Experimental evolution of field populations of Daphnia magna in response to parasite treatment

Although there is little doubt that hosts evolve to reduce parasite damage, little is known about the evolutionary time scale on which host populations may adapt under natural conditions. Here we study the effects of selection by the microsporidian parasite Octosporea bayeri on populations of Daphnia magna. In a field study, we infected replicated populations of D. magna with the parasite, leaving control populations uninfected. After two summer seasons of experimental evolution (about 15 generations), the genetic composition of infected host populations differed significantly from the control populations. Experiments revealed that hosts from the populations that had evolved with the parasite had lower mortality on exposure to parasite spores and a higher competitive ability than hosts that had evolved without the parasite. In contrast, the susceptibility of the two treatment groups to another parasite, the bacterium Pasteuria ramosa, which was not present during experimental evolution of the populations, did not differ. Fitness assays in the absence of parasites revealed a higher fitness for the control populations, but only under low population density with high resource availability. Overall, our results show that, under natural conditions, Daphnia populations are able to adapt rapidly to the prevailing conditions and that this evolutionary change is specific to the environment.     

Parasite-mediated selection in experimental Daphnia magna populations

Abstract It has been suggested that parasites are a strong selecting force for their hosts and therefore may alter the outcome of competition among host genotypes. We tested the extent to which parasite-mediated selection by different parasite species influenced competition among clones of the cyclic parthenogen Daphnia magna . We monitored clone frequency changes in laboratory microcosm populations consisting of 21 D. magna clones. Parasite treatments (two microsporidians, Glugoides intestinalis and Ordospora colligata ) and a parasite-free control treatment were followed over a nine-month period. A further treatment with the bacterium Pasteuria ramosa failed. We found significant differences in clonal success among the treatments: the two parasite treatments differed from the control treatment and from each other. Additionally, we measured the clone-specific population carrying capacity, competitive ability against tester clones, and reproductive success of infected and uninfected females to test whether they correlate with clonal success in the microcosms. The clone-specific competitive ability was a good predictor of clonal success in the microcosms, but clonal carrying capacity and host reproductive success were not. Our study shows that parasite-mediated selection can strongly alter the outcome of clonal competition. The results suggest that parasites may influence microevolution in Daphnia populations during periods of asexual reproduction.     

Plasticity of growth rate and metabolism in Daphnia magna populations from different thermal habitats

The aim of this study was to evaluate the effect of temperature on growth and aerobic metabolism in clones of Daphnia magna from different thermal regimes. Growth rate (increment in size), somatic juvenile growth rate (increment in mass), and oxygen consumption were measured at 15 and 25 degrees C in 21 clones from one northern and two southern sites. There were no significant differences in body size and growth rate (increase in length) at both 15 and 25 degrees C among the three sites. Clones from southern site 2 had a higher mass increment than clones from the other two sites at both temperatures. Clone had a significant effect on growth (body length) and body size at both temperatures. As expected, age at maturity was lower at 25 degrees C (4.5 days) than at 15 degrees C, (11.6 days) and body sizes, after the release of the third clutch, were larger at 15 degrees C than at 25 degrees C. Northern clones had higher oxygen consumption rates and specific dynamic action (SDA) than southern clones at 15 degrees C. By contrast, southern clones from site 1 had a higher oxygen consumption and SDA than subarctic clones at 25 degrees C. Clones from southern site 2 had high oxygen consumption rates at both temperatures. Our results reveal important differences in metabolic rates among Daphnia from different thermal regimes, which were not always reflected in growth rate differences.     

Genetic diversity of Daphnia magna populations enhances resistance to parasites

The diversity-disease hypothesis states that decreased genetic diversity in host populations increases the incidence of diseases caused by pathogens (= monoculture effect) and eventually influences ecosystem functioning. The monoculture effect is well-known from crop studies and may be partially specific to the artificial situation in agriculture. The effect received little attention in animal populations of different diversities. Compared with plants, animals are mobile and exhibiting social interactions. We followed the spread of a microsporidian parasite in semi-natural outdoor Daphnia magna populations of low and high genetic diversity. We used randomly selected, naturally occurring host genotypes. Host populations of low diversity were initially monoclonal, while the host populations of high diversity started with 10 genotypes per replicate. We found that the parasite spread significantly better in host populations of low diversity compared with host populations of high diversity, independent of parasite diversity. The difference was visible over a 3-year period. Host genotypic diversity did not affect host population density. Our experiment demonstrated a monoculture effect in independently replicated semi-natural zooplankton populations, indicating that the monoculture effect may be relevant beyond agriculture.     

