Daphnia Longispina Complex Research Articles

Daphnia japonica sp. nov. (Crustacea: Cladocera) an eastern Palearctic montane species with mitochondrial discordance.

The Daphnia longispina complex (Crustacea: Cladocera) contains several keystone freshwater species such as D. longispina O.F. Müller (D. rosea Sars is a junior synonym), D. galeata Sars, D. cucullata Sars, and D. dentifera Forbes. The complex is common throughout the Holarctic, but there are several geographic regions where local forms have been assigned to European species names based on a superficial morphological resemblance. Here we examine the species status of a form that was previously assigned to D. rosea from a montane bog pond on Honshu, Japan. We used two nuclear non-coding loci (nDNA), mitochondrial sequences (the ND2 protein-coding region) and morphology for evidence. The mitochondrial gene evidence supported the existence of a divergent lineage that is more closely related to D. galeata than to D. dentifera. However, morphology and the nuclear DNA data indicated a lineage that is most closely related to D. dentifera. As our evidence supported the existence of a cohesive divergent lineage, we described a new species, Daphnia japonica sp. nov. Recognition of local and subalpine diversity in this group is critical as ongoing anthropogenic disturbance has been associated with introductions, local extirpations, and hybridization.

Infection by a eukaryotic gut parasite in wild Daphnia sp. associates with a distinct bacterial community.

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.

Comparing recent and preindustrial cladoceran assemblages in 35 metal- and smelting-affected lakes near Sudbury (Ontario, Canada)

Abstract Base metal smelting activity around Sudbury (Ontario, Canada) has persisted for over a century, emitting metals and acidifying contaminants that affected lakes downwind of smelter stacks. Although considerable research has been directed toward the paleolimnological study of diatoms and chrysophytes in the region, relatively little comparable work has been completed on invertebrates. Here, cladoceran (Branchiopoda: Cladocera) assemblages are compared using a paleolimnological “top-bottom approach”, comparing preimpact and recent assemblages. Spatiotemporal differences were likely driven by acidification-related stressors and legacy effects of metal contamination. The replacement of the Daphnia longispina complex with the Daphnia pulex complex is attributable to diminished lake-water calcium concentrations. Increased abundances of early colonist taxa demonstrate that many acidified lakes have not fully recovered. Regional stressors further complicate recovery and the interpretation of temporal changes. Finally, clustering of recent cladoceran assemblages reflects the effects of modern multiple stressors and persistent historical stressors. This study shows that acidification history partially explains differences in cladoceran assemblages, but also that multiple stressors will likely continue to influence assemblage composition and recovery in Sudbury lakes. Defining appropriate reference conditions, considering historical and modern stressors, will be important when assessing the extent and trajectory of ecosystem recovery from widespread anthropogenic impacts.

Application of species-specific primers to estimate the in situ diet of Bythotrephes [Cladocera, Onychopoda] in its native European range via molecular gut content analysis.

The study of invasive species often focuses on regions of recent introduction rather than native habitats. Understanding an invasive species in its natural environment, however, can provide important insights regarding the long-term outcome of invasions. In this study we investigated the diet of the invasive spiny water flea, Bythotrephes longimanus, in two Austrian perialpine lakes, where it is native. The gut contents of wild-caught Bythotrephes individuals were estimated by quantitative polymerase chain reaction, targeting species-specific fragments of the barcoding region of the cytochrome c oxidase I gene of potential prey. The observed prey spectrum of Bythotrephes in the study lakes consisted primarily of Eudiaptomus gracilis and Diaphanosoma brachyurum. The Daphnia longispina complex, Leptodora kindtii and Mesocyclops leuckarti also contributed to the diet. Results indicate that Bythotrephes is a generalist feeder with a preference for epilimnetic prey.

Unexpected endemism in the Daphnia longispina complex (Crustacea: Cladocera) in Southern Siberia.

