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.     

The components of life-history traits variation in Daphnia magna population

The variation of six growth and reproduction traits in the parthenogenetic clones of Daphnia magna and its relationships with the genotypes at 5 isozyme loci were studied under controlled conditions at three food concentrations. A significant genetic component of age at maturity, growth rate and fecundity variation was detected. Nested ANOVA revealed the influences of the genotypes of the isozyme loci, that are more pronounced at the high food concentration. In some cases the genotype-environment interactions were found.The variation of body length at 6 days and total number of offspring includes a significant heterozygosity — environment interaction. The most heterozygous clones show the least differences in life history traits between food conditions.Life-history traits heritability, genotype-environment interactions and different reproductive strategies of the clones are discussed.     

Life history responses of <Emphasis Type="Italic">Daphnia longispina</Emphasis> to mosquitofish (<Emphasis Type="Italic">Gambusia holbrooki</Emphasis>) and pumpkinseed (<Emphasis Type="Italic">Lepomis gibbosus</Emphasis>) kairomones

In the present study, the effect of chemical cues from two fish species (mosquitofish and pumpkinseed), at different concentrations, was tested in life history experiments with Daphnia longispina. The two fish species used represent the most abundant planktivores of many Mediterranean shallow lakes (SW Europe), where the indigenous fish communities have been replaced by such exotic assemblages. Results have shown a similar response of D. longispina to both fish species: kairomones stimulated daphnids to produce more offspring, which resulted in higher fitness (r), relatively to a fishless control. Fish presence also induced an earlier first reproduction, a smaller size at maturity of daphnids, and the production of smaller-sized neonates. Significant correlations with fish concentration (indirect measure of fish kairomone concentration) were found for size at maturity and neonate size, for both fish species. These results are in accordance to the “positive response” observed by other authors, which represents a defence mechanism to face losses caused by fish predators. The chemically mediated size reduction of mature females and neonates is an adaptive response to the size-selective predation exerted by fish. Pumpkinseed introduction is very recent in the lake of origin of the daphnids used in the experiments and its kairomone produced similar effects to mosquitofish in the life history of D. longispina. These results are contrary to the existence of a species-specific kairomone and support the hypothesis of a general fish kairomone. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Egg size and composition in an ageing capital breeder – consequences for offspring performance

1. Egg size is often used as a proxy of egg quality although size and composition may vary, e.g. in insects egg size usually decreases as female ages. Whether this decrease in size reflects reduced concentrations of essential nutrients such as lipids and proteins of eggs laid by ageing females, or does reduced size per se explain often observed lower fitness of later laid eggs is poorly explored. 2. Egg properties were compared with fitness parameters of offspring laid on the first and fourth night during the oviposition period of a capital breeding moth, Cleorodes lichenaria (Hufnagel). The study aim was to explore whether decreased egg size is caused by decreased provisioning into later laid eggs measured as egg protein and lipid concentration, and whether it results in lower fitness of later laid offspring. 3. The fresh and dry weight of eggs decreased over the oviposition period, but the protein and lipid concentration remained constant. Survival of larvae was lower among the fourth night laid offspring on a low quality host Parmelia sulcata Taylor compared to a high quality host Ramalina fraxinea (L.) Ach. No differences were observed in egg fertility or hatchability, neonate survival without food and pupal mass between the offspring produced on different nights. 4. Decreased survival of offspring produced later was rather attributable to absolute provisioning (i.e. lower weight of eggs) than relative provisioning (i.e. decreased concentrations of nutrients in eggs). It is argued that lower survival of later laid smaller eggs on low quality diet is likely attributable to physical and chemical characteristics of host lichens and/or physical properties of tiny neonate larvae.     

Endogenous regulation of environmentally induced sexuality in a rotifer: a multigenerational parental effect induced by fertilisation

SUMMARY 1. Sexual reproduction in the heterogonic life cycle of many rotifers occurs when amictic females, which produce diploid eggs developing parthenogenetically into females, are environmentally induced to produce mictic females. Mictic females produce haploid eggs which develop parthenogenetically into males or, if fertilised, into resting eggs – encysted embryos which develop into amictic females after an obligatory diapause. 2. A Florida strain of Brachionus calyciflorus was used to test the prediction that amictic females hatching from resting eggs (Generation 1), and those from the next few parthenogenetic generations, have a lower propensity to produce mictic daughters in response to crowding than those from later parthenogenetic generations. In 10 replicate clones, populations initiated by amictic females from generations 1, 5, 8, 12 and 18 were exposed to a standardised crowding stimulus, and the proportion of mictic females in the populations was determined. These proportions varied significantly across generations and clones. They were very low in the early generations and gradually increased to a mean of about 0.5 at Generation 12. 3. The mechanism for the transgenerational plasticity in response to crowding is not known. One possibility is that resting eggs contain an agent from their fertilised mictic mother's yolk gland that prevents development into mictic females and is transmitted in increasingly low concentrations through successive parthenogenetic generations of amictic females. 4. This parental effect may contribute to clonal fitness by ensuring that a clone developing from a resting egg will attain a higher population size through female parthenogenesis before maximising its commitment to sexual reproduction, even in the presence of a crowding stimulus from a high population density of other clones. Therefore, the number of resting eggs to which a clone contributes its genes should be maximised. 5. The clonal variation in propensity to produce mictic females in this strain indicates genetic variation in the trade‐off between maximising population growth via female parthenogenesis and increasing the probability of producing at least some resting eggs before local extinction from the plankton.     

