Reaction Norm Analysis Research Articles

Partitioning the forms of genotype-by-environment interaction in the reaction norm analysis of stability

Key messageThe reaction norm analysis of stability can be enhanced by partitioning the contribution of different types of G × E to the variation in slope.The slope of regression in a reaction norm model, where the performance of a genotype is regressed over an environmental covariable, is often used as a measure of stability of genotype performance. This method could be developed further by partitioning variation in the slope of regression into the two sources of genotype-by-environment interaction (G × E) which cause it: scale-type G × E (heterogeneity of variance) and rank-type G × E (heterogeneity of correlation). Because the two types of G × E have very different properties, separating their effect would enable a clearer understanding of stability. The aim of this paper was to demonstrate two methods which seek to achieve this in reaction norm models. Reaction norm models were fit to yield data from a multi-environment trial in Barley (Hordeum vulgare), with the adjusted mean yield from each environment used as the environmental covariable. Stability estimated from factor-analytic models, which can disentangle the two types of G × E and estimate stability based on rank-type G × E, was used for comparison. Adjusting the reaction norm slope to account for scale-type G × E using a genetic regression more than tripled the correlation with factor-analytic estimates of stability (0.24–0.26 to 0.80–0.85), indicating that it removed variation in the reaction norm slope that originated from scale-type G × E. A standardisation procedure had a more modest increase (055–0.59) but could be useful when curvilinear reaction norms are required. Analyses which use reaction norms to explore the stability of genotypes could gain additional insight into the mechanisms of stability by applying the methods outlined in this study.

Reaction-norm analysis of neonatal lamb mortality suggests heritability varies with cold-stress: an example in the Elsenburg Merino selection lines

Context Coping with high levels of cold stress should be beneficial to survival of lambs, given the high mortality rate associated with severe winter storms. The Elsenburg Merino selection experiment involved divergent selection for reproduction. Phenotypic results comparing the positively selected H-Lines and negatively selected L-Lines suggested that cold-stress adaption could have contributed to the favourable genetic trends for survival of H-Line lambs. However, observing the genetic merit of better adapted animals depends on the presence of cold stress at the time of recording. A genotype by environment component (G × E) could, thus, be important when assessing survival/mortality phenotypes. Aim This study proposed the genetic analysis of this possible G × E component and compared the H- and L-Lines in this regard. Methods The sire model allowed the use of progeny phenotypes for neonatal mortality recorded during different levels of cold stress, and the possible G × E could be investigated through the reaction-norm approach. Genetic parameters were evaluated as random regression components by implementing a Gibbs sampling approach. A data set of 5723 individual lamb records was analysed as the progeny of 213 sires. Results A modelled G × E component played an important role in mortality outcomes, with the mean estimate (and 95% confidence interval) for the slope ( σ s b 2 = 0.113 [ 0.0019 – 0.28 ] AN22464_IE1.gif) only marginally smaller than the corresponding estimate for the intercept ( σ s a 2 = 0.124 [ 0.003 – 0.26 ] AN22464_IE2.gif). The reaction-norm model showed a higher heritability (h2 ± posterior standard deviation) for mortality at 3 days of age during high cold-stress (0.22 ± 0.16 at ~1100 KJm−2h−1) than during mild (0.13 ± 0.10 at ~960 KJm−2h−1) conditions, suggesting a greater ability to discriminate between sires at increasing stress levels. Conclusions Failure to account for this G × E component putatively contributes to the low h2 commonly reported for survival traits. The higher h2 at increased levels of cold stress could have played an important part in the higher survival of the H-Line progeny, who were better at coping with cold, wet and windy conditions. Implications Larger studies representing a wider environmental trajectory are recommended. This should be very feasible since cold stress can be derived from commonly available weather-station data.

Resurrection of dormant zooplankton grazers reveals multiple evolutionary responses to toxic cyanobacteria