The biological significance of regional cladoceran morphotypes in the montane regions of the central Palearctic remains poorly understood. In the Holarctic Daphnia longispina complex (Cladocera: Daphniidae), several variants, lineages and species have been proposed as endemic for Southern Siberia. Daphnia turbinata Sars, for example, named after its unusual head shape, is known only from Southern Siberia. Here we sequence DNA of Daphnia from three mitochondrial genes (12S rRNA, 16S rRNA, and NADH dehydrogenase subunit 2, ND2) from 57 localities in Russia and Mongolia (the majority being from Southern Siberia) and place them in evolutionary context with existing data. Our aim was to examine regional endemism of the Daphnia longispina complex in Southern Siberian; to improve the phylogenetic understanding with improved taxonomic and regional sampling, and to better understand the influence of Pleistocene glaciation on the biogeography of these lineages. At least three lineages showed genetic evidence for endemism in Southern Siberia. There was strong support for D. turbinata as a sister lineage to to D. longispina/D. dentifera. Another endemic, Siberian D. cf. longispina, is a sister group to the longispina group in general. Within D. longispina s. str. there was an endemic Siberian clade with a western range boundary near the Yenisei River Basin. Gene flow estimates among populations (based on FST values) were very low for clades of D. longispina on a regional (the original 12S dataset), and on a pan-Eurasian (the extended 12S dataset) scale. Negative values of Fu’s FS and Tajima’s D tests prevailed for the species examined with significant values found for two D. longispina clades, D. dentifera, D. galeata and D. cristata. Our results support the notion that Southern Siberia is an important biogeographic region for cladocerans as it contained unexpected diversity of endemics (such as D. turbinata, D. cf. longispina and lineages of D. umbra and D. longsipina s.str.) and from being the geographic meeting place of expanding postglacial lineages from eastern and western refugia.

Identification and Phylogeny of Cryptic Species of the Daphnia Longispina Complex (Anomopoda, Daphniidae) Using ITS2 Secondary Structure

Cladocerans of the genus Daphnia O.F. Muller (Crustacea: Cladocera) are typical members of different types of water bodies and are used as model organisms in various biological disciplines. Within the genus, a considerable number of closely related and cryptic species are noted, the identification of which by morphological traits often causes difficulties. In the present study, species identification in the Daphnia longispina complex, namely, D. longispina O.F. Muller, D. hyalina Leydig, D. umbra Taylor, Hebert et Colbourne, and D. cf. longispina, was performed using data on the predicted secondary structure of the second internal transcribed spacer (ITS2) of the nuclear rDNA. Phylogenetic analysis carried out using the Profile Neighbor-Joining algorithm and the analysis of compensatory base changes supported the species status of D. hyalina and D. cf. longispina. It was demonstrated that the data on the ITS2 secondary structure can be used to resolve complex taxonomic problems in the study of cladocerans in general and the species complexes in particular.

Establishment of a new microsporidian genus and species, Pseudoberwaldia daphniae (Microsporidia, Opisthosporidia), a common parasite of the Daphnia longispina complex in Europe.

Microsporidia are among the most common microparasites of cladocerans and have potentially significant impact on host populations. However, many of these pathogens are known only from molecular-based studies. We provide ultrastructural data supported by molecular phylogeny for a common microsporidium infecting the Daphnia longispina complex, important planktonic filter-feeders in reservoirs and ponds in the temperate Holarctic region. This parasite, previously characterized only by molecular means, infects adipose cells around the Daphnia midgut and eventually fills the centre of the host body with ovoid-shaped spores. A new microsporidian genus and species belonging to the Agglomeratidae superclade is described as Pseudoberwaldia daphniae gen. et sp. nov. Molecular data indicate its widespread presence in Central European reservoirs (reported as isolate "MIC1") but also in Swedish coastal rockpools ("Ängskärs-klubben"). The most closely related lineage was reported from a caddisfly larva; we thus speculate that this taxon may have an insect secondary host in its life cycle. Morphological characterization and differential diagnosis of most commonly encountered microsporidian taxa infecting hosts in the D. longispina complex in Europe opens new possibilities for studies of their ecological and evolutionary interactions.

