Daphnia Pulicaria Research Articles

Temperature and ultraviolet radiation on a high mountain daphnid: When do interactions become lethal to highly adapted populations?

ABSTRACT Aquatic organisms from high mountain lakes experience extreme influences of climate and solar radiation, especially ultraviolet radiation (UVR), manifested through alterations in physiology, life history, and phenology. Zooplankton, a pivotal component of lake ecosystems, is particularly sensitive to changes in environmental conditions because of their short life cycles and diminished mobility. In the high mountains of the tropical Andes, zooplankton populations are adapted to low temperatures and high UVR because most of the Andean lakes are >4000 m a.s.l. This study focuses on Daphnia pulicaria populations originating from high-mountain Andean lakes, investigating their responses to different temperatures and UVR exposure by means of lab experiments. Key findings indicate that temperature is the most important factor impacting population variables, with high temperatures and UVR exposure leading to unfavorable population outcomes; however, physiological variables (cell viability and pigmentation, measured as melanin) are similarly influenced by temperature and UVR exposure. Pigmentation increases with temperature and is accentuated by solar radiation, indicating an adaptive response to mitigate UVR damage. Conversely, cellular viability declines with elevated temperatures and UVR exposure, showing that higher temperatures may offset the protective effects of pigmentation. Overall, these findings underscore the vulnerability of daphnid populations in high-mountain Andean systems to anticipated climate change impacts, with potential consequences for ecological dynamics in these critical ecosystems. More importantly, they show the importance of studying temperature and UVR (and probably other environmental conditions) as interacting variables because the results will dramatically differ if each factor is considered separately.

Multiple signs of ecosystem change in the zooplankton community of a large temperate lake

Large lakes, such as Lake Simcoe, Ontario, Canada, are undergoing significant change due to local and global stressors. Uni- and multivariate analyses of Lake Simcoe's zooplankton community from 1986 to 2012 indicated multiple events of ecosystem change that were synchronous across three lake stations . In the mid-1990s, shifts in zooplankton species abundance and richness, and total cladoceran body size were strongly correlated with the invasion of the zooplanktivore, Bythotrephes cederstroemii. In the early 2000s, additional shifts in zooplankton abundance, as well as copepod body size, coincided with increased water clarity (linked to filter feeding by the invader Dreissena polymorpha) and hypolimnetic water temperature. Further community changes occurred in the 2000s when Bythotrephes declined and many vulnerable cladoceran species recovered. However, the Lake Simcoe community did not fully return to its pre-invasion state as the cold-water herbivores, Daphnia longiremis and Daphnia pulicaria, remained absent. The Lake Simcoe zooplankton community illustrates ongoing ecosystem change that propagated throughout the lake food web and may be reflected in other lakes experiencing global stressors of climate change and species invasions.

Impacts of sequential salinity and heat stress are recovery time-specific in freshwater crustacean, Daphnia pulicaria

Stressors can interact to affect animal fitness, but we have limited knowledge about how temporal variation in stressors may impact their combined effect. This limits our ability to predict the outcomes of pollutants and future dynamic environmental changes. Elevated salinity in freshwater ecosystems has been observed worldwide. Meanwhile, heatwaves have become more frequent and intensified as an outcome of climate change. These two stressors can jointly affect organisms; however, their interaction has rarely been explored in the context of freshwater ecosystems. We conducted lab experiments using Daphnia pulicaria, a key species in lakes, to investigate how elevated salinity and heatwave conditions collectively affect freshwater organisms. We also monitored the impacts of various recovery times between the two stressors. Daphnia physiological conditions (metabolic rate, Na+-K+-ATPase (NKA) activity, and lipid peroxidation level) and life history traits (survival, fecundity, and growth) in response to salt stress as well as mortality in heat treatment were examined. We found that Daphnia responded to elevated salinity by upregulating NKA activity and increasing metabolic rate, causing a high lipid peroxidation level. Survival, fecundity, and growth were all negatively affected by this stressor. These impacts on physiological conditions and life history traits persisted for a few days after the end of the exposure. Heat treatments caused mortality in Daphnia, which increased with rising temperature. Results also showed that individuals that experienced salt exposure were more susceptible to subsequent heat stress, but this effect decreased with increasing recovery time between stressors. Findings from this work suggest that the legacy effects from a previous stressor can reduce individual resistance to a subsequent stressor, adding great difficulties to the prediction of outcomes of multiple stressors. Our work also demonstrates that cross-tolerance/susceptibility and the associated mechanisms remain unclear, necessitating further investigation.

