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Eutrophication due to nutrient addition can result in major alterations in aquatic ecosystem productivity. Foundation species, individually and interactively, whether present as invasive species or as instruments of ecosystem management and restoration, can have unwanted effects like stabilizing turbid eutrophic states. In this study, we used whole-pond experimental manipulations to investigate the impacts of disturbance by nutrient additions in the presence and absence of two foundation species: Dreissena polymorpha (a freshwater mussel) and Myriophyllum spicatum (a macrophyte). We tracked how nutrient additions to ponds changed the prokaryotic and eukaryotic communities, using 16S, 18S, and COI amplicon sequencing. The nutrient disturbance and foundation species imposed strong selection on the prokaryotic communities, but not on the microbial eukaryotic communities. The prokaryotic communities changed increasingly over time as the nutrient disturbance intensified. Postdisturbance, the foundation species stabilized the prokaryotic communities as observed by the reduced rate of change in community composition. Our analysis suggests that prokaryotic community change contributed both directly and indirectly to major changes in ecosystem properties, including pH and dissolved oxygen. Our work shows that nutrient disturbance and foundation species strongly affect the prokaryotic community composition and stability, and that the presence of foundation species can, in some cases, promote the emergence and persistence of a turbid eutrophic ecosystem state. [ABSTRACT FROM AUTHOR]

Human activities have facilitated the invasion of freshwater ecosystems by various organisms. Especially, invasive bivalves such as the quagga mussels, Dreissena bugensis, have the potential to alter ecosystem function as they heavily affect the food web. Quagga mussels occur in high abundance, have a high filtration rate, quickly spread within and between waterbodies via pelagic larvae, and colonize various substrates. They have invaded various waterbodies across the Northern Hemisphere. In Central Europe, they have invaded multiple large and deep perialpine lakes with first recordings in Lake Geneva in 2015 and 2016 in Lake Constance. In the deep perialpine lakes, quagga mussels quickly colonized the littoral zone but are also abundant deeper (>80 m), where they are often thinner and brighter shelled. We analysed 675 quagga mussels using ddRAD sequencing to gain in‐depth insights into the genetic population structure of quagga mussels across Central European lakes and across various sites and depth habitats in Lake Constance. We revealed substantial genetic differentiation amongst quagga mussel populations from three unconnected lakes, and all populations showed high genetic diversity and effective population size. In Lake Constance, we detected no genetic differentiation amongst quagga mussels sampled across different sites and depth habitats. We also did not identify any convincing candidate loci evidential for adaptation along a depth gradient and a transplant experiment showed no indications of local adaptation to living in the deep based on investigating growth and survival. Hence, the shallow‐water and the deep‐water morphotypes seem to be a result of phenotypic plasticity rather than local adaptation to depth. In conclusion, our ddRAD approach revealed insight into the establishment of genetically distinct quagga mussel populations in three perialpine lakes and suggests that phenotypic plasticity and life history traits (broadcast spawner with high fecundity and dispersing pelagic larvae) facilitate the fast spread and colonization of various depth habitats by the quagga mussel. [ABSTRACT FROM AUTHOR]

Invasive species have far-reaching consequences for whole ecosystems. The two dreissenids zebra and quagga mussels are two of the most successful invaders in aquatic systems. Quagga mussels can colonize deeper parts of a lake while zebra mussels prefer shallow areas. Both species show large variability in shell morphology, but only quagga mussels show a shallow and deep-water morphotype. The aim of this study is to investigate the depth adaptation of quagga mussels and the potential filtration advantage of the quagga mussel compared to the zebra mussel. We conducted common garden experiments, where we measured filtration rates at different temperatures of quagga mussels collected from different water depths and compared them to zebra mussel from 1m depth. We found filtration rates to be strongly temperature dependent, and in particular, lower at 4 °C. Overall, zebra mussels fed less than quaggas. Quagga mussels collected from 1 m depth fed more at higher temperature than quaggas from 30 and 60 m. We hypothesize that deep-water quagga mussels are less sensitive to lower temperature and/or constrained to modify filtration rates at higher temperatures. [ABSTRACT FROM AUTHOR]

Hybridization following secondary contact of genetically divergent populations can influence the range expansion of invasive species, though specific outcomes depend on the environmental dependence of hybrid fitness. Here, using two genetically and ecologically divergent threespine stickleback lineages that differ in their history of freshwater colonization, we estimate fitness variation of parental lineages and hybrids in semi‐natural freshwater ponds with contrasting histories of nutrient loading. In our experiment, we found that fish from the older freshwater lineage (Lake Geneva) and hybrids outperformed fish from the younger freshwater lineage (Lake Constance) in terms of both growth and survival, regardless of the environmental context of our ponds. Across all ponds, hybrids exhibited the highest survival. Although wild‐caught adult populations differed in their functional and defence morphology, it is unclear which of these traits underlie the fitness differences observed among juveniles in our experiment. Overall, our work suggests that when hybrid fitness is insensitive to environmental conditions, as observed here, introgression may promote population expansion into unoccupied habitats and accelerate invasion success. [ABSTRACT FROM AUTHOR]

Host genotype may shape host-associated bacterial communities (commonly referred to as microbiomes). We sought to determine (a) whether bacterial communities vary among host genotypes in the water flea Daphnia galeata and (b) if this difference is driven by the genetic distance between host genotypes, by using D. galeata genotypes hatched from sediments of different time periods. We used 16S amplicon sequencing to profile the gut and body bacterial communities of eight D. galeata genotypes hatched from resting eggs; these were isolated from two distinct sediment layers (dating to 1989 and 2009) of a single sediment core of the lake Greifensee, and maintained in a common garden in laboratory cultures for 5 years. In general, bacterial community composition varied in both the Daphnia guts and bodies; but not between genotypes from different sediment layers. Specifically, genetic distances between host genotypes did not correlate with beta diversity of bacterial communities in Daphnia guts and bodies. Our results indicate that Daphnia bacterial community structure is to some extent determined by a host genetic component, but that genetic distances between hosts do not correlate with diverging bacterial communities. [ABSTRACT FROM AUTHOR]

Aim: Biological invasions are a major threat to biodiversity in aquatic and terrestrial habitats. Insects represent an important group of species in freshwater and terrestrial habitats, and they constitute a large proportion of non‐native species. However, while many non‐native insects are known from terrestrial ecosystems, they appear to be less represented in freshwater habitats. Comparisons between freshwater and terrestrial habitats of invader richness relative to native species richness are scarce, which hinders syntheses of invasion processes. Here, we used data from three regions on different continents to determine whether non‐native insects are indeed under‐represented in freshwater compared with terrestrial assemblages. Location: Europe, North America, New Zealand. Methods: We compiled a comprehensive inventory of native and non‐native insect species established in freshwater and terrestrial habitats of the three study regions. We then contrasted the richness of non‐native and native species among freshwater and terrestrial insects for all insect orders in each region. Using binomial regression, we analysed the proportions of non‐native species in freshwater and terrestrial habitats. Marine insect species were excluded from our analysis, and insects in low‐salinity brackish water were considered as freshwater insects. Results: In most insect orders living in freshwater, non‐native species were under‐represented, while they were over‐represented in a number of terrestrial orders. This pattern occurred in purely aquatic orders and in orders with both freshwater and terrestrial species. Overall, the proportion of non‐native species was significantly lower in freshwater than in terrestrial species. Main conclusions: Despite the numerical and ecological importance of insects among all non‐native species, non‐native insect species are surprisingly rare in freshwater habitats. This is consistent across the three investigated regions. We review hypotheses concerning species traits and invasion pathways that are most likely to explain these patterns. Our findings contribute to a growing appreciation of drivers and impacts of biological invasions. [ABSTRACT FROM AUTHOR]