Reorganization of aquatic communities from low-nutrient lakes in northwestern New Brunswick, Canada

Sparse monitoring of New Brunswick (Canada) lakes creates challenges for understanding mechanisms of deteriorating water quality, such as recent instances of increased cyanobacterial biomass in low-nutrient systems. To assess long-term environmental change experienced by low-nutrient, dimictic New Brunswick lakes we use sedimentary remains of algal pigments and Cladocera in dated cores from impact (prone to late-summer algal blooms, Lac Unique) and reference (no observed algal blooms, First and States) lakes. Overall, all three lakes now exhibit greater bosminid abundance and fewer daphniids, lower Cladocera richness, and smaller average cladoceran body size, although some lake-specific differences in assemblage response exist that cannot be related solely to top-down or bottom-up forces. Zooplankton trends are most pronounced at Lac Unique and First Lake, where algal production is generally greater. Across all three lakes, the only significant (IndVal: p < 0.05) cladoceran bioindicator of the post-1990 period is the pelagic bosminid group, whereas prior to ~ 1990, Daphnia longispina-complex and several littoral taxa were significant (IndVal: p < 0.05) bioindicators. These temporal shifts suggest that the smaller-bodied bosminid group may now be favored over its larger-bodied pelagic competitor Daphnia sp. in these lakes, irrespective of heterogeneous long-term algal patterns inferred from stable sedimentary pigments. We suggest that the indirect effects of climate change, principally during the spring and summer quarters in New Brunswick, may be associated with marked shifts in limnological structure, as evidenced by changes in dominant zooplankton of the pelagic zone. However, further research is needed to rule out whether or not size-selective predation, and its potential interactions with climate change and other stressors, are key mechanisms favoring increased bosminids in low-nutrient, dimictic lakes.

The establishment of hybrids of the Daphnia longispina complex explained by a mathematical model incorporating different overwintering life history strategies.

Interspecific hybridization (i.e. mating between species) occurs frequently in animals. Among cyclical parthenogens, hybrids can proliferate and establish through parthenogenetic reproduction, even if their sexual reproduction is impaired. In water fleas of the Daphnia longispina species complex, interspecific hybrids hatch from sexually produced dormant eggs. However, fewer hybrid genotypes contribute to the dormant egg bank and their hatching rate from dormant eggs is reduced, compared to eggs resulting from intraspecific crosses. Therefore, Daphnia hybrids would benefit from adaptations that increase their survival over winter as parthenogenetic lineages, avoiding the need to re-establish populations after winter from sexually produced dormant eggs. Here, we constructed a mathematical model to examine the conditions that could explain the frequently observed establishment of hybrids in the D. longispina species complex. Specifically, we compared the outcome of hybrid and parental taxa competition given a reduced contribution of hybrids to hatchlings from the sexually produced dormant egg bank, but their increased ability to survive winter as parthenogenetic lineages. In addition, different growth rates of parental species and differences in average annual temperatures were evaluated for their influence on hybrid production and establishment. Our model shows that increased overwinter performance as parthenogenetic females can compensate for reduced success in sexual reproduction, across all tested scenarios for varying relative growth rates of parental species. This pattern holds true for lower annual temperatures, but at higher temperatures hybrids were less successful. Consequently, hybrids might become less abundant as temperatures rise due to climate change, resulting in reduced diversity and faster differentiation of the parental species.

Zooplankton communities and Bythotrephes longimanus in lakes of the montane region of the northern Alps

Lakes in the Alps represent a considerable fraction of nutrient-poor lakes in Central Europe, with unique biodiversity and ecosystem properties. Although some individual lakes are well studied, less knowledge is available on large-scale patterns essential to general understanding of their functioning. Here, we aimed to describe crustacean zooplankton communities (Cladocera, Copepoda) and identify their environmental drivers in the pelagic zone of 54 oligotrophic lakes in the montane region of the Alps (400–1200 m) in Austria, Germany, and Switzerland, covering a spatial scale of 650 km. Moreover, we aimed to provide data on the distribution and ecological requirements of the North American invader Bythotrephes longimanus in its Central European native range. Communities were mainly dominated by widespread species typical of lowland habitats, and only a few true specialists of oligotrophic alpine lakes were present. The most frequent taxa were the Daphnia longispina complex and Eudiaptomus gracilis, with 48 and 45 occurrences, respectively. Species richness decreased with altitude and increased with lake area. The main structuring factors of community composition were chlorophyll a concentration and depth, which drove an apparent separation of mesotrophic and oligotrophic communities. Bythotrephes had 13 occurrences, showing a preference for deep oligotrophic lakes. Its presence was not coupled with lower crustacean species richness, as was repeatedly observed in North America. Additionally, it frequently co-occurred with the other large predatory cladoceran, Leptodora kindtii. B. longimanus might be considered a truly montane species in Central Europe, given its absence in lowland and alpine lakes.

