Phenotypic plasticity of Daphnia life history traits in response to predator kairomones: genetic variability and evolutionary potential

Cladoceran populations can respond to changingpredation regimes by a phenotypical response as wellas by shifts in genotype frequencies. In this study,we investigated the phenotypic plasticity exhibited bylife history traits of D. galeata in response tothe presence of predator kairomones, as well as theextent to which natural selection may act on thesetraits and their phenotypic plasticity. In alife-table experiment, seven clones of a natural D. galeata population were subjected to kairomonesfrom fish (Perca), from an invertebrate predator(Chaoborus) or a mixture of both. Life historytraits were affected by the kairomones of bothpredators, but effects of Chaoborus wereneutralised by Perca in the kairomone mix. Noapparent trade-off was found between growth- andreproduction related traits: although daphnids fromthe Chaoborus treatment grew faster thandaphnids from the other treatments, no reduction inthe reproductive output was observed. Broad-senseheritabilities were found to be relatively high forsome life history traits (size at maturity, neonatesize, number of neonates) as well as for thephenotypic plasticity response of these traits. Thisreflects the evolutionary potential of life historytraits and their phenotypic response to predatorkairomones in the D. galeata population.Publication number 2334 of The Netherlands Institute of Ecology, Centre for LimnologyPublication number 2334 of The Netherlands Institute of Ecology, Centre for Limnology     

Assessment of the importance of fish predation versus copepod predation on life history traits of Daphnia hyalina

Daphnia hyalina is a cladoceran present throughthe whole year except for late summer in Maranhão,a meso-eutrophic reservoir in central Portugal. Apartfrom the influence of food, both vertebrate andinvertebrate predation pressures seem to have aneffect on D. hyalina population dynamics.Enclosure experiments were designed to assess therelative importance of both types of predation. Afterthe summer crash, D. hyalina reached highernumbers in the fishless enclosures than in the lakedespite of high predation pressure upon juveniles byAcanthocyclops robustus. Fish predation upon thelargest individuals, especially large egg bearingfemales, was responsible for the lower fertility ofthe open water population when compared with theenclosure population. In the enclosures an increase intail spine length was observed. The longer tail spineprobably offered protection from copepod predation,allowing at least some of the juveniles to coexistwith their potential predator and reach the adultstage, less susceptible to copepod predation.     

Phenotypic plasticity for plant development, function and life history

A single genotype can produce different phenotypes in different environments. This fundamental property of organisms is known as phenotypic plasticity. Recently, intensive study has shown that plants are plastic for a remarkable array of ecologically important traits, ranging from diverse aspects of morphology and physiology to anatomy, developmental and reproductive timing, breeding system, and offspring developmental patterns. Comparative, quantitative genetics and molecular approaches are leading to new insights into the adaptive nature of plasticity, its underlying mechanisms and its role in the ecological distribution and evolutionary diversification of plants.     

Antipredator reaction norms for life history traits in Daphnia pulex: dependence on temperature and food

Synergistic effects between temperature and food level on the vulnerability of Daphnia life histories to predation have previously received little attention, despite their potential significance for summer population dynamics. In this investigation, most traits in the early life history of Daphnia pulex altered in response to increasing concentrations of fish kairomone. Although there was some variation attributable to experimental temperature and food treatments, traits exhibited reaction norms which suggested at least some degree of kairomone dose-dependence. Temperature, food level and their interaction affected the mean values of every trait examined and in some cases also influenced the antipredator response via a three-way interaction with fish kairomone. The insertion of an extra juvenile instar resulted in later maturation at a larger size for most females raised under conditions of low temperature and low food, but this response was largely suppressed in the presence of fish kairomone. Earlier maturation due to deletion of an instar was observed most frequently under high food and temperature conditions with fish kairomone present, suggesting an effect of kairomone on the size threshold for reproductive development. Principal components analysis was used to produce an index reflecting the net potential vulnerability to fish predation of the suite of life history traits. Vulnerability generally declined with increasing kairomone level as a result of the apparently adaptive alterations in most life history traits. Raised temperature and food level also generally reduced potential vulnerability, but a highly significant interaction between these factors was also found. Potentially important implications of these results for optimal vertical migration and summer population dynamics are discussed.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Predator-mediated plasticity in morphology, life history, and behavior of Daphnia: the uncoupling of responses

