adaptiveness of life history plasticity to irradiance during the seedling establishment in Picea omorika plants raised in a growth-room. Siblings of a synthetic population comprising 21 families from six natural populations were exposed to contrasting light levels to explore variation in phenotypic expression of three seedling traits: days from germination to cotyledon opening (DGTOC), days from cotyledon opening to epicotyl appearance (DCTOE), and epicotyl length at 6 weeks (EPL6). Ambient light conditions significantly affected DCTOE and EPL6, but not DGTOC. Phenotypic selection analysis revealed that DGTOC was under negative directional selection in both radiation environments, suggesting that canalization of DGTOC was promoted across different light conditions, as well as that the observed pattern of canalization might be regarded as adaptive. DCTOE was also found to be under negative directional selection in both light treatments, but the plastic responses of this trait were opposite to the values favoured by selection within environments. Since there was evidence for selection against plasticity in DCTOE, the pattern of plastic responses in DCTOE to variation in light conditions could be diagnosed as maladaptive. Multiple regression analysis revealed a cost of canalization in DGTOC regardless of light environment, as well as a cost of plasticity in DCTOE under high light intensity. All genetic correlations across light environments were significantly different from unity, indicating the existence of heritable variation for plasticity in these traits. However, since DGTOC and DCTOE were involved in a genetic trade-off with respect to both trait mean and plasticity, these early life histories would never reach their optimal values across radiation environments.     

克隆植物的表型可塑性与等级选择

表型可塑性是指生物个体生长发育过程中遭受不同环境条件作用时产生不同表型的能力。进化的发生有赖于自然选择对种群遗传可变性产生的效力以及各基因型的表型可塑性。有足够的证据说明表型可塑性的可遗传性,它实际上是进化改变的一个成分。一般通过优化模型、数量遗传模型和配子模型来研究表型可塑性的进化。植物的构型是相对固定的,并未完全抑制表型可塑性。克隆植物因其双构件性而具有更广泛的、具有重要生态适应意义的表型可塑性。构件性使克隆植物具有以分株为基本单位的等级结构,从而使克隆植物的表型选择也具有等级性。构件等级一般包含基株、克隆片段或分株系统以及分株3个典型水平。目前认为克隆植物的自然选择有两种模式,分别以等级选择模型和基因型选择模型表征。等级选择模型认为:不同的等级水平同时也是表型选择水平,环境对各水平具有作用,各水平之间也有相互作用,多重表型选择水平的净效应最终通过繁殖水平——分株传递到随后的世代中。基因型选择模型指出:克隆生长引起分株的遗传变异,并通过基株内分株间以及基株间的非随机交配引起种子库等位基因频率的改变,产生微进化。这两种选择模式均突出强调了分株水平在自然选择过程中的变异性以及在进化中的重要性,强调了克隆生长和种子繁殖对基株适合度的贡献。基因型选择模型包含等级选择模型的观点,是对等级选择模型的重要补充。克隆植物的表型可塑性表现在3个典型等级层次上,由于各层次对自然选择压力具有不同的反应,其表型变异程度一般表现出“分株层次>分株片段层次>基株层次”的等级性反应模式。很多证据表明,在构件有机体中构件具有最大的表型可塑性,植物的表型可塑性实际上是构件而非整个遗传个体的反应。这说明克隆植物的等级反应模式可能具有普适性。如果该反应模式同时还是构件等级中不同“个体”适应性可塑性反应的模式,那么可以预测:1)在克隆植物中,分株层次受到的自然选择强度也最大,并首先发生适应性可塑性变化,最终引起克隆植物微进化;2)由于较弱的有性繁殖能力,克隆植物在进化过程中的保守性可能大于非克隆植物。克隆植物等级反应模式的普适性亟待验证。     

Genetic and phenotypic influences on clone-level success and host specialization in a generalist parasite

Studying resource specialization at the individual level can identify factors constraining the evolution of generalism. We quantified genotypic and phenotypic variability among infective stages of 20 clones of the parasitic trematode Maritrema novaezealandensis and measured their infection success and post-infection fitness (growth, egg output) in several crabs and amphipods. First, different clones varied in their infection success of different crustaceans. Second, neither genetic nor phenotypic traits had consistent effects on infection success across all host species. Although the results suggest a relationship between infection success and phenotypic variability, phenotypically variable clones were not better at infecting more host species than less variable ones. Third, genetic and phenotypic traits also showed no consistent correlations with post-infection fitness measures. Overall, we found no consistent clone-level specialization, with some clones acting as specialists and others, generalists. The trematode population therefore maintains an overall generalist strategy by comprising a mixture of clone-level specialists and generalists.     