Host-associated bacterial communities play an important role in host fitness and resistance to diseases. Yet, few studies have investigated tripartite interaction between a host, parasite and host-associated bacterial communities in natural settings. Here, we use 16S rRNA gene amplicon sequencing to compare gut- and body- bacterial communities of wild water fleas belonging to the Daphnia longispina complex, between uninfected hosts and those infected with the common and virulent eukaryotic gut parasite Caullerya mesnili (Family: Ichthyosporea). We report community-level changes in host-associated bacteria with the presence of the parasite infection; namely decreased alpha diversity and increased beta diversity at the site of infection, i.e. host gut (but not host body). We also report decreased abundance of bacterial taxa proposed elsewhere to be beneficial for the host, and an appearance of taxa specifically associated with infected hosts. Our study highlights the host-microbiota-infection link in a natural system and raises questions about the role of host-associated microbiota in natural disease epidemics as well as the functional roles of bacteria specifically associated with infected hosts. [ABSTRACT FROM AUTHOR]

The dreissenids, quagga mussel Dreissena bugensis and zebra mussel D. polymorpha, are invasive freshwater mussels in Europe and North America. These species strongly impact aquatic ecosystems, such as the food web through their high abundance and filtration rate. They spread quickly within and between waterbodies, and have the ability to colonize various substrates and depths. The zebra mussel invaded and established in Swiss lakes in the 1960s, whereas the quagga mussel was not detected until 2014. We collected all available data from cantonal as well as local authorities and other institutions to describe the colonization pattern of quagga mussels in perialpine lakes north of the Alps. We also collected data regarding the distribution of larval stages of the mussels, the so-called veliger larvae. We observed that in lakes colonized by the quagga mussel, veligers are present the whole year round, whereas they are absent in winter in lakes with only zebra mussels. Additionally, we present detailed information about the invasion and colonization pattern of quagga mussels in Lake Constance. Quagga mussels colonized the lakeshore within a few years (~ 2016-2018), outcompeted zebra mussels, and have reached densities > 5000 ind. m-2 in the littoral zone, even at 80 m densities above 1000 ind. m-2 were found at some locations. At the end of the article, we discussed possibilities on how the spread of quagga mussels within and among northern perialpine lakes should be monitored and prevented in the future. [ABSTRACT FROM AUTHOR]

B I Max Planck Institute for Limnology (MPIL), Department of Ecophysiology i b B (scientifically instructed and officially supervised by W. Lampert) b B Brendelberger, Heinz (1988) b Emeritus Professor of Limnology and Zoology, University Kiel (Germany) B Rothhaupt, Karl Otto (1988) b Emeritus Professor of Limnology, University Konstanz (Germany). The passing of Winfried Lampert on the 6th of March 2021 has reverberated in the aquatic science community world-wide. In all, more than 150 scientists from at least 26 countries worked in the Ecophysiology Department of MPIL during Winfried's tenure, including diploma students, PhD students, postdocs, staff research scientists, and visitors. 2 Box PhD students (year of PhD) of W. Lampert and the senior scientists of his MPIL Ecophysi... B I Limnological Institute Lake Constance (Albert-Ludwigs-Universität Freiburg) i b B (scientifically instructed by W. Lampert, officially supervised by department head, H. J. Elster) b B Muck, Peter (1980) b B Pott, Eckhart (1980) b Author of various nature travel guides and ornithological illustrated books. [Extracted from the article]

Eutrophication is a persistent threat to aquatic ecosystems worldwide. Foundation species, namely those that play a central role in the structuring of communities and functioning of ecosystems, are likely important for the resilience of aquatic ecosystems in the face of disturbance. However, little is known about how interactions among such species influence ecosystem responses to nutrient perturbation. Here, using an array (N = 20) of outdoor experimental pond ecosystems (15,000 L), we manipulated the presence of two foundation species, the macrophyte Myriophyllumspicatum and the mussel Dreissena polymorpha, and quantified ecosystem responses to multiple nutrient disturbances, spread over two years. In the first year, we added five nutrient pulses, ramping up from 10 to 50 μg P/L over a 10‐week period from mid‐July to mid‐October, and in the second year, we added a single large pulse of 50 μg P/L in mid‐October. We used automated sondes to measure multiple ecosystems properties at high frequency (15‐minute intervals), including phytoplankton and dissolved organic matter fluorescence, and to model whole‐ecosystem metabolism. Overall, both foundation species strongly affected the ecosystem responses to nutrient perturbation, and, as expected, initially suppressed the increase in phytoplankton abundance following nutrient additions. However, when both species were present, phytoplankton biomass increased substantially relative to other treatment combinations: non‐additivity was evident for multiple ecosystem metrics following the nutrient perturbations in both years but was diminished in the intervening months between our perturbations. Overall, these results demonstrate how interactions between foundation species can cause surprisingly strong deviations from the expected responses of aquatic ecosystems to perturbations such as nutrient additions. [ABSTRACT FROM AUTHOR]

Danube Delta shallow lakes experience cyanobacteria blooms that can negatively affect the aquatic ecosystem. Although there are several studies on Danube Delta cyanobacteria, little is known about their spatial–temporal patterns and the potential predictive role they can offer. We therefore analyzed the distribution of cyanobacteria in 19 lakes belonging to three lake complexes and tested whether their seasonal dynamics are in line with the predictions of the PEG model. Furthermore, we investigated to which extent cyanobacteria diversity and abundance were related to lake hydrogeomorphological characteristics such as surface, water level, connectivity, water retention, flood risk, and transparency. Although lakes had different seasonal cyanobacterial assemblages, the biovolume and genus richness had a geographical pattern, decreasing from south-east (lakes forming the fluvial delta) towards north-west (lakes forming the maritime delta). Cyanobacteria biovolume reflected largely the PEG model peaking in summer (the fluvial delta) and autumn (the maritime delta). Genus richness followed the same pattern. Cyanobacteria distribution was predicted by various abiotic (e.g., risk of flooding, connectivity) and biotic factors (e.g., submersed macrophytes, phytoplankton diversity, peat deposits). Our study contributes to the understanding of cyanobacteria diversity and distribution in shallow interconnected lakes by revealing the complexity of predictors for geographical and seasonal patterns. [ABSTRACT FROM AUTHOR]