AbstractThe cyanobacterial blooms caused by eutrophication exert strong selection on zooplankton grazers. Resurrection ecology, coupled with multivariate reaction norm analysis, can be used to parse the relative contributions of plasticity and evolution to mediating adaptation to environmental changes in natural populations. Here, we tested whether increasing levels of toxic cyanobacteria result in multivariate reaction norm changes in a natural zooplankton population. We resurrected dormant eggs of Ceriodaphnia cornuta (Cladocera) from three sediment layers in Dianshan Lake from 1988 to 2018, a period during which the abundances of cyanobacteria continuously increased, and conducted a common garden experiment to quantify the multivariate reaction norms of nine morphological, life history, and behavioral traits involved in the response to the toxic cyanobacterium Microcystis aeruginosa. The youngest subpopulation coinciding with the high‐eutrophic period was characterized by smaller size at birth and maturity and fewer offspring in their second broods compared with the oldest subpopulation coinciding with the low‐eutrophic period. Patterns of covariance in multivariate trait space provided strong evidence of adaptive evolution in multiple traits over time. The total trait change generated through both constitutive evolution and evolution of plasticity ranged from 1% to 64% depending on the trait and subpopulations being compared. These results suggest that multivariate phenotypic shifts in response to environmental changes in the study population are achieved by integrating plasticity, evolution of trait means, and evolution of plasticity.

Patterns of morphological divergence in fruit fly: response to nutritional variations through changes in allometric relationships and trait sizes

AbstractThe morphological structures of organisms may be subject to divergence when exposed to varied nutritive conditions, due to the occurrence of developmental plasticity and its fixation due to the process of genetic accommodation. The allometric relationship of traits with respect to each other and to the whole body defines the shape of an individual. Developmental plasticity in different nutritional regimes affects the absolute value of traits and its relative scaling with respect to body size. We investigated the diversification in wing dimensions and body size in Drosophila ananassae by utilizing isofemale lines of Bilaspur and karyotypically characterized stocks of Varanasi, by rearing flies in nutritional regimes having differences in protein and carbohydrate compositions. The effect of dietary protein variations on plasticity of absolute size of body and wing dimensions was found to be highly genotype‐dependent as compared to sucrose, in both the sexes and in both the populations. This suggests that variation in proteins in natural environment may induce greater selective pressure on traits and may lead to diversification in morphological structures due to the process of genetic accommodation. We found highly significant genetic variation in plasticity for allometric relationship in Bilaspur population for variations in protein as well as carbohydrate in dietary media. However, constraints in plasticity of allometric slopes for wing dimensions in karyotypic stocks of Varanasi were found in response to both the nutritional variations. The analysis of reaction norms also suggested that there is involvement of alpha inversion present in 2L chromosome of Varanasi population, in adapting to different nutritional environmental conditions due to changes in static allometry of different wing dimensions. Overall, this study reveals that the nutritional ecology and evolutionary allometry govern the morphological divergence of traits size and shape in a genotypic‐dependent manner, thereby resulting in morphological evolutions.

Reaction norm analysis of pig growth using environmental descriptors based on alternative traits.

Contemporary group (CG) estimates of different phenotypes have not been widely explored for pigs. The objective of this study was to extend the traits used to derive environmental descriptors of the growing pig, to include CG estimates of early growth between birth and start of feed intake test (EADG), growth during feed intake test (TADG), lifetime growth (ADG), daily feed intake (DFI), backfat (BF) and muscle depth (MD). Pedigree and performance records (n=7,746) from a commercial Australian piggery were used to derive environmental descriptors based on CG estimates of these six traits. The CG estimates of growth traits described different aspects of the environment from the CG estimates of carcass traits (r<0.10). These definitions of the environment then were used in reaction norm analysis of growth, to evaluate sire-by-environment interaction (Sire×E) for growth. The most appropriate reaction norm model to evaluate Sire×E for growth was dependent on the environmental descriptor used. If the trait used to derive an environmental descriptor was distinctly different from growth (e.g., BF and MD), CG as an additional random effect was required in the model. If not included, inflated common litter effect and sire intercept variance suggest there was unaccounted environmental variability. There was no significant Sire×E using any of the definitions of the environment, with estimated variance in sire slopes largest when environments were defined by BF ( =97±83(g/day)2 ), followed by environments defined by DFI ( =39±101(g/day)2 ). While there appears to be differences in ability to detect Sire×E, improved data structure is required to better assess these environmental descriptors based on alternative traits. The ideal trait, or combination of traits, used to derive environmental descriptors may be unique for individual herds. Therefore, multiple phenotypes should be further explored for the evaluation of Sire×E for growth in the pig.

Genotype–environment interaction for age at first calving in buffaloes, using the reaction norm model

The objective of this study was to evaluate the genotype-environment interaction effect on age at first calving in buffaloes. The records were analysed using two approaches: (a) standard animal model and (b) reaction norm model. For the reaction norm analysis, two environmental gradients were formed, using age of first calving or milk yield group contemporary average. The results showed differences in the heritability estimates when using the two approaches. The reaction norm model indicated high heritability in more favourable environments and low magnitude genetic correlations between extreme environments. Based on our findings, we verified the significance of the genotype-environment interaction effect on age at first calving in buffaloes.