Morphological canalization,integration, and plasticity in response to population density in Abutilon theophrasti: Influences of soil conditions and growth stages

Phenotypic integration and developmental canalization have been hypothesized to constrain the degree of phenotypic plasticity, but little evidence exists, probably due to the lack of studies on the relationships among the three processes, especially for plants under different environments. We conducted a field experiment by subjecting plants of Abutilon theophrasti to three densities, under infertile and fertile soil conditions, and analyzing correlations among canalization, integration, and plasticity in a variety of measured morphological traits after 50 and 70 days, to investigate the relationships among the three variables in response to density and how these responses vary with soil conditions and growth stages. Results showed trait canalization decreased and phenotypic integration and the degree of plasticity (absolute plasticity) in traits increased with density. Phenotypic integration often positively correlated with absolute plasticity, whereas correlations between trait canalization and plasticity were insignificant in most cases, with a few positive ones between canalization and absolute plasticity at low and medium densities. As plants grew, these correlations intensified in infertile soil and attenuated in fertile soil. Our findings suggested the complexity of the relationship between canalization and plasticity: Decreased canalization is more likely to facilitate active plastic responses under more favorable conditions, whereas increased level of integration should mainly be an outcome of plastic responses. Soil conditions and growth stage may affect responses of these correlations to density via modifying plant size, competition strength, and plastic responses in traits. We also predicted that decreased canalization can be advantageous or disadvantageous, and the lack of response to stress may demonstrate a stronger ability of adaptation than passive response, thus should be adaptive plasticity as active response.     

Plasticity and genotypic variation in some Mentha × verticillata hybrids

Leaf and flower oil terpene composition and several plant morphological characteristics of 17 Mentha × verticillata hybrids were analysed during two growing seasons (1988 and 1989). The data obtained were used to study the phenotypic plasticity, the genotypic variation and the genetic variation for phenotypic plasticity. All plants showed high leveis of phenotypic plasticity for both oil chemical and morphometrical parameters. Higher degrees of genotypic variation were found among the plants for oll components while a higher phenotypic plasticity was observed for morphological parameters. Temperatures and rainfall data were collected during the growing seasons and correlated to the data obtained from plant oil and morphology. Low levels of phenotypic plasticity and high degrees of genotypic variation were found to form outliers in the population of M. x verticillata hybrids. The results obtained confirm a significant effect of environmental conditions on the physiology and morphology of the genus Mentha.     

Local adaptation and ecological genetics of host-plant specialization in a leaf beetle

The tendency of insect species to evolve specialization to one or a few plant species is probably a major reason for the remarkable diversity of herbivorous insects. The suggested explanations for this general trend toward specialization include a range of evolutionary mechanisms, whose relative importance is debated. Here we address two potentially important mechanisms: (i) how variation in the geographic distribution of host use may lead to the evolution of local adaptation and specialization; (ii) how selection for specialization may lead to the evolution of trade‐offs in performance between different hosts. We performed a quantitative genetic experiment of larval performance in three different populations of the alpine leaf beetle Oreina elongata reared on two of its main host plants. Due to differences in host availability, each population represents a distinctly different selective regime in terms of host use including selection for specialization on one or the other host as well as selection for utilizing both hosts during the larval stage. The results suggest that selection for specialization has lead to some degree of local adaptations in host use: both single‐host population had higher larval growth rate on their respective native host plant genus, while there was no difference between plant treatments in the two‐host population. However, differences between host plant treatments within populations were generally small and the degree of local adaptation in performance traits seems to be relatively limited. Genetic correlations in performance traits between the hosts ranged from zero in the two‐host population to significantly positive in the single‐host populations. This suggests that selection for specialization in single host populations typically also increased performance on the alternative host that is not naturally encountered. Moreover, the lack of a positive genetic correlation in the two host‐population give support for the hypothesis that performance trade‐offs between two host plants may typically evolve when a population have adapted to both these plants. We conclude that although there is selection for specialization in larval performance traits it seems as if the genetic architecture of these traits have limited the divergence between populations in relative performance on the two hosts.     