Changing reproductive strategy from investment in current (asexual eggs) to future (sexual ephippia) reproduction depending on environmental cues is an important fitness trait in zooplankton species like water fleas, Daphnia. Different cues are reported for ephippia production, for example food limitation, changes in photoperiod and temperature, the presence of predators, nutrient depletion or signals of crowding. However, the reliability of these cues strongly depends on the local habitat. In Daphnia galeata, ephippia production might not be subject to any of the environmental cues outlined. Here, we directly test whether changes in crowding alone provides a reliable cue for D. galeata to switch reproductive mode. We conducted life history assays using 10 genotypes of D. galeata from a Swiss Lake to investigate density dependent reproduction and production of ephippia. We started populations at different densities and eliminated competition for food by systematically increasing food availability with population size. Our results suggest that population density depresses population growth rate via a change in energy allocation from current into future reproduction in D. galeata. Thus, signals of crowding produced at high population density constitute a reliable and sufficient cue for D. galeata to switch from asexual to sexual reproduction. [ABSTRACT FROM AUTHOR]

A major challenge in ecology is to understand determinants of ecosystem functioning and stability in the face of disturbance. Some important species can strongly shape community structure and ecosystem functioning, but their impacts and interactions on ecosystem-level responses to disturbance are less well known. Shallow ponds provide a model system in which to study the effects of such species because some taxa mitigate transitions between alternative ecosystem states caused by eutrophication. We performed pond experiments to test how two foundation species (a macrophyte and a mussel) affected the biomass of planktonic primary producers and its stability in response to nutrient additions. Individually, each species reduced phytoplankton biomass and tended to increase rates of recovery from disturbance, but together the species reversed these effects, particularly with larger nutrient additions. This reversal was mediated by high cyanobacterial dominance of the community and a resulting loss of trait evenness. Effects of the foundation species on primary producer biomass were associated with effects on other ecosystem properties, including turbidity and dissolved oxygen. Our work highlights the important role of foundation species and their interactive effects in determining responses of ecosystem functioning to disturbance. [ABSTRACT FROM AUTHOR]

Eutrophication of shallow lakes often triggers a series of cascading ecological effects. Among these are shifts in the zooplankton community structure due to phytoplankton changes, or shifts in the fish community reducing size-selective feeding of planktivorous fish. In such conditions, larger zooplankton (e.g. Daphnia) can have a selective advantage. Re-oligotrophication can reverse such food-web interactions. Europe's largest wetland system (the Danube Delta) went through a period of eutrophication and is now slowly recovering again. However, changes in the Daphnia populations triggered by eutrophication in this system have remained unstudied. We used different sampling strategies to screen 24 lakes (which differ in their ecological state) for the presence of Daphnia as well as for biotic and abiotic parameters potentially explaining Daphnia abundance. Daphnia densities were surprisingly low. We found D. magna ephippia in the sediment, but no D. magna in the water column. Microsatellite analyses on pelagic Daphnia populations confirmed the presence of the D. longispina complex and provided evidence for significant hybridisation events. FluoroProbe data showed that Daphnia was mainly present in lakes with a higher phytoplankton production. Our study provides insights into the spatial and temporal distribution of Daphnia in a very dynamic wetland system. [ABSTRACT FROM AUTHOR]

We compared taxon composition of the Daphnia longispina hybrid community, as reconstructed from dormant eggs (retrieved from sediment samples) and the pelagic population (retrieved from formaldehyde-preserved zooplankton samples), from the same lake and of the same time period. As microsatellite markers do not work on largely fragmented DNA, such as of formaldehyde-preserved samples, both types of samples (dormant eggs and pelagic Daphnia) were screened with single-nucleotide polymorphism (SNP) markers. Here, we designed a genotyping panel of short SNP-bearing amplicons and, to facilitate screening, we developed a multiplex genotyping protocol. The results of this comparison confirmed differences between dormant and pelagic samples. Specifically, D. galeata was overrepresented in the sedimentary egg bank in comparison to the pelagic population, indicating that this taxon is more involved in sexual reproduction than other taxa. In addition to being successfully applied on formaldehyde-preserved samples, SNP-genotyping was more efficient than microsatellites on sedimentary eggs, and was more sensitive for hybrid detection. In conclusion, the SNP-based genotyping panel presented here enables to study the genetic structure of past populations from common formaldehyde-preserved collections. It is also promising for genotyping old dormant eggs, which can extend the temporal range of Daphnia community reconstructions. [ABSTRACT FROM AUTHOR]

The phylum Cyanobacteria comprises a non-photosynthetic lineage. The diversity and distribution of non-photosynthetic cyanobacteria (NCY) across aquatic environments are currently unknown, including their ecology. Here, we report about composition and phylogenetic diversity of two clades of NCY in ten lakes of the European peri-Alpine region, over the past ∼100 years. Using 16S rDNA sequences obtained from dated sediment cores, we found almost equal proportion of taxa assigned to Melainabacteria and the deepest-branching group Sericytochromatia (ML635J-21) (63 total detected taxa). The topology of our reconstructed phylogenies reflected evolutionary relationships expected from previous work, that is, a clear separation between the deepest branching Sericytochromatia, the Melainabacteria, and the photosynthetic cyanobacteria clades. While different lakes harbored distinct NCY communities, the diversity of NCY assemblages within and between lakes (alpha and beta diversity) did not significantly change over the last century. This is in contrast with what was previously reported for photosynthetic cyanobacteria. Unchanged community phylogenetic similarity over geographic distance indicated no dispersal limitation of NCY at the regional scale. Our results solicit studies linking in-lake environmental factors to the composition of these microorganisms' communities, whose assembly appeared not to have been influenced by large-scale anthropogenic environmental changes. This is the first attempt to study the diversity and distribution of NCY taxa across temperate lakes. It provides a first step towards understanding their distribution and ecological function in pelagic aquatic habitats, where these organisms seem to be prevalent. [ABSTRACT FROM AUTHOR]

Background: Regulatory circuits of infection in the emerging experimental model system, water flea Daphnia and their microparasites, remain largely unknown. Here we provide the first molecular insights into the response of Daphnia galeata to its highly virulent and common parasite Caullerya mesnili, an ichthyosporean that infects the gut epithelium. We generated a transcriptomic dataset using RNAseq from parasite-exposed (vs. control) Daphnia, at two time points (4 and 48 h) after parasite exposure. Results: We found a down-regulation of metabolism and immunity-related genes, at 48 h (but not 4 h) after parasite exposure. These genes are involved in lipid metabolism and fatty acid biosynthesis, as well as microbe recognition (e.g. c-type lectins) and pathogen attack (e.g. gut chitin). Conclusions: General metabolic suppression implies host energy shift from reproduction to survival, which is in agreement with the known drastic reduction in Daphnia fecundity after Caullerya infection. The down-regulation of gut chitin indicates a possible interaction between the peritrophic matrix and the evading host immune system. Our study provides the first description of host transcriptional responses in this very promising host-parasite experimental system. [ABSTRACT FROM AUTHOR]