Reaction norm analysis reveals rapid shifts toward delayed maturation in harvested Lake Erie yellow perch (Perca flavescens).

Harvested marine fish stocks often show a rapid and substantial decline in the age and size at maturation. Such changes can arise from multiple processes including fisheries‐induced evolution, phenotypic plasticity, and responses to environmental factors other than harvest. The relative importance of these processes could differ systematically between marine and freshwater systems. We tested for temporal shifts in the mean and within‐cohort variability of age‐ and size‐based maturation probabilities of female yellow perch (Perca flavescens Mitchill) from four management units (MUs) in Lake Erie. Lake Erie yellow perch have been commercially harvested for more than a century, and age and size at maturation have varied since sampling began in the 1980s. Our analysis compared probabilistic maturation reaction norms (PMRNs) for cohorts when abundance was lower and harvest higher (1993–1998) to cohorts when abundance was higher and harvest lower (2005–2010). PMRNs have been used in previous studies to detect signs of evolutionary change in response to harvest. Maturation size threshold increased between the early and late cohorts, and the increases were statistically significant for the youngest age in the western MU1 and for older ages in the eastern MU3. Maturation envelope widths, a measure of the variability in maturation among individuals in a cohort, also increased between early and late cohorts in the western MUs where harvest was highest. The highest rates of change in size at maturation for a given age were as large or larger than rates reported for harvested marine fishes where declines in age and size at maturation have been observed. Contrary to the general observation of earlier maturation evolving in harvested stocks, female yellow perch in Lake Erie may be rapidly evolving delayed maturation since harvest was relaxed in the late 1990s, providing a rare example of possible evolutionary recovery.

The effect of a porcine reproductive and respiratory syndrome outbreak on genetic parameters and reaction norms for reproductive performance in pigs1.

The objective of this study was to estimate genetic parameters of antibody response and reproductive traits after exposure to porcine reproductive and respiratory syndrome virus. Blood samples were taken approximately 60 d after the outbreak. Antibody levels were quantified as the sample-to-positive ratio (S/P ratio) using a fluorescent microsphere assay. Reproductive traits included total number born (TNB), number born alive (NBA), number stillborn (NSB), number mummified (NBM), and number born dead (NBD). Mortality traits were log transformed for genetic analyses. Data were split into prior, during, and after the disease outbreak phases using visual appraisal of the estimates of farm-year-week effects for each reproductive trait. For NBA, data from all phases were combined into a reaction norm analysis with regression on estimates of farm-year-week effects for NBA. Heritability for S/P ratio was estimated at 0.17 ± 0.05. Heritability estimates for reproduction traits were all low and were lower during the outbreak for NBA but greater for mortality traits. TNB was not greatly affected during the outbreak, as many sows that farrowed during the outbreak were mated prior to the outbreak. Heritability for TNB decreased from 0.13 (prior) to 0.08 (after). Genetic correlation estimates between prior to and during the outbreak were high for TNB (0.86 ± 0.23) and NBA (0.98 ± 0.38) but lower for mortality traits: 0.65 ± 0.43, −0.42 ± 0.55, and 0.29 ± 1.39 for LNSB, LNBM, and LNBD, respectively. TNB prior to and after the outbreak had a lower genetic correlation (0.32 ± 0.33). In general, genetic correlation estimates of S/P ratio with reproductive performance during the outbreak were below 0.20 in absolute value, except for LNSB (−0.73 ± 0.29). Based on the reaction norm model, estimates of genetic correlations between the intercept and slope terms ranged from 0.24 ± 0.50 to 0.54 ± 0.35 depending on the parameterization used, indicating that selection for the intercept may result in indirect selection for steeper slopes, and thus, less resilient animals. In general, estimates of genetic correlations between farm-year-week effect classes based on the reaction norm model resembled estimates of genetic correlations from the multivariate analysis. Overall, compared to previous studies, antibody S/P ratios showed a lower heritability (0.17 ± 0.05) and low genetic correlations with reproductive performance during a porcine reproductive and respiratory syndrome outbreak, except for the LNSB.