GENETIC VARIATION AND COVARIATION IN RESPONSES TO HOST PLANTS BY ALSOPHILA POMETARIA (LEPIDOPTERA: GEOMETRIDAE)

Progeny of uncommon parthenogenetic genotypes of the polyphagous geometric Alsophila pometaria were reared on four host plant species representing the same genus, different confamilial genera, and different families. On the supposition that uncommon asexual genotypes have arisen recently from the sympatric sexual population, they may be viewed as a representation of variation in the sexual population that has been captured by parthenogenesis. In both the laboratory and the field, significant effects of genotype, host, and their interaction were found for survival. Live weight displayed significant effects of genotype, host, and genotype x host interaction in the laboratory, and significant main effects in the field. The broad heritability of live weight within hosts was significantly greater than zero in all cases, ranging from 0.18 to 0.33. Heritability values in the field were similar to those in the laboratory. Genetic correlations between weight attained on pairs of host plants, calculated from uncorrected family means, were all positive and many were statistically significant. When the data were corrected for differences in generalized vigor by taking the deviations from genotype means on a standard host plant, one of nine genetic correlations was significant and positive, and one was significant and negative. Within their limits of precision, these data suggest that genetic factors enhancing performance on one host do not generally have strong antagonistic pleiotropic effects on performance on the other hosts in this population. The relevance of these observations to the evolution of resource specialization in general and host specialization by phytophagous insects in particular is discussed. The common assumption that trade-offs in efficiency of utilization cause the evolution of specialized resource use requires more empirical evidence than seems to exist.     

Phenotypic integration plasticity in Daphnia magna: an integral facet of G × E interactions

Phenotypic integration can be defined as the network of multivariate relationships among behavioural, physiological and morphological traits that describe the organism. Phenotypic integration plasticity refers to the change in patterns of phenotypic integration across environments or ontogeny. Because studies of phenotypic plasticity have predominantly focussed on single traits, a G × E interaction is typically perceived as differences in the magnitude of trait expression across two or more environments. However, many plastic responses involve coordinated responses in multiple traits, raising the possibility that relative differences in trait expression in different environments are an important, but often overlooked, source of G × E interaction. Here, we use phenotypic change vectors to statistically compare the multivariate life‐history plasticity of six Daphnia magna clones collected from four disparate European populations. Differences in the magnitude of plastic responses were statistically distinguishable for two of the six clones studied. However, differences in phenotypic integration plasticity were statistically distinguishable for all six of the clones studied, suggesting that phenotypic integration plasticity is an important component of G × E interactions that may be missed unless appropriate multivariate analyses are used.     

Population differentiation for plasticity to light in an annual herb: Adaptation and cost

Phenotypic plasticity allows plants to cope with environmental heterogeneity. Environmental variation among populations may select for differentiation in plasticity. To test this idea, we used the annual plant Geranium carolinianum, which inhabits old fields that are densely vegetated and lack canopy cover and wood margins with tree shade but less neighbor shade. Individuals from three populations of each habitat were planted in natural low and high light environments, and morphological traits important for light acquisition were measured. Old-field plants were more plastic, with greater elongation of petioles and internodes in low light than those from wood margins. This larger shade avoidance response suggests evolution of greater plasticity to neighbor shade than to the tree canopy. Fitness of old-field plants was high across both light environments, whereas fitness of wood-margin plants was reduced in low light. Selection favored longer internodes in low than high light. Finally, plasticity for internode length was negatively associated with fitness in high light, suggesting a cost of plasticity for this trait. Together these results indicate that shade-avoidance plasticity of petiole and internode length is adaptive. However, greater elongation of internode length may be constrained by the cost of plasticity expressed in high light. The evolution of plasticity appears to reflect a balance between its adaptive nature and its cost to fitness.