Abstract: According to the Red Queen hypothesis, clonal diversity in asexual populations could be maintained by negative frequency‐dependant selection by coevolving parasites. If common clones are selected against and rare clones gain a concomitant advantage, we expect that clonal turnover should be faster during parasite epidemics than between them. We tested this hypothesis exploring field data of the Daphnia–Caullerya host–parasite system. The clonal make‐up and turnover of the Daphnia host population was tracked with high temporal resolution from 1998 until 2013, using first allozyme and later microsatellite markers. Significant differences in the clonal composition between random and infected subsamples of Daphnia populations were detected on six of seven tested occasions, confirming genetic specificity of the host–parasite interaction in this system. We used time series analysis to compare the rates of host clonal turnover to the incidence of parasitism, and found that Caullerya prevalence was significantly associated with microsatellite‐based clonal turnover. As alternate hypotheses, we further tested whether turnover was related to a variety of biotic, abiotic, and host demographic parameters. Other significant correlates of turnover were cyanobacterial biomass and (weakly) temperature. Overall, parasitism seems to be a strong driver of host clonal turnover, in support of the Red Queen hypothesis. [ABSTRACT FROM AUTHOR]

Eutrophication generally favours the growth of cyanobacteria over eukaryotic green algae in freshwater lakes. Cyanobacteria constitute a poor food source for the waterflea Daphnia, an important primary consumer of phytoplankton in lakes. While it is known that some Daphnia species are adapted to eutrophic conditions and can cope with cyanobacteria in their diet, it is less known whether cyanobacterial community composition can influence Daphnia population structure in lakes. We studied the variation in genetic diversity of Daphnia resting eggs and cyanobacterial DNA preserved in sediment cores from three European lakes impacted by eutrophication. Our retrospective analysis confirms that D. galeata invaded the two pre-alpine lakes around the middle of the twentieth century, hybridized with and became dominant over D. longispina. This coincides with the presence in all lakes and the increase in the proportion of colonial and filamentous cyanobacteria in the pre-alpine lakes. The recent re-oligotrophication of the lakes did not reverse the cyanobacterial and Daphnia assemblages to their pre-eutrophication composition and genetic structure, suggesting that both changed irreversibly due to anthropogenic influence on the ecosystems. Genetic analyses applied to lake sedimentary archives have the potential to unveil how different compartments of the food web covary in a changing environment. [ABSTRACT FROM AUTHOR]

Daphnia perform diel vertical migration (DVM), a predator-avoidance strategy to migrate towards deeper and colder layers in the water column in the morning and movement to the algae-rich surface layers in the evening. However, individuals performing DVM incur several trade-offs since they might suffer from resource limitation and a slower instantaneous birth rate in deeper depths. DVM patterns may be modified by abiotic factors such as temperature, food concentration, or pH and vary among different Daphnia species and genotypes. Furthermore, Daphnia host a variety of microparasites that might pose an additional factor influencing DVM behaviour. For infected individuals, migration into cooler temperature layers might slow down parasite growth. Moreover, parasites can increase opacity of their hosts. Non-migrating individuals might then be selectively purged from the upper layers by visually hunting predators. With these premises we asked, whether epidemics of the ichthyosporean parasite Caullerya mesnili affect or are affected by the DVM behaviour of Daphnia in Lake Greifensee, Switzerland by analysing the vertical distribution of Daphnia during day and night on two dates. Furthermore, we were interested whether a potential interaction depends on host genotype. We therefore studied the genotypic composition of the integrated population in regular sampling intervals over the course of one year and on a fine-grained vertical resolution during the Caullerya epidemic in late summer. Since Caullerya-infected Daphnia migrated equally well as uninfected ones, the findings of this study suggest that Caullerya epidemics neither affected nor were affected by the DVM behaviour of Daphnia. We observed clonal succession in the lake but could not link this succession to the Caullerya epidemic; all except one of the common multilocus genotypes were under-infected. In addition, outbreak and course of this Caullerya epidemic seemed to rely mainly on environmental cues. Because this first study only provides a snapshot of time, we hope that further studies will be done to verify our results. [ABSTRACT FROM AUTHOR]

We investigated the evolutionary response of an ecologically important freshwater crustacean, Daphnia, to a rapidly changing toxin environment. From the 1920s until the 1960s, the use of leaded gasoline caused the aquatic concentration of Pb to increase at least fivefold, presumably exerting rapid selective pressure on organisms for resistance. We predicted that Daphnia from this time of intense pollution would display greater resistance than those hatched from times of lower pollution. This question was addressed directly using the resurrection ecology approach, whereby dormant propagules from focal time periods were hatched and compared. We hatched several Daphnia genotypes from each of two Swiss lakes, during times of higher (1960s /1980s) and lower (2000s) lead stress, and compared their life histories under different laboratory levels of this stressor. Modern Daphnia had significantly reduced fitness, measured as the population growth rate (λ), when exposed to lead, whereas those genotypes hatched from times of high lead pollution did not display this reduction. These phenotypic differences contrast with only slight differences measured at neutral loci. We infer that Daphnia in these lakes were able to rapidly adapt to increasing lead concentrations, and just as rapidly lost this adaptation when the stressor was removed. [ABSTRACT FROM AUTHOR]

Biological invasions are a global issue with far-reaching consequences for single species, communities and whole ecosystems. Our understanding of modes and mechanisms of biological invasions requires knowledge of the genetic processes associated with successful invasions. In many instances, this information is particularly difficult to obtain as the initial phases of the invasion process often pass unnoticed and we rely on inferences from contemporary population genetic data. Here, we combined historic information with the genetic analysis of resting eggs to reconstruct the invasion of Daphnia pulicaria into Lower Lake Constance ( LLC) in the 1970s from the resting egg bank in the sediments. We identified the invader as 'European D. pulicaria' originating from meso- and eutrophic lowland lakes and ponds in Central Europe. The founding population was characterized by extremely low genetic variation in the resting egg bank that increased considerably over time. Furthermore, strong evidence for selfing and/or biparental inbreeding was found during the initial phase of the invasion, followed by a drop of selfing rate to low levels in subsequent decades. Moreover, the increase in genetic variation was most pronounced during early stages of the invasion, suggesting additional introductions during this period. Our study highlights that genetic data covering the entire invasion process from its beginning can be crucial to accurately reconstruct the invasion history of a species. We show that propagule banks can preserve such information enabling the study of population genetic dynamics and sources of genetic variation in successful invasive populations. [ABSTRACT FROM AUTHOR]

Forecasts of ecological dynamics in changing environments are increasingly important, and are available for a plethora of variables, such as species abundance and distribution, community structure and ecosystem processes. There is, however, a general absence of knowledge about how far into the future, or other dimensions (space, temperature, phylogenetic distance), useful ecological forecasts can be made, and about how features of ecological systems relate to these distances. The ecological forecast horizon is the dimensional distance for which useful forecasts can be made. Five case studies illustrate the influence of various sources of uncertainty (e.g. parameter uncertainty, environmental variation, demographic stochasticity and evolution), level of ecological organisation (e.g. population or community), and organismal properties (e.g. body size or number of trophic links) on temporal, spatial and phylogenetic forecast horizons. Insights from these case studies demonstrate that the ecological forecast horizon is a flexible and powerful tool for researching and communicating ecological predictability. It also has potential for motivating and guiding agenda setting for ecological forecasting research and development. [ABSTRACT FROM AUTHOR]