Genetic diversity of two Daphnia-infecting microsporidian parasites, based on sequence variation in the internal transcribed spacer region.

BackgroundMicrosporidia are spore-forming obligate intracellular parasites that include both emerging pathogens and economically important disease agents. However, little is known about the genetic diversity of microsporidia. Here, we investigated patterns of geographic population structure, intraspecific genetic variation, and recombination in two microsporidian taxa that commonly infect cladocerans of the Daphnia longispina complex in central Europe. Taken together, this information helps elucidate the reproductive mode and life-cycles of these parasite species.MethodsMicrosporidia-infected Daphnia were sampled from seven drinking water reservoirs in the Czech Republic. Two microsporidia species (Berwaldia schaefernai and microsporidium lineage MIC1) were sequenced at the internal transcribed spacer (ITS) region, using the 454 pyrosequencing platform. Geographical structure analyses were performed applying Fisher’s exact tests, analyses of molecular variance, and permutational MANOVA. To evaluate the genetic diversity of the ITS region, the number of polymorphic sites and Tajima’s and Watterson’s estimators of theta were calculated. Tajima’s D was also used to determine if the ITS in these taxa evolved neutrally. Finally, neighbour similarity score and pairwise homology index tests were performed to detect recombination events.ResultsWhile there was little variation among Berwaldia parasite strains infecting different host populations, the among-population genetic variation of MIC1 was significant. Likewise, ITS genetic diversity was lower in Berwaldia than in MIC1. Recombination signals were detected only in Berwaldia.ConclusionGenetic tests showed that parasite populations could have expanded recently after a bottleneck or that the ITS could be under negative selection in both microsporidia species. Recombination analyses might indicate cryptic sex in Berwaldia and pure asexuality in MIC1. The differences observed between the two microsporidian species present an exciting opportunity to study the genetic basis of microsporidia-Daphnia coevolution in natural populations, and to better understand reproduction in these parasites.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1584-4) contains supplementary material, which is available to authorized users.

Temporal changes in cladoceran assemblages subjected to a low calcium environment: combining the sediment record with long-term monitoring data

Lakewater calcium (Ca) decline affects softwater lakes across the Canadian Shield. Ca decline is one consequence of acid deposition, and has impeded biological recovery in formerly acidified lakes. Reduced Ca availability may advantage taxa better adapted to low Ca waters. Crosson Lake in south-central Ontario (Canada) has an extensive monitoring record and has experienced strong Ca decline since the late 1970s, recently falling below 1.5 mg L−1, a threshold value for some large Daphnia O.F. Muller, 1785 taxa. Paleolimnological analysis of the sedimentary cladoceran assemblages revealed that changes associated with the Ca decline began in the early 1970s. Relative abundances of the jelly-clad Holopedium glacialis Rowe, 2007 and the Daphnia pulex complex increased while Ca-sensitive members of the Daphnia longispina complex decreased. Zooplankton net hauls (1981–2010) corroborate the paleolimnological analysis, revealing that increases in the D. pulex complex were due to Daphnia catawba Coker, 1926 (a taxon tolerant of low Ca). Lakewater Ca, dissolved organic carbon and total phosphorus explain a significant amount of variation within the cladoceran community; however, the relationship between Ca concentration and the daphniid community was most apparent. The Crosson Lake paleolimnological and direct monitoring data may describe ecological changes that are also occurring in many other softwater lakes across the Canadian Shield and elsewhere.

Parental and hybrid Daphnia from the D.longispina complex: long-term dynamics in genetic structure and significance of overwintering modes.