Temporal genetic stability in natural populations of the waterflea Daphnia magna in response to strong selection pressure

Studies monitoring changes in genetic diversity and composition through time allow a unique understanding of evolutionary dynamics and persistence of natural populations. However, such studies are often limited to species with short generation times that can be propagated in the laboratory or few exceptional cases in the wild. Species that produce dormant stages provide powerful models for the reconstruction of evolutionary dynamics in the natural environment. A remaining open question is to what extent dormant egg banks are an unbiased representation of populations and hence of the species’ evolutionary potential, especially in the presence of strong environmental selection. We address this key question using the water flea Daphnia magna, which produces dormant stages that accumulate in biological archives over time. We assess temporal genetic stability in three biological archives, previously used in resurrection ecology studies showing adaptive evolutionary responses to rapid environmental change. We show that neutral genetic diversity does not decline with the age of the population and it is maintained in the presence of strong selection. In addition, by comparing temporal genetic stability in hatched and unhatched populations from the same biological archive, we show that dormant egg banks can be consulted to obtain a reliable measure of genetic diversity over time, at least in the multidecadal time frame studied here. The stability of neutral genetic diversity through time is likely mediated by the buffering effect of the resting egg bank.     

Phototaxis in Daphnia magna: the influence of temperature and acidity on the phototactic behaviour of Daphnia genotypes

The influence of culture temperature, temperature shocks andpH shocks on the phototactic behaviour of four Daphnia magnaclones was studied. Test animals tended to become more positivelyphototactic when the pH of the medium was raised, and less positivelyphototactic when cultured at high (27°C) temperature. Responsesto temperature changes at the start of the experiment were clonedependent. Differences between genotypes remained highly significantdespite the broader environmental circumstances in which experimentswere carried out. Broad heritability estimates for the phototacticbehaviour of Daphnia under the different environmental conditionsstudied ranged from 0.78 to 0.88.     

Differential response of Daphnia genotypes to oxygen stress: respiration rates,hemoglobin content and low-oxygen tolerance

Summary Laboratory respiration rate experiments using three electrophoretically identified clones of the fresh water, planktonic cladoceran, Daphnia pulex, from an eutrophic farm pond, indicated that clones acclimated to both low and high oxygen levels, regulated oxygen consumption across a wide range of oxygen concentrations (1.0–9.0 mg· liter^-1). A threshold oxygen level of 0.5–1.0 mg·liter^-1 was reached, where animals succumbed to oxygen stress, regardless of hemoglobin content. No significant clonal differences in respiration rates were found. These data suggest that members of this Daphnia population are able to regulate oxygen metabolism across a wide range of ambient oxygen concentrations, and indicate a well-adapted respiratory system.Low-oxygen tolerance experiments and hemoglobin measurements indicated further that physiological differences indeed exist between clones; one clone produced the lowest amount of hemoglobin and was least tolerant of low oxygen levels. These data imply that spatial and temporal changes in dissolved oxygen concentration may be an important selective force influencing the clonal (genotypic) composition of natural cladoceran populations.     

Egg size and egg mass of Daphnia magna: response to food availability

The influence of different food availability on egg size and egg mass in Daphnia magna Straus was studied in long-term experiments using a flow-through system. Daphnia were either kept a constant high or low food levels or subjected to alternating periods of high food and starvation. Some animals were starved continuously after they had deposited their first clutch of eggs. Eggs were measured and weighed and their density (dry mass per volume) was determined. The results support the model of Glazier (1992), which defines a region of reproductive constraint at very low food concentrations and a region of adaptive response as food concentration increase. Egg sizes were largest under continuously low food concentrations (0.1 mg Cl^–1), which indicates that the maximum of Glazier's non-linear response curve is at very low food levels. Eggs produced during starvation were small, probably as a result of reproductive constraints. Egg density was about 0.37 mg dry weight mm^–1 and did not differ between treatments.     