In order to trace community dynamics and reticulate evolution in hybrid species complexes, long-term comparative studies of natural populations are necessary. Such studies require the development of tools for fine-scale genetic analyses. In the present study, we developed species-diagnostic SNP-based markers for hybridizing freshwater crustaceans: the multispecies Daphnia longispina complex. Specifically, we took advantage of transcriptome data from a key species of this hybrid complex, the annotated genome of a related Daphnia species and well-defined reference genotypes from three parental species. Altogether eleven nuclear loci with several species-specific SNP sites were identified in sequence alignments of these reference genotypes from three parental species and their interspecific hybrids. A PCR-RFLP assay was developed for cost-efficient large population screening by SNP-based genotyping. Taxon assignment by RFLP patterns was nearly perfectly concordant with microsatellite genotyping across several screened populations from Europe. Finally, we were able to amplify two short regions of these loci in formaldehyde-preserved samples dating back to the year 1960. The species-specific SNP-based markers developed here provide valuable tools to study hybridization over time, including the long-term impact of various environmental factors on hybridization and biodiversity changes. SNP-based genotyping will finally allow eco-evolutionary dynamics to be revealed at different time scales. [ABSTRACT FROM AUTHOR]

Despite the relevance of resting eggs for ecology and evolution of many aquatic organisms and their exposure to contaminants accumulating in sediments, ecotoxicological studies using resting eggs are vastly underrepresented. The authors established a method to perform exposure assays with resting eggs produced by the Daphnia longispina species complex, key species in large lake ecosystems. A mixture of organic contaminants previously detected in sediments of Lake Greifensee was selected to test the potential effect of organic contaminants present in sediments on the hatching process. Resting eggs were exposed to a mix of 10 chemicals, which included corrosion inhibitors, biocides, pesticides, and personal care products, for a period of 15 d. Using an automated counting software, the authors found a significant increase in hatching success in the exposed resting eggs compared with controls. Such an effect has not yet been reported from ecotoxicological assays with resting eggs. Possible mechanistic explanations as well as the potential implications on the ecology and evolution of aquatic species that rely on a resting egg banks are discussed. Observed increased mortality and developmental abnormalities for hatchlings in the exposure treatments can be explained by toxic contaminant concentrations. The results of the present study highlight the need for additional studies assessing the effects of organic contaminants on resting egg banks and aquatic ecosystems. Environ Toxicol Chem 2014;9999:1-8. © 2014 SETAC [ABSTRACT FROM AUTHOR]

Parasite environments are heterogeneous at different levels. The first level of variability is the host itself. The second level represents the external environment for the hosts, to which parasites may be exposed during part of their life cycle. Both levels are expected to affect parasite fitness traits. We disentangle the main and interaction effects of variation in the immediate host environment, here the diatom Asterionella formosa (variables host cell volume and host condition through herbicide pre-exposure) and variation in the external environment (variables host density and acute herbicide exposure) on three fitness traits (infection success, development time and reproductive output) of a chytrid parasite. Herbicide exposure only decreased infection success in a low host density environment. This result reinforces the hypothesis that chytrid zoospores use photosynthesis-dependent chemical cues to locate its host. At high host densities, chemotaxis becomes less relevant due to increasing chance contact rates between host and parasite, thereby following the mass-action principle in epidemiology. Theoretical support for this finding is provided by an agent-based simulation model. The immediate host environment (cell volume) substantially affected parasite reproductive output and also interacted with the external herbicide exposed environment. On the contrary, changes in the immediate host environment through herbicide pre-exposure did not increase infection success, though it had subtle effects on zoospore development time and reproductive output. This study shows that both immediate host and external environment as well as their interaction have significant effects on parasite fitness. Disentangling these effects improves our understanding of the processes underlying parasite spread and disease dynamics. [ABSTRACT FROM AUTHOR]

Host density can increase infection rates and reduce host fitness as increasing population density enhances the risk of becoming infected either through increased encounter rate or because host condition may decline. Conceivably, potential hosts could take high host density as a cue to up-regulate their defence systems. However, as host density usually covaries with food availability, it is difficult to examine the importance of host density in isolation. Thus, we performed two full-factorial experiments that varied juvenile densities of Daphnia magna (a freshwater crustacean) and food availability independently. We also included a simulated high-density treatment, where juvenile experimental animals were kept in filtered media that previously maintained Daphnia at high-density. Upon reaching adulthood, we exposed the Daphnia to their sterilizing bacterial parasite, Pasteuria ramosa, and examined how the juvenile treatments influenced the likelihood and severity of infection (Experiment I) and host immune investment (Experiment II). Neither juvenile density nor food treatments affected the likelihood of infection; however, well-fed hosts that were well-fed as juveniles produced more offspring prior to sterilization than their less well-fed counterparts. By contrast, parasite growth was independent of host juvenile resources or host density. Parasite-exposed hosts had a greater number of circulating haemocytes than controls (i.e., there was a cellular immune response), but the magnitude of immune response was not mediated by food availability or host density. These results suggest that density dependent effects on disease arise primarily through correlated changes in food availability: low food could limit parasitism and potentially curtail epidemics by reducing both the host’s and parasite’s reproduction as both depend on the same food. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including systematics and distribution, morphology, and cytology.

The occurrence of members of the highly diverse Daphnia longispina complex in Southern and Central Asian high-mountain lakes has been recognized for more than a century. Until now, however, no molecular data have been available for these populations inhabiting the 'Roof of the World.' Here, we present the first identification for D. gr. longispina from that region based on a molecular phylogeny. Our findings show that alpine lakes in the Pamir and Himalaya mountains host populations of widespread species of the complex, for which these are the highest known localities. A spineless morph from the Himalaya region, previously labeled as D. longispina var . aspina, was clustering tightly with D. dentifera, while a population from the Pamir mountain range was grouped with D. longispina. In addition, we analyzed ecological data available for lakes in the Khumbu region (Himalaya) to investigate ecological preferences of non-pigmented D. gr. longispina. The identified factors can at least partly be related to avoidance of high UV conditions by this species. We conclude that the widespread species D. dentifera and D. longispina also colonized the Asian high-mountain lakes, and identify the need for further research to trace the possible effect of rapid environmental changes in this region on the diversity and ecology of high-altitude Daphnia populations. [ABSTRACT FROM AUTHOR]