We studied the way 12 traits responded to fish kairomones in a set of 16 Daphnia magna clones derived from four different habitats-two where daphnids co-occur with fish and two without fish. These clones differed widely in their response to predator kairomones, with none of the clones showing a significant response in all traits and all clones showing a response for at least one trait. Most of the clones showed a significant response in one to four traits, with no evidence for an association between different traits. Clones from fish habitats were slightly more responsive to the presence of fish kairomones than clones from fishless locations. We conclude that most clones show an induced response to the presence of their predators (fish) but that there is a large genetic variability with respect to the traits for which clones show a response. Our results indicate that the major distinction is not between inducible and noninducible genotypes but rather that the genotypes differ in the combination of traits for which they show inducible responses.     

Daphnia vertical distribution and the presence of toxic cyanobacteria

Daphnia can alter its vertical position in the water column in response to chemical cues from predators. In this study we tested the hypothesis that a Daphnia pulex clone with little evolutionary exposure to cyanobacteria would move away from patches of cyanobacteria (Anabaena affinis and A. flos-aquae) which contain potent endotoxins. Daphnia was censused at 2 h intervals for 6 h in laboratory columns in which there was a steep vertical gradient of cyanobacteria. Data were analyzed by repeated-measures ANOVA. Control (no Anabaena) and experimental columns showed no significant differences in Daphnia distributions. Daphnia in experimental columns frequently moved into areas with dense concentrations of Anabaena and stayed there for long periods of time. Our results show that this D. pulex clone does not exhibit a rapid (within 6 h) avoidance response to toxic Anabaena.     

Phenotypic plasticity in two marine snails: constraints superseding life history

In organisms encountering predictable environments, fixed development is expected, whereas in organisms that cannot predict their future environment, phenotypic plasticity would be optimal to increase local adaptation. To test this prediction we experimentally compared phenotypic plasticity in two rocky-shore snail species; Littorina saxatilis releasing miniature snails on the shore, and Littorina littorea releasing drifting larvae settling on various shores, expecting L. littorea to show more phenotypic plasticity than L. saxatilis. We compared magnitude and direction of vectors of phenotypic difference in juvenile shell traits after 3 months exposure to different stimuli simulating sheltered and crab-rich shores, or wave-exposed and crab-free shores. Both species showed similar direction and magnitude of vectors of phenotypic difference with minor differences only between ecotypes of the nondispersing species, indicating that plasticity is an evolving trait in L. saxatilis. The lack of a strong plastic response in L. littorea might be explained by limits rather than costs to plasticity.     

Colonization dynamics and life history traits of seven Daphnia pulex genotypes

Summary A series of experiments revealed significant differences in the potential ability of seven Daphnia pulex genotypes to colonize two lakes in which this species does not naturally reside. Life table experiments, in which individuals from each genotype were raised separately on water and natural phytoplankton from the two lakes, revealed several significant differences among genotypes in life history traits, including age and size at first reproduction, clutch size and offspring body size. Significant differences among genotypes were also found in mean genotype fitness and rate of population increase, although all genotypes were able to increase in absolute numbers. Significant genotypexlake interaction was found for several life history traits and mean fitness, indicating that the relative success of invading genotypes may depend on habitat characteristics. Enclosure experiments, in which all seven genotypes were introduced together into enclosures in both lakes, revealed that some genotypes increased greatly while others declined in relative abundance, and that the most successful genotypes differed between lakes. In addition, the most successful genotypes in the enclosures were not necessarily the genotypes that displayed the highest fitness in the life table experiments, possibly because individuals in the enclosures competed for food resources, leading to exclusion of certain genotypes.     