In recent decades, hybridization has become a focus of attention because of its role in evolutionary processes. However, little is known about changes in genetic structure within and between parental species and hybrids over time. Here, we studied processes of genetic change in parental species and hybrids from the Daphnia longispina complex (Crustacea, Cladocera) over a period of six years across ten habitats. These cyclical parthenogens respond to fluctuating environments by switching from asexual to sexual reproduction. Importantly, sexually produced diapausing eggs, which resist extreme conditions such as low temperatures and serve as dispersal stages, are produced to a lower extent by hybrids. Long-term microsatellite data revealed clear differences between hybrids and parental species. In hybrids, clonal diversity values were lower, whereas heterozygosity and linkage disequilibrium values were higher compared to parental species. Clonal diversity of hybrids responded to the strength of the winter, with cold winters resulting in few genotypes in the following spring. In time windows when only asexual hybrid females survive, priority effects will favour the establishment of the hybrid offspring before hatchlings from parental diapause eggs can enter the community. The constant high levels of heterozygosity maintained by clonal reproduction in hybrids might lead to their successful establishment over time, when they are able to escape competition from both parental species. Although we found evidence that hybrids diversity depends on fluctuating environments, a direct link between hybrid abundance and the strength of winter was missing. Because of reduced adaptability in clonally reproducing hybrids, multiple factors must contribute to promoting their long-term success in fluctuating environments.

Clonal structure and depth selection during a Caullerya mesnili epidemic in a hybridizing population of the Daphnia longispina complex

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.

Is bigger better? A possibility for adaptation of Daphnia to filamentous cyanobacteria in the face of global warming

Body size is a key determinant of fitness in Daphnia. Bigger size means higher feeding efficiency and reproduction. However, filamentous cyanobacteria have a more detrimental effect on larger daphnids. Predicted global warming is expected to reduce size of Daphnia and facilitate frequent occurrence of cyanobacteria. Therefore, we asked two questions: (i) does elevated temperature cause reduced size of daphnids, and (ii) is temperature-dependent size reduction adaptive in an environment dominated by filamentous cyanobacteria? To address these issues, we obtained 8 clones of the Daphnia longispina complex from artificially heated lakes and 8 clones from control lakes, and exposed them to a favorable food Scenedesmus obliquus and a mixture of S. obliquus and the filamentous cyanobacterium Cylindrospermopsis raciborskii in life table experiments at 16, 20, and 24°C. Individuals from heated lakes attained larger body size than those from control lakes. Moreover, exposure to the filamentous cyanobacterium led to reduced fecundity of Daphnia from the non-heated lakes and did not affect reproduction of Daphnia from the heated lakes. We conclude that Daphnia display some evolutionary adaptations to cope with filamentous cyanobacteria, linked to long-term exposition to elevated temperature. Our results violate broadly accepted assumptions that ectotherms are smaller in warmer environments.

New possibilities arise for studies of hybridization: SNP-based markers for the multi-species Daphnia longispina complex derived from transcriptome data

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.

Filamentous cyanobacteria alter the relative fitness in a Daphnia hybrid species complex

Summary Interspecific hybridisation is common in freshwater cladocerans. Within the Daphnia longispina complex hybrids frequently dominate over parental species, and environmental heterogeneity in the pelagial of lakes has been invoked to explain the coexistence of hybrids and parental species. In laboratory conditions, I investigated the effects of filamentous cyanobacteria on the relative fitness of Daphnia hyalina, Daphnia galeata and Daphnia hyalina×galeata hybrids. For each taxon, the performance of three clones was measured using animals fed with two diets: (i) a mixture of the filamentous cyanobacterium Cylindrospermopsis raciborskii and the green alga Scenedesmus obliquus and (ii) Scenedesmus obliquus in non‐limiting concentration. Inter‐ and intraspecific differences in ability to cope with cyanobacterial filaments were found; hybrids were, on average, superior to parental species in the absence of filaments, but in the presence of filaments one of the parental species (D. hyalina) was superior, while performance of hybrids did not differ from D. galeata. No relationship was found between size at first reproduction (range 1.3–2.0 mm) and reduced fitness due to the presence of cyanobacterial filaments. Shifts in relative fitness of daphnids due to variation in food quality, mediated by the presence of filamentous cyanobacteria, may promote coexistence of hybrids and parental species of the D. longispina complex.