Parasites play an important role in the regulation of host population growth. How these ubiquitous stressors interact with anthropogenic stressors is less often studied. In a full factorial experiment we explored the independent and combined effects of the widely used herbicide diuron and a chytrid parasite on the fitness of genetically different monoclonal diatom populations. Furthermore, we evaluated how herbicide exposure influenced infection dynamics, parasite fitness and the impact of infectious disease on host populations. We found no evidence of host genetic variation for diuron sensitivity and parasite resistance. Instead, host population phenotype was a decisive factor in controlling parasite growth. Although herbicide exposure initially posed a constraint on disease transmission, it enhanced the spread of disease over time. Consequently, the nature of the parasite-toxicant stressor interaction shifted from antagonistic (on exponential host growth) towards additive (on final uninfected host density). We conclude that herbicide exposure can modify infection dynamics and impact of disease on host populations through the complex interplay between host and parasite growth dynamics and host population phenotype. [ABSTRACT FROM AUTHOR]

1. Environmental stressors can influence population and community structure. However, the majority of experimental work on multiple environmental stressors has been done at the individual level only. 2. In this work, we followed changes in experimental assemblages of Daphnia (waterfleas; consisting of two taxa and three clones per taxon) after exposure to the fungal parasite Metschnikowia sp. and/or the pesticide diazinon. 3. We found a significant shift in taxonomic and clonal composition under both stressors. Strikingly, in the parasite and parasite + pesticide treatments, one taxon went extinct. While the pesticide had no effect on total parasite prevalence, the taxon that was more susceptible to extinction was, at the same time, more infected when additionally exposed to the pesticide. Furthermore, the density of all adult females was significantly reduced in the parasite treatment, but not in the pesticide treatment. 4. Our results demonstrate that the dynamics of Daphnia assemblages are altered by parasites and pesticide exposure. Given the key role of Daphnia in aquatic food webs in transferring primary production into fish food, this could be of wide significance in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Organisms are exposed to multiple biotic and abiotic environmental stressors, which can influence the dynamics of individual populations and communities. Populations may also genetically adapt to both natural (e.g. disease) and anthropogenic (e.g. chemical pollution) stress. In the present study, we studied fitness consequences of exposure to both a parasite (i.e. biotic) and a pesticide (i.e. abiotic) for the water flea Daphnia. In addition, we investigated whether these fitness consequences change through time as a population evolves. Thus, we exposed Daphnia magna clones, hatched from dormant eggs isolated from different time layers of a natural dormant egg bank, to the parasite Pasteuria ramosa and the insecticide diazinon in a multifactorial experiment. While our experimental treatments for unknown reasons failed to induce disease symptoms in the Daphnia, we did observe a reduced survival of D. magna when simultaneously exposed to both the parasite and the pesticide. No increased mortality upon exposure to individual stressors was observed. We did not observe an evolutionary change in fitness response of the Daphnia clones hatched from different time horizons upon exposure to stressors. [ABSTRACT FROM AUTHOR]

The article presents research which assessed the effects of temperature on the prevalence of parasite in the Daphnia longispina complex. The researchers examined the infection results between six genotypes of host that represents the hybrid of this species and the parental taxa. They found that both the host clone and temperature clone within the taxa has enhanced the infection rate of Daphnia longispina.

The Red Queen coevolutionary hypothesis predicts that parasites drive oscillations in host genotype frequencies due to frequency-dependent selection where common hosts are at disadvantage. However, examples of this phenomenon in natural populations are scarce. To examine if the Red Queen theory operates in the wild, we studied the genetic structure of populations of the crustacean waterflea ( Daphnia), in relation to their infection levels, for which we collected multiple samples from a variety of lakes. The most common clone in a given population was often underinfected. This advantage, however, did not remain stable over time. Instead, the most common clone decreased in frequency over subsequent generations, indicating that parasites can track common clones. Such decreases were not observed in uninfected populations. Moreover, host clonal evenness was higher across the set of infected lakes compared to uninfected lakes; suggesting that any common clone is selected against when parasites are present. These results strongly suggest that Red Queen dynamics do operate in the wild. [ABSTRACT FROM AUTHOR]

The overenrichment (eutrophication) of aquatic ecosystems with nutrients leading to algal blooms and anoxic conditions has been a persistent and widespread environmental problem. Although there are many studies on the ecological impact of elevated phosphorus (P) levels (e.g., decrease in biodiversity and water quality), little is known about the evolutionary consequences for animal species. We reconstructed the genetic architecture of a Daphnia species complex in 2 European lakes using diapausing eggs that were isolated from sediment layers covering the past 100 years. Changes in total .P were clearly associated with a shift in species composition and the population structure of evolutionary lineages. Although environmental conditions were largely reestablished after peak eutrophication during the 1970s and 1980s, original species composition and the genetic architecture of species were not restored, but evolved along new evolutionary trajectories. Our data demonstrate that anthropogenically induced temporal alterations of habitats are associated with long-lasting changes in communities and species via interspecific hybridization and introgression. [ABSTRACT FROM AUTHOR]

A collaborative research effort was undertaken to evaluate the robustness of a recently developed genetic tool for species identification of members in the morphologically variable Daphnia longispina species complex. This genetic method, based on restriction fragment length polymorphism (RFLP) of the internal transcribed spacer region (ITS) of nuclear ribosomal DNA (rDNA) with restriction enzymes Mwo I and Sau96 I [Billiones et al., 2004. Hydrobiologia 526: 43–53], was applied to many different European populations. Results were compared with two or more independently obtained characters (morphology, allozymes, mitochondrial DNA (mtDNA), or cloned rDNA-ITS sequences). Individuals of most taxa were readily identified, but unexpected ITS-RFLP patterns were found in many individuals indicated by other markers to be D. galeata or one of its hybrids. Among 43 investigated D. galeata populations (902 specimen analysed by ITS-RFLP), deviant RFLP fragment patterns occurred in 26 (i.e., more than half) of the populations. The deviant patterns could be attributed to the loss of one single restriction site in the ITS2 region. This loss made the distinction of D. galeata from other species unreliable, and F1 hybrids could not be identified. Future users should be aware of this shortcoming of the Billions et al. [2004. Hydrobiologia 526: 43–53] protocol. As a solution to this problem, we present an improved genetic identification protocol based on a simple double digestion of the rDNA-ITS region with the restriction enzymes BsrB I and EagI. Sequence analyses of rDNA-ITS clones and preliminary testing indicate that the new protocol is unaffected by the rDNA variation which troubled the Mwo I/ Sau96 I protocol. Further, the new protocol identifies all European species of the D. longispina complex, as well as their F1 hybrids. However, a wider screening is required to verify its general utility for all species, since yet unknown variation may occur. [ABSTRACT FROM AUTHOR]

Suspended particles are abiotic factors that can affect the abundance of cladocerans such as daphnids. Ultra-oligotrophic Lake Brienz, situated in the front ranges of the Swiss Alps, is dominated by two major inflows that annually transport over 300,000 tons of suspended glacial material into the lake. A laboratory flow-through experiment was performed to test whether these suspended particles have an influence on the fitness of Daphnia hyalina from Lake Brienz, measured as body size, fecundity and juvenile growth rate, especially when they are simultaneously exposed to low food concentrations. Our results show that the concentration of suspended particles present in Lake Brienz does not reduce the fitness of daphnids, even at very low food concentration. In fact, a low concentration of suspended particles increased the fitness. Reduction of fitness could only be observed at a suspended particle concentration of over 25 mg l−1, a level that has never been recorded in Lake Brienz. [ABSTRACT FROM AUTHOR]