Genetic (co)variance and plasticity of behavioural traits in Lidia bovine breed

Scores for four behavioural traits – aggressiveness, ferocity, face hiding and nobility – recorded between 1984 and 2010 in Lidia bovine breed (the property of a single breeder in Spain), were analysed using the multi-trait and reaction norm models. The multi-trait analysis results revealed heritability values of between 0.23 for nobility and 0.33 for ferocity. Principal components analysis of the genetic correlation matrix identified two factors that explained 91.6% of the total variance. Reaction norm analysis revealed genetic correlation values across sexes and time to be <|1|; this indicates the existence of an interaction genotype x environment. Plasticity, defined as the difference between breeding values over time periods and across the sexes, can be a useful tool for taking into account the influence of the environment on the breeding goal. This allows this interaction to be managed within the framework of a selection programme. With the exception of aggressiveness, which showed significant plasticity, the behaviour of the traits was robust over time and across the sexes. This methodology therefore allows the detection of groups of animals with more robust or plastic responses, particularly with respect to aggressiveness. The use of plasticity as a selection criterion might be beneficial in Lidia bovine breed breeding programmes.

Evolutionary divergence of reaction norms in ecological context: a commentary.

Evolutionary divergence of reaction norms in ecological context: a commentary.

Variation in reaction norms: Statistical considerations and biological interpretation

Analysis of reaction norms, the functions by which the phenotype produced by a given genotype depends on the environment, is critical to studying many aspects of phenotypic evolution. Different techniques are available for quantifying different aspects of reaction norm variation. We examine what biological inferences can be drawn from some of the more readily applicable analyses for studying reaction norms. We adopt a strongly biologically motivated view, but draw on statistical theory to highlight strengths and drawbacks of different techniques. In particular, consideration of some formal statistical theory leads to revision of some recently, and forcefully, advocated opinions on reaction norm analysis. We clarify what simple analysis of the slope between mean phenotype in two environments can tell us about reaction norms, explore the conditions under which polynomial regression can provide robust inferences about reaction norm shape, and explore how different existing approaches may be used to draw inferences about variation in reaction norm shape. We show how mixed model-based approaches can provide more robust inferences than more commonly used multistep statistical approaches, and derive new metrics of the relative importance of variation in reaction norm intercepts, slopes, and curvatures.

Genotype×environment interaction for weaning weight in Nellore cattle using reaction norm analysis

This study aimed to evaluate the existence of genotype×environment interaction (G×E) for weaning weight in Nellore cattle, using reaction norms. Thus, 58,032 records of weaning weight adjusted to 205 days (WW) were analyzed, from animals born between 1987 and 2005, originating from three herds in different regions of Brazil. A random regression model was used to describe changes in breeding values according to the environmental gradient formed by different contemporary groups. Analyses were carried out utilizing INTERGEN software under a Bayesian approach. The solutions of contemporary groups distributed in the environmental gradient varied from −58 to +56kg, and estimates of direct heritability coefficients ranged from 0.05 to 0.44 through the environmental gradient. Genetic correlation between intercept and slope of the reaction norms was 0.75, and the correlation between breeding values of bulls in different environments ranged from −0.38 to 0.99, being higher in highly favorable environments. The results showed the effect of genotype×environment interaction on weaning weight in the herds of this study, especially on the bulls’ rankings. Thus, the authors emphasize the importance of considering the G×E in the genetic evaluation of Nellore cattle, which will result in increased selection efficiency in the weaning period.

Disentangling genetic change from phenotypic response in reproductive parameters of Flemish Cap cod Gadus morhua

Flemish Cap cod Gadus morhua, as with many other fish populations subjected to high fishing rates, has experienced variation in size and age at maturation. Following intense fishing activity and concurrent with a substantial decline in population biomass, cohorts in the early 1990s exhibited a reduction in the length and age at which there is a 50% chance of being mature (L50 and A50 respectively) with values lower than ever estimated before. A further decline in A50 (not in L50) was found after the cod fishing moratoria, in late 1990s and early 2000s cohorts. At the same time, the water temperature showed substantial changes and the abundance of the main prey species of juvenile cod experienced large variation. Shrimp replaced hyperiids as the main prey item since the late 1990s, which was followed by large changes in feeding habits, fish condition and mean size at age. The analysis of maturation reaction norms and countertrend variations in growth rate, water temperature and condition support the hypothesis of genetic change as the main explanation for the decline in size and age at maturation of the cohorts of the late 1980s and early 1990s. However the change in A50 in the late 1990s and early 2000s cohorts is suggested to be caused mainly by a phenotypic response as a consequence of variation in feeding opportunities, water temperature and density-dependent changes in growth rate. If food availability decreases and there is a decrease in water temperature, a decrease in the reproductive potential and survivorship may be expected.

Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 375:229-238 (2009) - DOI: https://doi.org/10.3354/meps07760 Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic Manjusri P. Wijekoon1, Velmurugu Puvanendran2,*, Danny W. Ings1, Joseph A. Brown1,† 1Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Newfoundland A1C 5S7, Canada 2Nofima, Muninbakken 9-13, 9291 Tromsø, Norway *Corresponding author. Email: velpuvy@nofima.no †Deceased ABSTRACT: To test the countergradient variation (CnGV) hypothesis and determine any genetic differences among populations, we conducted an experiment with juvenile cod originating from 3 regions in the NW Atlantic, 3Ps (Placentia Bay, Newfoundland, Canada; 48°N, 54°W), 4T (southern Gulf of St. Lawrence; 46°N, 61°W), and 4X (44°N, 67°W), at 2 temperatures (7 and 11°C) for 15 wk. Results indicated that temperature influenced the growth rates of individuals in all 3 populations, and juveniles reared at 11°C were significantly bigger (length and weight) than those reared at 7°C. No significant differences in growth and food conversion efficiency were found among the 3 populations. Although 3Ps juveniles were significantly smaller in length and lighter in weight than 4T and 4X juveniles at the start of the experiment, length and weight did not differ among populations at the end of the experiment. Survival of both 3Ps and 4T juveniles was significantly higher than that of 4X cod in both high and low water temperature treatments. Although statistical analyses showed no significant differences in growth and food conversion efficiency among juveniles from the 3 populations, reaction norm analysis showed a strong genotypic influence on growth and survival (no genotype × environment interactions) but a genotypic × environment interaction in food conversion efficiency. Our results suggest that Atlantic cod juveniles from these 3 regions do not show CnGV in growth and food conversion efficiency. However, performance differences among populations may be stage specific, and this may have masked CnGV in traits among Atlantic cod populations in our study. KEY WORDS: Growth · Geographic variation · Survival · Adaptation · Atlantic cod Full text in pdf format PreviousNextCite this article as: Wijekoon MP, Puvanendran V, Ings DW, Brown JA, † (2009) Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic. Mar Ecol Prog Ser 375:229-238. https://doi.org/10.3354/meps07760 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 375. Online publication date: January 26, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Evolutionary genetics of ageing in the wild: empirical patterns and future perspectives

Summary 1 Classical evolutionary theory states that senescence should arise as a consequence of the declining force of selection late in life. Although the quantitative genetic predictions of hypotheses derived from this theory have been extensively tested in laboratory studies of invertebrate systems, relatively little is known about the genetics of ageing in the wild. 2 Data from long-term ecological studies is increasingly allowing quantitative genetic approaches to be used in studies of senescence in free-living populations of vertebrates. We review work to date and argue that the patterns are broadly consistent with theoretical predictions, although there is also a clear need for more empirical work. 3 We argue that further advances in this field of research might be facilitated by increased use of reaction norm models, and a decreased emphasis on attempting to discriminate between mutation accumulation and antagonistic pleiotropy models of senescence. We also suggest a framework for the better integration of environmental and genetic effects on ageing. 4 Finally, we discuss some of the difficulties in applying quantitative genetic models to studies of senescence outside the laboratory. In particular we highlight the problems that viability selection can cause for an accurate estimation of parameters used in the prediction of age-trajectory evolution.

Aerial dispersal plasticity under different wind velocities in a salt marsh wolf spider

Dispersal can be regarded as a process operating both between and within patches of suitable habitat. For uncontrolled dispersal processes, the risk of crossing the borders of the habitat patch and arriving in the unsuitable landscape matrix will increase with decreasing patch area, in particular when the distance between isolated habitat patches is larger than the species’ average dispersal capacity. Ballooning dispersal in spiders can be considered as a passive dispersal process, in which dispersed distances depend on the prevalent wind velocity. We executed a reaction norm analysis to analyze how dispersal propensity of the salt marsh wolf spider Pardosa purbeckensis depended on population characteristics (patch size) and the environment (wind velocity). Dispersal propensity was affected by the interaction between wind velocity and maternal patch size. Ballooning propensities decreased with decreasing salt marsh size. Interestingly, genotypes from large salt marshes show higher ballooning propensities under higher wind velocities, whereas those from small habitat patches show their highest dispersal propensity under low wind velocities. Crossing reaction norms and subsequently strong genotype 3 environment interaction variation was observed in all populations but tended to be lower in genotypes from large salt marshes. It is likely that this pattern results from differences in wind velocity–related costs of within-habitat dispersal in salt marshes of different sizes. Key words: ballooning, behavioral plasticity, Lycosidae, Pardosa purbeckensis, reaction norm. [Behav Ecol]