Environmental correlates of the intrinsic rate of natural increase in primates

Morphological and life history traits of two clones of the cladoceran Daphnia pulex were measured in the presence and absence of size-selective insect predators, the midge larva Chaoborus flavicans, which preys on small Daphnia, and the water bug Notonecta glauca, which preys on large Daphnia. The aim was to detect predator-induced phenotypic changes, particularly the effect of simultaneous exposure to both types of predators. Other work has shown that in the presence of Chaoborus americanus, Daphnia pulex produce a socalled neck spine which may carry several teeth. The morphological modifications are supposed to serve as an anti-predator device. Furthermore, females exposed to Chaoborus often delay their maturation; this has been interpreted as a cost that balances the benefits of the neck teeth. In this investigation, females of both clones produced fewer but larger offspring than control animals when reared in the presence of Chaoborus flavicans. The offspring showed the typical neck spine and delayed first reproduction. In the presence of Notonecta glauca, one of the clones produced more and smaller offspring, and maturation occurred at earlier instars. The other clone also produced more offspring than the control but there was no size difference. When both predators were present, in most cases the reactions of the daphnids were similar to those in the Notonecta experiment. The response to Chaoborus appeared to be suppressed. The observed modifications are interpreted as evolved strategies that reduce the impact of size-selective predation. They are consistent with predictions of life-history theory.     

The influence of <Emphasis Type="Italic">Myriophyllum verticillatum</Emphasis> and artificial plants on some life history parameters of <Emphasis Type="Italic">Daphnia magna</Emphasis>

Submerged macrophytes are known to serve as refuge for zooplankton but also seem to suppress the zooplankton growth. Thus, there is a conflict between the positive and negative role of macrophytes for zooplankton. We tested the influence of physical structure using artificial macrophytes, chemicals released by a macrophyte (Myriophyllum verticillatum), as well as the combined effects of these two factors on the life history of Daphnia magna. Daphnids matured at a smaller size and produced fewer eggs but larger individual offspring in the presence of artificial and real plants. Thus, under conditions with plants, we observed a trade off between number of eggs produced and the individual size of the offspring. Daphnids grown in the presence of exudates without plants were larger at maturity and showed no reduction in clutch size as compared with the control. We suggest that the macrophytes (real and artificial) negatively affected the daphnids in two ways: (1) food particles were settling down faster on the plant structures and this reduced the available food for Daphnia, (2) the plant structures were obstacles for the daphnids causing them to spent more energy during swimming. Both effects resulted in a reduced somatic growth, whereas the increased individual offspring size was probably a response to the reduced food level. Daphnia was not negatively affected by Myriophyllum exudates.     

Life history characteristics of Daphnia magna clones differing in phototactic behaviour

In an analysis of a life table experiment involving positively, intermediately and negatively phototactic Daphnia magna clones, life history traits such as the average duration of the adult instar, neonate and adult body size were found to be correlated with phototactic behaviour. The size of the eggs and neonates was positively correlated with adult body size, and with egg development time. Adult body size was positively correlated with the size of the second and subsequent clutches. I argue that the intrinsic positive correlation between offspring size and egg development time is a key factor structuring the differences in life history patterns observed between the positively and intermediately phototactic Daphnia genotypes, and that the two life history patterns are to be considered alternatives suited for different environmental conditions (e.g. habitats with and without strong predation pressure on adults).     

Egg size plasticity corresponding to maternal food conditions in an annual fish,Ayu <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>

The classic model of Smith and Fretwell predicts that the optimal egg size will vary according to the shape of the relationship between offspring size and offspring fitness, which may vary among environments. Adaptive significance of intrapopulation egg size variation was examined using Ayu (Plecoglossus altivelis). The species has an annual and migratory life history. Fish under controlled rearing conditions become sexually mature with a trend that smaller females produced larger eggs later in the season. Observed egg size variation was explained by the maternal specific growth rate, which was composed of maternal body size and growing period. Hatchlings from larger eggs had a larger notochord length, larger yolk-sac and grew faster. Such offspring traits provide general advantages of increased larval size, which confer competitive ability for assuring early survivorship. In conclusion, egg size plasticity in Ayu suggests higher offspring fitness through enhancement of their accessibility to food.     

Genetic variation for maternal effects on parasite susceptibility

The expression of infectious disease is increasingly recognized to be impacted by maternal effects, where the environmental conditions experienced by mothers alter resistance to infection in offspring, independent of heritability. Here, we studied how maternal effects (high or low food availability to mothers) mediated the resistance of the crustacean Daphnia magna to its bacterial parasite Pasteuria ramosa. We sought to disentangle maternal effects from the effects of host genetic background by studying how maternal effects varied across 24 host genotypes sampled from a natural population. Under low‐food conditions, females produced offspring that were relatively resistant, but this maternal effect varied strikingly between host genotypes, i.e. there were genotype by maternal environment interactions. As infection with P. ramosa causes a substantial reduction in host fecundity, this maternal effect had a large effect on host fitness. Maternal effects were also shown to impact parasite fitness, both because they prevented the establishment of the parasites and because even when parasites did establish in the offspring of poorly fed mothers, and they tended to grow more slowly. These effects indicate that food stress in the maternal generation can greatly influence parasite susceptibility and thus perhaps the evolution and coevolution of host–parasite interactions.