Parasites influence host life-history traits and therefore might crucially shape host populations in natural systems. In a series of laboratory experiments, we studied the impact of an oomycete brood parasite on its Daphnia (waterflea) host. We asked whether Daphnia dump the infected brood and subsequently are able to reproduce again as was occasionally observed in a preliminary study. No viable offspring developed from infected clutches, but 78% of the infected females produced healthy offspring after releasing the infected brood while molting. Neither those offsprings’ development success nor their mothers’ reproductive potential was affected by the brood parasite. However, infected Daphnia had a reduced life-span and suffered an increased susceptibility to another parasite, an unidentified bacterium. Additionally, we studied the prevalence of this brood parasite and the unidentified bacterium in a natural Daphnia assemblage in a pre-alpine lake, across changing demographic and environmental conditions. The brood parasite epidemic seemed to be host-density dependent. Our results show that the brood parasite’s impact on the host population is enhanced when combined with the unidentified bacterium. [ABSTRACT FROM AUTHOR]

1. Previous studies have shown that interspecific hybridisation is common among taxa from the Daphnia galeata/ hyalina/ cucullata species complex. We investigated the influence of predator kairomones on the morphology and life histories of nine clones belonging to three taxa (pure D. galeata, F1 hybrids between D. galeata and D. hyalina, and backcrossed D. hyalina) of this species complex. Predators exerting positive (fish) and negative ( Chaoborus larvae) size-selective predation were tested. 2. The most responsive traits were size at maturity and size of neonates. Despite large between-clone variation, discriminant analysis revealed that the three taxa were distinct from each other in key life-history traits. F1 hybrids did not react in an intermediate way compared to the other taxa: the multivariate distances between F1 hybrids and either taxon were larger than between pure D. galeata and backcrossed D. hyalina. 3. The average plasticity (calculated across all traits) was similar for all three taxa. With regard to the size at maturity and neonate body size, the strength of the response was a function of the intrinsic values of these traits expressed in the control. For example, for size at maturity, smaller individuals showed a significantly stronger reaction to Chaoborus kairomones than larger ones. 4. Finally, we monitored seasonal changes in body size, egg number and population density of pure D. galeata and F1 hybrids in Greifensee (Switzerland). The two taxa experienced similar seasonal changes in body size but, on some sampling dates, they differed in mean egg number. The observed seasonal changes in Daphnia body size were consistent with what would be expected if the predator assemblage shifted from fish to Chaoborus over the course of the summer. The fluctuations in the frequencies of Daphnia taxa, however, were not related to seasonal variation in Daphnia body size. 5. Experimental data suggest that temporally heterogeneous predation regimes might be an important condition stabilising the co-occurrence of Daphnia hybrids with parental taxa. Predation regimes, however, cannot solely explain dynamic changes in taxon frequency in Greifensee. [ABSTRACT FROM AUTHOR]

1. Hybridization between species is a common phenomenon in plants and animals. If parasite prevalence differs for hybrids and parental species (i.e. taxa) there may be considerable consequences for relative hybrid fitness. Some studies have investigated hybrid complexes for infection, and complex-specific differences in parasite prevalence have been detected. 2. Based on the results of a field study on a hybridizing Daphnia population from a single lake, it has been hypothesized that permanently over- or under-infected hybrids do not exist. The observed field-patterns can only be temporal because taxa, in addition to single genotypes, might be the subject of parasite driven host frequency-dependent selection. Thus, parasites will track any common taxon within a hybrid complex. 3. In the present study, hybridizing Daphnia populations from 43 lakes were screened for parasite infections to obtain indirect evidence for coevolutionary cycles. It was hypothesized that, due to time lags between the evolution of resistance in host populations and the evolution of the parasite towards tracking of a common host taxon, the same Daphnia taxon will be over-infected in some lakes, while being under-infected in others. 4. Two of the four parasite species were specialists: their prevalence differed among coexisting Daphnia taxa. The varying infection patterns detected across spatially segregated hybridizing Daphnia populations are consistent with theoretical predictions for coevolutionary cycles. Thus the infection patterns, as observed under natural conditions, are temporal and unstable. 5. Additionally, the spatial distribution of the four parasite species was analysed with respect to habitat differences. The results show that the presence of a particular parasite on a host taxon was determined not only by the host-specificity of the parasite, but also by host-habitat relations. [ABSTRACT FROM AUTHOR]

Lakes and reservoirs disrupt the longitudinal connectivity of streams, considerably affecting benthic macroinvertebrate assemblages and diversity. Changes in assemblage composition within fragments can result from habitat alteration and reduced dispersal between fragments. We investigated the effects of habitat fragmentation in 10 Alpine streams, examining 69 taxa of benthic macroinvertebrates from 22 sites in fragmented and freeflowing streams. Total taxon richness (α-diversity) ranged from 6 to 27 in individual sites, and total richness was not significantly affected by fragmentation. However, Ephemeroptera and Diptera (excluding Simuliidae) richness was significantly reduced in stream fragments. Beta-diversity indicated a high degree of taxon turnover among sites within streams, but was not significantly different between fragmented and unfragmented streams. Characterizing the biological, physiological, and ecological traits of Ephemeroptera showed that communities in reservoir-fragmented streams had a higher affinity for fine sediments, increased temperatures, and reduced current velocity. Taxon assemblages in fragments were not nested subsets of unfragmented site assemblages. Thus, species turnover and species replacement in fragments is common, suggesting that most taxa are able to freely disperse among fragments. We suggest that habitat alteration was the primary cause of changes in assemblage structure in these streams. Consequently, habitat-based conservation is likely to be successful in maintaining populations of all but the weakest dispersers. [ABSTRACT FROM AUTHOR]

1. In this paper we test the usefulness of acoustic backscatter measurements from a 614 kHz Acoustic Doppler Current Profiler (ADCP) for the qualitative and quantitative characterisation of zooplankton distributions in lakes. ADCP-based backscatter estimates were compared with frequent net hauls obtained during a calibration experiment in which the acoustic backscatter was strongly dominated by vertical migratingChaoborus flavicanslarvae.2. The correlation between backscatter estimates and theC. flavicansconcentration was very good. Vertical swimming speed of larvae, measured directly by the ADCP, was up to a maximum of 5 mm s−1 and agreed very well with the observed vertical movement of the backscatter contour lines. Although the strong backscatter fromC. flavicansoverwhelmed the signal from the remaining zooplankton, a good correlation between backscatter strength and the total remaining zooplankton concentration, dominated byCyclopsspp., was found for the depth and time intervals where noC. flavicanswere present.3. In addition to the calibration experiment, longer-term ADCP measurements from different lakes revealed a strong temporal correlation between the onset of the up- and downward migration of zooplankton and the local sunset and sunrise.4. We conclude that ADCPs can be used to monitor plankton distributions both temporally and spatially. It also seems possible to estimate plankton densities after appropriate calibration. [ABSTRACT FROM AUTHOR]