Diversity of the Daphnia longispina species complex in Chinese lakes: a DNA taxonomy approach

Cladocerans of the Daphnia longispina complex are often the key components of pelagic food webs in northern temperate lakes and ponds. In the present study, we aimed to fill the gap in knowledge on the biogeography and diversity of this species complex, through analysis (by sequencing two mitochondrial genes) of the taxonomic structure of 18 lake populations across China, covering a large geographical and ecological range. Based on the patterns observed in the Western Palaearctic and Japan, we hypothesized that several species of the D. longispina complex should coexist and that some cryptic lineages are present in China. In the lakes studied, however, we detected haplotypes of only two species (D. galeata and D. dentifera). Daphnia galeata was detected in eutrophic lakes at lower altitudes (in central and eastern China), whereas D. dentifera was found in high-altitude oligotrophic lakes of the Tibetan Plateau in western China. The two lineages coexisted in only one of the lakes studied, in Tibet. The variation in mitochondrial DNA differed between the two species. Daphnia dentifera haplotypes were lake specific, while D. galeata haplotypes were often shared across lakes and geographic regions, indicating a recent rapid expansion of the latter species.

Hybridizing Daphnia communities from ten neighbouring lakes: spatio-temporal dynamics, local processes, gene flow and invasiveness.

BackgroundIn natural communities of cyclical parthenogens, rapid response to environmental change is enabled by switching between two reproduction modes. While long periods of asexual reproduction allow some clones to outcompete others, and may result in “clonal erosion”, sexual reproduction restores genetic variation in such systems. Moreover, sexual reproduction may result in the formation of interspecific hybrids. These hybrids can then reach high abundances, through asexual clonal reproduction. In the present study, we explored genetic variation in water fleas of the genus Daphnia. The focus was on the short-term dynamics within several clonal assemblages from the hybridizing Daphnia longispina complex and the impact of gene flow at small spatial scales.ResultsDaphnia individuals belonged either to the parental species D. galeata and D. longispina, or to different hybrid classes, as identified by 15 microsatellite markers. The distribution and genotypic structure of parental species, but not hybrids, corresponded well with the geographical positions of the lakes. Within parental species, the genetic distance among populations of D. galeata was lower than among populations of D. longispina. Moreover, D. galeata dominance was associated with higher phosphorous load. Finally, there was no evidence for clonal erosion.ConclusionsOur results suggest that the contemporary structure of hybridizing Daphnia communities from ten nearby lakes is influenced by colonization events from neighbouring habitats as well as by environmental factors. Unlike the parental species, however, there was little evidence for successful dispersal of hybrids, which seem to be produced locally. Finally, in contrast to temporary Daphnia populations, in which a decrease in clonal diversity was sometimes detectable over a single growing season, the high clonal diversity and lack of clonal erosion observed here might result from repeated hatching of sexually produced offspring. Overall, our study provides insights into spatio-temporal dynamics in a hybridizing Daphnia species complex in a recently established lake system, and relates genetic similarities of populations to a scenario of secondary invasion enhanced by environmental factors.

Long-term Trends in Cladoceran Assemblages Related to Acidification and Subsequent Liming of Middle Lake (Sudbury, Canada)

In the mid-20th century, similar to many lakes in the vicinity of Sudbury, Canada, Middle Lake was severely acidified due to nearby smelting operations. However, this lake is of particular interest because it was limed in 1973, and later fertilized as part of a restoration effort. Here, we use paleolimnological methods to track cladoceran assemblage responses to acidification, liming, and subsequent recovery in a ∼250-year lake sediment record. Cladoceran assemblage changes, notably increases in Chydorus brevilabris, coincided with the late 1800s establishment of open-pit ore roasting in the region. As acidification progressed, the Daphnia pulex complex was replaced by the Daphnia longispina complex. At the height of acidification, and with similar timing to the liming, C. brevilabris increased abruptly in relative abundance in the sediment record, followed by a rapid decline. Invertebrate predation was investigated using Bosmina mucro length; however, no significant trends were evident. Our results suggest that complete biological recovery has not occurred. Specifically, species richness (rarefied) is ∼64 % lower after the onset of acidification, and many rare species present prior to the onset of acidification have not yet returned to pre-impact levels despite dispersal events of these rare taxa being observed during contemporary zooplankton monitoring. Factors impeding the complete biological recovery of the cladocerans in Middle Lake may include biotic resistance, ongoing metal contamination, and a warming climate.