Phenotypic plasticity for life history traits in Drosophila melanogaster. II. Epigenetic mechanisms and the scaling of variances

Abstract We manipulated developmental time and dry weight at eclosion in 15 genotypes of Drosophila melanogaster by growing the larvae in 9 environments defined by 3 yeast concentrations at 3 temperatures. We observed how the genetic and various environmental components of phenotypic variation scaled with the mean values of the traits. Temperature, yeast, within-environmental factors and genotype influenced the genotypic and environmental standard deviations of the two traits in patterns that point to very different modes of physiological and developmental action of these factors. Since different factors affected the environmental and genetic components of the phenotypic variation either in parallel or inversely, we conclude that environmental heterogeneity may have small or large effects on evolutionary rates depending on which factors cause the heterogeneity. The analysis also suggests that the scaling of variances with the mean is not as trivial as is often assumed when coefficients of variation are computed to “standardize” variation.     

Phenotypic plasticity in Daphnia magna Straus: variable maturation instar as an adaptive response to predation pressure

Life history responses of four Daphnia magna clones at two food levels were studied to assess the importance of maturation instar on the plasticity of fitness responses under simulated mortality regimes. Females of the clones studied could vary offspring size with consequent effects on their maturation time. Significant genetic variability in life history and fitness responses, measured as the intrinsic rate of population increase, within and across food levels was observed, but most of this variation could be attributed to maturation instar differences among clones within and across environments. In the laboratory, without extrinsic mortality, females maturing earlier always had higher fitness than those maturing later, indicating a clear fitness cost of delaying maturity. Nevertheless using a model, we showed that the observed maturation instar effects on life history responses can lead to differences in fitness under different size-selective predation regimes, such that females with delayed maturity have higher fitness under invertebrate predation while females maturing earlier have higher fitness under fish predation regimes. These results suggest that intraclonal variation in offspring size and hence in the number of maturation instars can be an adaptation to living in habitats subject to temporal fluctuations in fish and invertebrate predation pressure.     

Phenotypic plasticity in Daphnia pulicaria as an adaptation to high biomass of colonial and filamentous cyanobacteria: experimental evidence

We investigated the ability of the water flea, Daphnia, to adaptthe size and structure of its filtering apparatus as a responseto experimentally increased biomass of inedible filamentousand colonial cyanobacteria in a large in situ enclosure experiment.Predator-induced phenotypic plasticity in Daphnia has been extensivelydocumented, but only a small number of studies have focusedon morphological changes induced by food quantity and quality.Here we show that Daphnia responded to increased biomass ofinedible phytoplankton in its environment by enlarging the areaand the mesh size of its filtering apparatus. These observationssuggest that Daphnia responds to increased concentrations ofinedible particles in the same fashion as it does in a verylow food environment. In our study, daphnids exposed to a highbiomass of inedible algae, in fertilized enclosures, had significantlylarger (12–15%) filter screens attached to their thirdand fourth limbs in comparison to daphnids exposed to a lowbiomass of inedible algae. The mesh size also increased in thesame conditions. These results suggest that daphnids used theirphenotypic plasticity to respond to changes in their food qualityand quantity. By using this strategy, daphnids can maximizetheir food uptake and hence compensate for the scarcity of suitablefood encountered in very oligotrophic conditions or even ineutrophic conditions when phytoplankton communities are dominatedby large inedible species.     

Variation in life history responses of Daphnia to toxic Microcystis aeruginosa

Three clones of Daphnia pulex and two clones of Daphnia longispinawere exposed to toxic Microcystis aeruginosa for 21 days ina lifetable experiment. The growth and reproduction of individualdaphnids were followed daily to study the long-term effectsof toxic Microcystis. Exposure to Microcystis increased mortality,decreased growth, delayed maturation and decreased offspringproduction, indicating nutritional deficiency and toxic effects.We found variation in life history responses between speciesand among clones. Our results suggest that toxic cyanobacteriamay act as a modifying agent in zooplankton communities at boththe species and clonal level.     