Fisheries-induced trends in reaction norms for maturation in North Sea plaice

We analyse how intensive exploitation may have caused evolutionary changes in the age and length at maturation in North Sea plaice Pleuronectes platessa. Such evolutionary change in the onset of maturation is expected, given that fishing mortality is more than 4 times higher than natural mortality. In order to disentangle phenotypic plasticity from evolutionary change, we employ the probabilistic reaction-norm approach. This technique allows us to estimate the probabilities of maturing at each relevant age and size, and to disentangle the plasticity in age and size at maturation that results from changes in growth rates from evolutionary changes in maturation propensities themselves. This recently developed method is applied here to females of 41 cohorts (1955 to 1995) of North Sea plaice. We focus on trends in fishing mortality, in growth rates, and in the probabilities of maturing, and test the hypothesis that the decrease in age and length at maturation is partly caused by fisheries-induced adaptive change. We find that the reaction norm for age and length at maturation has indeed significantly shifted towards younger age and smaller length. The reaction-norm analysis suggests a picture in which short-term fluctuations originating from plastic responses are superimposed on a persistent long-term trend resulting from genetic responses and higher body growth.

Natural selection for grazer resistance to toxic cyanobacteria: evolution of phenotypic plasticity?

We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor-quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcvstis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj). We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log-transformed) changes in gj. Log(gj) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in gj and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in gj among genotypes), a significant genotype x food-type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log-transformed data, a significant sediment-genotype-age x food-type interaction. The latter shows that phenotypic plasticity evolved over the period studied. Two constraints act on response to selection in the D. galeata-Lake Constance system. First, gj on a diet containing poor food is highly correlated with gj on a diet of good food, thus evolving resistance also meant evolving an increase in gj on both diets. Second, because genotypes with a high gj also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential gj after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized gj and led to animals of small adult body size even if their genotypes had the potential for high gj and large size. With realized gj reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.

Seasonality and genetic architecture of development time and body size of the birch feeding sawfly Priophorus pallipes.

We tested, using the sawfly Priophorus pallipes feeding on leaves of mountain birch, whether the expression of genetic (co)variation of larval development time and body size can be altered by exposing larvae to diets with differential seasonal changes in quality. In nature, larvae feed mainly on mature leaves, but occasionally they are forced to consume senescing leaves. Sixty families were assayed on three experimentally simulated diets: mature leaves of high quality, senescing leaves of rapidly declining quality, and senesced leaves of low quality. The intuitively obvious positive phenotypic and genetic correlations between development time and final mass were observed when the larvae consumed leaves of stable high quality, but low and declining food quality prevented long-growing individuals and families from achieving high final mass, switching the correlations to close to zero or negative in these treatments. The amount of genetic variation for body size showed a non-linear change across the diet quality gradient, whereas genetic variation for development time increased with decreasing diet quality. The among-trait difference in the degree reaction norms crossed along the diet gradient caused the changes in the expression of genetic (co)variation within the environments. Our results show that seasonally varying diet quality induces dramatic changes in the genetic (co)variation of development time and body size, and that simultaneous analysis of reaction norms and environment-specific expression of genetic (co)variation is necessary for the understanding of the genetic characteristics underlying the construction of phenotypes in heterogeneous environments.

NATURAL SELECTION FOR GRAZER RESISTANCE TO TOXIC CYANOBACTERIA: EVOLUTION OF PHENOTYPIC PLASTICITY?

We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor-quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcystis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj).We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log-transformed) changes in gj. Log(gj) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in gj and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in gj among genotypes), a significant genotype × food-type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log-transformed data, a significant sediment-genotype-age × food-type interaction. The latter shows that phenotypic plasticity evolved over the period studied.Two constraints act on response to selection in the D. galeata–Lake Constance system. First, gj on a diet containing poor food is highly correlated with gj on a diet of good food, thus evolving resistance also meant evolving an increase in gj on both diets. Second, because genotypes with a high gj also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential gj after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized gj and led to animals of small adult body size even if their genotypes had the potential for high gj and large size. With realized gj reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.Corresponding Editor: A. Caballero