1. We investigated whetherDaphnia galeata × hyalinahybrids of Lake Constance and Lake Greifensee show the same pattern of life history parameters as previously reported forD. galeata × cucullatahybrids and whether such a pattern is consistent betweenDaphniapopulations from those two lakes.2. Hybrids in Lake Constance were intermediate in size compared with the parental species. Hybrids in Lake Greifensee were smaller thanD. galeata. The intrinsic growth rate (r) of hybrids from Lake Constance was not significantly different from the faster growing parental taxonD. galeata. However,rof hybrids from Lake Greifensee was significantly lower than that ofD. galeata.3. The observed juvenile body length differences between the taxa varied with the clutch number. The first clutch juvenile lengths of the three taxa did not differ for Lake Constance. First clutch juveniles of Lake GreifenseeD. galeatawere smaller than hybrid first clutch juveniles. The third clutch juvenile length did not differ between taxa from Lake Greifensee, butD. galeatajuveniles from Lake Constance were bigger than those ofD. hyalina.4. The life history pattern found in Lake Constance corresponds to previous findings from other studies. The hybrids in this lake combine the faster population growth of one parental species with a relatively small size. In the case of Lake Greifensee hybrids, the relatively large size of first clutch juveniles and the small size of the adults could be interpreted as dual adaptations to invertebrate and fish predation. We speculate that the lower population growth rate of the hybrids is a trade-off for this twofold protection. [ABSTRACT FROM AUTHOR]

Daphnia often occur in species complexes that consist of two or more co-occurring species and their hybrids. Hybrid individuals are often capable of sexual reproduction and so backcrossing with introgression occurs. To better understand hybridization and backcrossing frequency, we sought to develop PCR-based, species-specific markers in the Daphnia galeata–hyalina species complex using amplified fragment length polymorphism (AFLP). This technique produces large numbers of reproducible markers for assessing diversity across the nuclear genome and provides several advantages over mtDNA and microsatellite approaches. We examined 28 clones of D. galeata, D. hyalina, and their hybrids isolated from Lake Constance on the Swiss-German border. Using a single AFLP primer combination we found five potential species-specific markers, defined as bands that occurred in >80% of one parental species and <20% of the other. Two bands appeared to be co-dominant and were present (homozygous) in D. galeata, absent in D. hyalina, and heterozygous in the hybrid. We conclude AFLP could provide enough PCR-based, species-specific markers to identify species, hybrids, and backcrosses from even small amounts of tissue (i.e. resting eggs). [ABSTRACT FROM AUTHOR]

Many polluted lakes in Europe are being restored and phosphorus concentrations have dropped dramatically in these lakes. We studied the genetic structure of Daphnia galeata over the past 30 years in Lake Greifensee, Switzerland, a period during which the phosphorus concentration in the lake reduced dramatically. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from six different time horizons in dated sediments. We compared juvenile growth, size and egg production of D. galeata reared on high-P and low-P algae to investigate whether Daphnia have evolved to grow better on phosphorus-limited algae. Our results indicate that life histories of D. galeata differed significantly between both food types. We also found significant clone effects for size and egg number. But we found no significant interaction between the depth from which the clones were selected and food quality. This means that we found no evidence for adaptive micro-evolution in response to P reduction in the lake. We discuss our results in relation to other studies that found evidence for adaptive micro-evolution in comparable time frames. [ABSTRACT FROM AUTHOR]

In Greifensee (Switzerland), Daphnia galeata x hyalina hybrids cooccur with both parental taxa. Hybrids are the most abundant taxon, suggesting that hybrids have greater fitness. In addition to many known factors that promote hybrids, specific environmental conditions favoring parentals must also exist to explain their cooccurrence. We investigated the influence of the protozoan gut parasite Caullerya mesnili on the D. galeata x hyalina species complex. Up to 22% of the Daphnia population was infected with C. mesnili in October 2002. C. mesnili dramatically reduced the fecundity of its hosts. Only 2% of infected individuals carried eggs compared with 70% in the uninfected group, which suggests that C. mesnili exerts a strong selection pressure. Our results indicate that hybrids were frequently infected, whereas parental D. galeata were almost never infected. We also found genetic variation for infection within hybrids, evidenced by significant differences in clonal composition between the infected and uninfected parts of the taxon. Resistance of D. galeata might counterbalance the greater fitness of hybrids and therefore contribute to the maintenance of coexistence of hybrids and parentals in this lake. [ABSTRACT FROM AUTHOR]

Sexual reproduction in Daphnia results in the production of diapausing eggs, which are enclosed in a structure called ephippium. Ephippia, accumulated by sedimentation, can be preserved for decades and offer the opportunity for microevolutionary studies as well as the study of former pelagic populations. In a Swiss subalpine lake (Greifensee), we studied the genetic structure of the pelagic Daphnia galeata x hyalina hybrid species complex. We examined sexual females, males, and ephippia production. Eggs from ephippia were hatched, and the genotypes and taxa composition of all daphnids were determined using four polymorphic allozyme loci, two of which are each diagnostic for D. galeata and D. hyalina. We found significant differences between the genetic composition and the backcross level of pelagic asexual females, sexual females, males, and ephippial eggs (Daphnia hatchlings). The asexual daphnids were dominated by hybrids. In contrast, sexual females, especially Daphnia hatched from ephippial eggs, are dominated by D. galeata. We conclude that hybrid Daphnia have a lower sexual reproductive success than the parental D. galeata. The recent hybrid dominance suggests that D. galeata that hatch from diapausing eggs are not able to alter the pelagic population. The genotypic class composition of the diapausing eggs does not reflect the extant pelagic population; therefore, Daphnia diapausing egg banks do not always represent the past lake taxa structure. [ABSTRACT FROM AUTHOR]

Diel vertical migration (DVM) is a dynamic behavioral pattern found extensively in the world's oceans and lakes, yet the role of food and temperature distribution on DVM is still unclear. While DYM has been mostly studied in systems with surface food maxima, deep-water food maxima are quite common in lakes and oceans. In such ecosystems, optimal conditions of temperature and food are uncoupled. In a Swiss high-mountain lake (Oberer Arosasee) with a deep-water food maximum, we found that Daphnia galeata adults and juveniles exhibit DVM behavior almost throughout the year and migrated upwards, out of food-rich environments, at night. In a large indoor mesocosm experiment, we were able to show that Daphnia respond to fish-mediated cues by migrating into deeper water layers. In the presence of fish and using natural vertical food and temperature distributions in the mesocosms, we could reproduce the vertical distribution of adult Daphnia observed in the field. The indoor experiments show that food and temperature modulate the actual depth at which the animals stop migrating, whereas fish and ultraviolet radiation likely determine the timing of migration (its synchronization with dawn and dusk). Overall, our results show that the nighttime movement into the surface waters contributes to the fitness of Daphnia, given the costs and benefits associated with the trade-off between food and temperature. [ABSTRACT FROM AUTHOR]