Phenotypic plasticity for life history traits in Drosophila melanogaster. I. Effect on phenotypic and environmental correlations

All 36 possible crosses among 6 homozygous lines of Drosophila melanogaster were tested for their phenotypic response in developmental time and dry weight at eclosion to variation in temperature and yeast concentration. This method was chosen because it allows one to produce the same heterozygous offspring repeatedly for testing under more conditions than could be handled at once. We estimated the effects of yeast concentration and temperature and their interaction on both the phenotypic and the environmental components of variation and covariation of the two traits. Development was slower at low temperatures and yeast concentrations, and dry weight and viability were lower at higher temperatures and lower yeast levels. Interactions of the two factors with the crosses and with each other indicated that there were genetic differences in plasticity and that the sensitivity of a trait to one factor depended on the level of the other. The covariation of the two traits was generally weak within an environment. Across environments, its sign depended on the factor that changed between the environments: positive for temperature, negative for yeast concentration. These findings can be explained in terms of an established growth model for Drosophila larvae. We conclude that for plastic traits with moderate or low heritability, the relationship between the phenotypic and genetic covariance matrices may be a complex function of the environmental factors that affect the traits. Some implications for the prediction of the evolution in fluctuating environments are outlined.     

Effects of UVB-irradiated algae on life history traits of Daphnia pulex

1. The impact of ultraviolet-B (UVB)-irradiated phytoplankton on the life history parameters of Daphnia was studied. Three species of Chlorophyceae ( Chlamydomonas reinhardtii, Scenedesmus acutus and S. subspicatus ) and one species of Cryptophyceae ( Cryptomonas pyrenoidifera ) were cultured with and without exposure to UVB radiation (biologically effective dose of 3 kJ m^–2 day^–1 DNA weighted) and fed to Daphnia pulex.
2. The daphnids feeding on UVB-irradiated algae tended to be smaller in size, to produce less and smaller offspring, and to have a reduced population growth rate.
3. Fatty acid (FA) analysis of the phytoplankton showed a reduced total FA content under UVB radiation for all four species. Changes in the content of unsaturated fatty acids (UFA) were species specific.
4. The results indicate that the energy transfer between the primary producers' and grazers' level might be negatively influenced by UVB radiation. Direct effects on food quality are modest but detectable, changes in quantity or species composition may be more important in the energy transfer.     

Predator-induced phenotypic plasticity in Daphnia pulex: uncoupling morphological defenses and life history shifts

Chemical cues from a predator Chaoborus sp. induce morphological defense (neck spine) and life history shifts (later reproduction, decreased fecundity but larger juvenile size) in the waterflea Daphnia pulex. These shifts have been interpreted either as costs of defense or as separate adaptation. In order to investigate if the life history shifts can be separated from the morphological defense, Daphnia pulex individuals were exposed to chemical cues from Chaoborus at different stages of life for variable periods. The daphnids that were exposed to Chaoborus started their reproduction later than the controls, although the differences were not statistically significant. Neck spine was induced only if daphnids were exposed to Chaoborus in an early stage of their life. Numbers of eggs produced were not affected by the different treatments, but egg mortality was higher in mothers exposed to Chaoborus. With these treatments it was possible to see neck spine induction without measurable life history changes or costs. On the other hand, irrespective of neck spine presence, the Chaoborus chemical(s) had an effect on Daphnia pulex mothers.Publication no 2159. Netherlands Institute of Ecology, Centre for LimnologyPublication no 2159. Netherlands Institute of Ecology, Centre for Limnology     

Sensitivity of Daphnia to toxic cyanobacteria: effects of genotype and temperature

1. We studied the effects of both acute and chronic exposure of Daphnia pulex to toxic Microcystis aeruginosa . We focused on the effects of Daphnia genotype and temperature (19 and 24 °C).
2. The study revealed variation among ten Daphnia pulex clones in survivorship under acute Microcystis exposure, measured as EC₅₀. An increase in temperature caused a clear decrease in EC₅₀, although the ranking of clones according to sensitivity remained the same at both temperatures.
3. In the chronic exposure of two of the clones, toxic Microcystis reduced survival and reproduction. The two clones differed in their responses, indicating different means of coping with toxic cyanobacteria. Toxic cyanobacteria reduced slightly more at 24 °C than 19 °C.
4. The clonal difference in sensitivity to toxic cyanobacteria at acute exposure was reversed at chronic exposure. This indicates that the results from short-term toxicity cannot be used to predict life history responses under sublethal exposure.     