The response of European Daphnia magna Straus and Australian Daphnia carinata King to changes in geomagnetic field

This study investigates the effects of lifelong exposure to reversed geomagnetic and zero geomagnetic fields (the latter means absence of geomagnetic field) on the life history of Daphnia carinata King from Australia and Daphnia magna Straus from Europe. Considerable deviation in the geomagnetic field from the usual strength, leads to a decrease in daphnia size and life span. Reduced brood sizes and increased body length of neonates are observed in D. magna exposed to unusual magnetic background. The most apparent effects are induced by zero geomagnetic field in both species of Daphnia. A delay in the first reproduction in zero geomagnetic field is observed only in D. magna. No adaptive maternal effects to reversed geomagnetic field are found in a line of D. magna maintained in these magnetic conditions for eight generations. Integrally, the responses of D. magna to unusual geomagnetic conditions are more extensive than that in D. carinata. We suggest that the mechanism of the effects of geomagnetic field reversal on Daphnia may be related to differences in the pattern of distribution of the particles that have a magnetic moment, or to moving charged organic molecules owing to a change in combined outcome and orientation of the geomagnetic field and Earth's gravitational field. The possibility of modulation of self-oscillating processes with changes in geomagnetic field is also discussed.     

Evolution of life histories and yields in experimental populations of Daphnia magna

An experiment on life-history evolution is described in which replicated populations of the Cladoceran Daphnia magna , made up of a standard mixture of clones, were subjected to two contrasting culling regimes, involving removal of small or large individuals. After approximately 150 days of culling, analysis of the life histories showed that genetic differences had emerged between culling regimes. Clones selected by culling small sized individuals grew rapidly through small size classes, whereas those selected by culling large sizes grew slowly through small size classes, with the result that the age at which they became vulnerable to harvesting was delayed. In addition, there was some redistribution of reproduction towards size classes that were not culled. This evolution is consistent with a major decline observed in the yield from populations in which large individuals were culled. We argue that changes of this general kind are likely to take place in exploited populations and that serious consideration should be given to evolutionary aspects of the management of such populations.     

Extinction hazards in experimental Daphnia magna populations: effects of genotype diversity and environmental variation

Extinction is ubiquitous in natural systems and the ultimate fate of all biological populations. However, the factors that contribute to population extinction are still poorly understood, particularly genetic diversity and composition. A laboratory experiment was conducted to examine the influences of environmental variation and genotype diversity on persistence in experimental Daphnia magna populations. Populations were initiated in two blocks with one, two, three, or six randomly selected and equally represented genotypes, fed and checked for extinction daily, and censused twice weekly over a period of 170 days. Our results show no evidence for an effect of the number of genotypes in a population on extinction hazard. Environmental variation had a strong effect on hazards in both experimental blocks, but the direction of the effect differed between blocks. In the first block, variable environments hastened extinction, while in the second block, hazards were reduced under variable food input. This occurred despite greater fluctuations in population size in variable environments in the second block of our experiment. Our results conflict with previous studies, where environmental variation consistently increased extinction risk. They are also at odds with previous studies in other systems that documented significant effects of genetic diversity on population persistence. We speculate that the lack of sexual reproduction, or the phenotypic similarity among our experimental lines, might underlie the lack of a significant effect of genotype diversity in our study.     

Enzyme variability in natural populations of Daphnia magna. IV. Ecological differentiation and frequency changes of genotypes at Audley End.

Genotypic frequencies were analysed for two years in a permanent population of the cladoceran crustacean, Daphnia magna, which was polymorphic for an esterase and for malate dehydrogenase. Large temporal changes in genotypic frequencies occurred at both loci. There was no evidence of a seasonal pattern in the frequency changes. In most samples, genotypes at the two enzyme loci were non-randomly associated; these associations showed temporal changes. On some occasions marked spatial heterogeneity in genotypic frequencies existed within the population. Genotypic differences in parthenogenetic and sexual egg production were observed. In a primarily parthenogenetically reproducing population, non-random associations between genotypes of structural and regulatory loci will be the rule. The allozyme variants themselves may or may not be under selection. The relevance of these observations to ecological studies on Daphnia is considered.     