Sensitivity of Daphnia to toxic cyanobacteria: effects of genotype and temperature

1. We studied the effects of both acute and chronic exposure of Daphnia pulex to toxic Microcystis aeruginosa . We focused on the effects of Daphnia genotype and temperature (19 and 24 °C).
2. The study revealed variation among ten Daphnia pulex clones in survivorship under acute Microcystis exposure, measured as EC₅₀. An increase in temperature caused a clear decrease in EC₅₀, although the ranking of clones according to sensitivity remained the same at both temperatures.
3. In the chronic exposure of two of the clones, toxic Microcystis reduced survival and reproduction. The two clones differed in their responses, indicating different means of coping with toxic cyanobacteria. Toxic cyanobacteria reduced slightly more at 24 °C than 19 °C.
4. The clonal difference in sensitivity to toxic cyanobacteria at acute exposure was reversed at chronic exposure. This indicates that the results from short-term toxicity cannot be used to predict life history responses under sublethal exposure.     

Effects of toxic and non-toxic cyanobacteria on the life history of tropical and temperate cladocerans

1. This study compares the effects of four toxic strains of Microcystis aeruginosa on tropical and temperate Cladocera. Survival was tested in acute toxicity experiments using Microcystis alone or in mixtures with the edible green algae Ankistrodesmus falcatus. The effect of chronic exposure on population growth was estimated in life‐table experiments by varying the proportion of Microcystis and the green alga. Nutritional deficiency was assessed using a non‐toxic cyanobacterium in a zooplankton growth experiment. Feeding inhibition was tested using a C‐labelled green alga as a tracer in mixtures with toxic Microcystis.
2. Toxicity varied consistently between Microcystis strains, while sensitivity varied consistently between cladoceran species. However, no relationship was found between sensitivity and geographical origin or cladoceran body size. Two small‐bodied cladocerans from the same tropical lake, Ceriodaphnia cornuta and Moinodaphnia macleayi, were the least sensitive and most sensitive species, respectively.
3. Surprisingly, two small tropical cladocerans survived longer without food than did three large Daphnia species and a third small tropical species.
4. Each of the three tropical Microcystis strains strongly reduced the population growth rate (little ‘r’) and reproductive output of each cladoceran, this reduction being proportional to the percentage of toxic cells in the diet.
5. As the sole food source, the non‐toxic cyanobacterium, Synechococcus elongatus, supported poor growth in M. macleayi. The nutritional deficiency was overcome when Synechococcus was mixed with either Ankistrodesmus or an emulsion rich in omega‐3 fatty acids.
6. Microcystis inhibited the feeding rate of two cladocerans, even when it comprised only 5% of a mixture with the green algae A. falcatus.
7. Differences in sensitivity to the toxic cyanobacterium appear to be associated with differences in life history between the cladoceran species rather than differences between tropical and temperate taxa. Slow‐growing species that are resistant to starvation appear less sensitive to toxic Microcystis than fast‐growing species, which also tend to die more quickly in the absence of food.     

Phenotypic plasticity and constancy of life-history traits in laboratory clones of Daphnia magna Straus: effects of neonatal length

1. The phenotypic constancy of four laboratory Daphnia magna clones in fitness-related life-history traits, such as age and clutch size at maturity, was studied among consecutive experimental runs in differing food environments.
2. A significant part of the observed clonal and genetic-by-environmental variation in age and clutch size at maturity was explained by experimentally uncontrollable variations in neonatal body length.
3. Despite food availability, neonatal length determined the number of instars invested to maturity and thus maturation age. Clonal differences in neonatal length and thus in maturation instar occurrence across environments explained most of the clonal variability observed in maturation age.
4. Although interclonal differences in clutch size existed, most of the phenotypic plasticity observed for clutch size was mediated by clonal differences in neonatal length.
5. Clonal differences in neonatal length and in the occurrence of maturation instars across environments dramatically affected the body length of instar IM-2 where provisioning of eggs take place. Since clutch size is determined from clutch mass and clutch mass was strongly related to the body length of instar IM-2, clonal differences across environments in body length of instar IM-2 mirrored clonal differences across environments in clutch size.
6. The results reported in the present study show that maternally mediated traits such as neonatal length affect how genotypes respond to different environmental selection regimes (genetic-by-environmental interaction). Future research needs to focus on the effects of neonatal length on the heritability or genetic variation of the reaction norms, since prediction of the response to selection is a key research objective in quantitative genetic studies.