The effect of diapause on the genetic structure of Daphnia magna populations

For intermittent Daphnia magna populations embryonic diapause following bisexual reproduction is the means of surviving in harsh environmental conditions. Another advantage of bisexuality is genetic recombination which produces a variety of genotypes. Crowding and short-day photo-periods promote bisexual reproduction, but photoperiod, at least, can not be an obligatory factor in D. magna since some bisexual females have been found also during the periods of long daylength. Because the catastrophe dates vary considerably, the most advantageous reproductive strategy must be one with some flexibility in reacting to irregular catastrophes by producing ephippial resting eggs whenever the probability of a catastrophe exceeds a certain threshold value. Extensive genetic changes occurring during diapause suggest that there are considerable genetic differences in ephippial egg production and egg survival. Genetic drift alone can not account for the great genetic changes observed, although in populations reacting slowly to changing conditions the number of diapausing ephippia can be relatively low. If the ephippia hatch over a long period of time, they can act as a stabilizing mechanism for genotype frequencies.     

Daphnia magna microRNAs respond to nutritional stress and ageing but are not transgenerational

Maternal effects, where the performance of offspring is determined by the condition of their mother, are widespread and may in some cases be adaptive. The crustacean Daphnia magna shows strong maternal effects: offspring size at birth and other proxies for fitness are altered when their mothers are older or when mothers have experienced dietary restriction. The mechanisms for this transgenerational transmission of maternal experience are unknown, but could include changes in epigenetic patterning. MicroRNAs (miRNAs) are regulators of gene expression that have been shown to play roles in intergenerational information transfer, and here, we test whether miRNAs are involved in D. magna maternal effects. We found that miRNAs were differentially expressed in mothers of different ages or nutritional state. We then examined miRNA expression in their eggs, their adult daughters and great granddaughters, which did not experience any treatments. The maternal (treatment) generation exhibited differential expression of miRNAs, as did their eggs, but this was reduced in adult daughters and lost by great granddaughters. Thus, miRNAs are a component of maternal provisioning, but do not appear to be the cause of transgenerational responses under these experimental conditions. MicroRNAs may act in tandem with egg provisioning (e.g., with carbohydrates or fats), and possibly other small RNAs or epigenetic modifications.     

Population dynamics of Daphnia magna as modified by chronic bromide stress

Two chronic toxicity experiments were conducted with Daphnia magna. In a semi-static experiment with cohorts, the no effect level for bromide in respect of the intrinsic rate of natural increase (derived from age-specific survival and fecundity) was 10 mg l^–1. A second test was started with small populations in an intermittent-flow system. These populations had a stable age distribution, were composed of cohorts of different ages, and showed an almost perfect logistic growth. Model calculations showed that bromide reduced the upper numerical limit (carrying capacity). It also increased the time-lag required to attain the maximum reproduction rate. For the first parameter, a no effect level of 14 mg l^–1 was calculated. For the latter a threshold could not be detected. The EC₅₀ and EC₁₀ were 27 and 18 mg l^–1, respectively. Additional experiments showed that individual growth of D. magna in time could also be described by a logistic equation. The age structure of the populations changed when food became limiting. This was parallelled by a reduction of the mean brood size. In conclusion it is stated that the results of the toxicity studies with populations support Halbach's view (1984), that population dynamics can be used like a magnifying glass to detect small sublethal ecotoxicological effects of environmental pollutants.     

Haemoglobin synthesis in Daphnia magna Straus (Crustacea: Cladocera): ecological differentiation between neighbouring populations

SUMMARY. 1. Variations in the haemoglobin index of two neighbouring populations of Daphnia magna were recorded over a range of dissolved oxygen concentrations (0.5–4.0ml O₂ 1⁻¹, 20°C). Reciprocal transfer experiments between habitats compared haemoglobin synthesis in situ.
2. An inverse relationship was found between the oxygen content of pond water and the haemoglobin indices of laboratory and natural populations.
3. Significant genetic differences in the synthesis of haemoglobin were found between the two populations. Animals from the poorly oxygenated habitat (0.8±0.18ml O₂ 1⁻¹) had consistently higher haemoglobin contents (maximum HI, 87.7±4.5) at all experimental and in situ oxygen levels. D. magna from the well oxygenated pond (4.3±0.59 ml O₂1⁻¹) had a lowered physiological ability to synthesize haemoglobin (maximum HI, 48.3±4.2). The process of ecological differentiation in Daphnia populations is discussed.