Pesticide exposure enhances dominance patterns in a zooplankton community.

Exposure to pesticides can profoundly alter community dynamics. It is expected that dominance patterns will be enhanced or reduced depending on whether the dominant species is less or more sensitive to the pesticide than the subdominant species. Community dynamics are, however, also determined by processes linked to population growth as well as competition at carrying capacity. Here, we used a mesocosm experiment to quantify the effect of chlorpyrifos exposure on the population dynamics of four cladoceran species (Daphnia magna, Daphnia pulicaria, Daphnia galeata and Scapholeberis mucronata) in mixed cultures, testing for direct effects of chlorpyrifos and indirect effects mediated by interactions with other species on the timing of population growth and dominance at carrying capacity. We also quantified whether the pesticide-induced changes in community dynamics affected top-down control of phytoplankton. By adding a treatment in which we used different genotype combinations of each species, we also tested to what extent genetic composition affects community responses to pesticide exposure. Immobilization tests showed that D. magna is the least sensitive to chlorpyrifos of the tested species. Chlorpyrifos exposure first leads to a reduction in the abundance of D. galeata to the benefit of D. pulicaria, and subsequently to a reduction in densities of D. pulicaria to the benefit of D. magna. This resulted in D. magna being more dominant in the pesticide than in the control treatment by the end of the experiment. There was no effect of genotypic differences on community patterns, and top-down control of phytoplankton was high in all treatments. Our results suggest that in this community dominance patterns are enhanced in line with the observed among-species differences in sensitivity to the pesticide. Our results also show that the development of the community in pesticide treatment is a complex interaction between direct and indirect effects of the pesticide.

The Transcriptomic Signature of Cyclical Parthenogenesis.

Cyclical parthenogenesis, where females can engage in sexual or asexual reproduction depending on environmental conditions, represents a novel reproductive phenotype that emerged during eukaryotic evolution. The fact that environmental conditions can trigger cyclical parthenogens to engage in distinct reproductive modes strongly suggests that gene expression plays a key role in the origin of cyclical parthenogenesis. However, the genetic basis underlying cyclical parthenogenesis remains understudied. In this study, we characterize the female transcriptomic signature of sexual versus asexual reproduction in the cyclically parthenogenetic microcrustacean Daphnia pulex and Daphnia pulicaria. Our analyses of differentially expressed genes (DEGs), pathway enrichment, and gene ontology (GO) term enrichment clearly show that compared with sexual reproduction, the asexual reproductive stage is characterized by both the underregulation of meiosis and cell cycle genes and the upregulation of metabolic genes. The consensus set of DEGs that this study identifies within the meiotic, cell cycle, and metabolic pathways serves as candidate genes for future studies investigating how the two reproductive cycles in cyclical parthenogenesis are mediated at a molecular level. Furthermore, our analyses identify some cases of divergent expression among gene family members (e.g., doublesex and NOTCH2) associated with asexual or sexual reproductive stage, suggesting potential functional divergence among gene family members.

Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon

Eilers J, Miller R, Loomis D, Vogel A. 2023. Effects of cyprinid removal and reintroduction: Diamond Lake, Oregon. Lake Reserv Manage. 39:156–173. Diamond Lake, located in the Oregon Cascade Range, was treated with rotenone in 2006 to remove invasive populations of cyprinids, Gila bicolor and Notemigonus crysoleucas. The treatment successfully removed all fish, and the lake was restocked in 2007 with rainbow trout (Oncorhynchus mykiss). The treatment resulted in large increases in transparency, large cladocerans (Daphnia pulicaria), and benthic invertebrates. The previous cyanobacterial blooms were comprised almost exclusively of Anabaena [Dolichospermum], whereas the current populations of cyanophytes include Gloeotrichia. Cyprinids were reintroduced into the lake and documented in 2008 (Notemigonus crysoleucas) and 2015 (Gila bicolor), likely contributing to a decline of several metrics of water quality. Piscivorous trout (Salmo trutta and Salmo trutta x Salvelinus fontinalis) were introduced into the lake starting in 2016 to control the introduced cyprinids. The cyprinid populations have stabilized, and most metrics of lake status (Secchi disk transparency, phytoplankton biovolume, abundance of large cladocerans, zoobenthic biomass, trout condition factor) indicate that the lake has improved substantially since the treatment and introduction of piscivorous trout. It is unclear whether the cyprinid populations are constrained by behavioral mechanisms associated with the introduction of the piscivorous trout or whether other factors currently keep the cyprinids in check. The success of this biomanipulation project requires continued monitoring and use of adaptive management strategies to respond to changes in fish composition.

Resurrection genomics provides molecular and phenotypic evidence of rapid adaptation to salinization in a keystone aquatic species

Ecologists and evolutionary biologists are increasingly cognizant of rapid adaptation in wild populations. Rapid adaptation to anthropogenic environmental change is critical for maintaining biodiversity and ecosystems services into the future. Anthropogenic salinization of freshwater ecosystems is quickly emerging as a primary threat, which is well documented in the northern temperate ecoregion. Specifically, many northern temperate lakes have undergone extensive salinization because of urbanization and the associated increase in impervious surfaces causing runoff, and the extensive use of road deicing salts (e.g., NaCl). It remains unclear whether increasing salinization will lead to extirpation of species from these systems. Using a "resurrection genomics" approach, we investigated whether the keystone aquatic herbivore, Daphnia pulicaria, has evolved increased salinity tolerance in a severely salinized lake located in Minnesota, USA. Whole-genome resequencing of 54 Daphnia clones from the lake and hatched from resting eggs that represent a 25-y temporal contrast demonstrates that many regions of the genome containing genes related to osmoregulation are under selection in the study population. Tolerance assays of clones revealed that the most recent clones are more tolerant to salinity than older clones; this pattern is concomitant with the temporal pattern of stabilizing salinity in this lake. Together, our results demonstrate that keystone species such as Daphnia can rapidly adapt to increasing freshwater salinization. Further, our results indicate that rapid adaptation to salinity may allow lake Daphnia populations to persist in the face of anthropogenic salinization maintaining the food webs and ecosystem services they support despite global environmental change.

Evolution of population dynamics following invasion by a non-native predator.

Invasive predatory species are frequently observed to cause evolutionary responses in prey phenotypes, which in turn may lead to evolutionary shifts in the population dynamics of prey. Research has provided a link between rates of predation and the evolution of prey population growth in the lab, but studies from natural populations are rare. Here, we tested for evolutionary changes in population dynamics parameters of zooplankton Daphnia pulicaria following invasion by the predator Bythotrephes longimanus into Lake Kegonsa, Wisconsin, US. We used a resurrection ecological approach, whereby clones from pre‐ and post‐invasive periods were hatched from eggs obtained in sediment cores and were used in a 3‐month growth experiment. Based on these data, we estimated intrinsic population growth rates (r), the shape of density dependence (θ) and carrying capacities (K) using theta‐logistic models. We found that post‐invasion Daphnia maintained a higher r and K under these controlled, predation‐free laboratory conditions. Evidence for changes in θ was weaker. Whereas previous experimental evolution studies of predator–prey interactions have demonstrated that genotypes that have evolved under predation have inferior competitive ability when the predator is absent, this was not the case for the Daphnia. Given that our study was conducted in a laboratory environment and the possibility for genotype‐by‐environment interactions, extrapolating these apparent counterintuitive results to the wild should be done with caution. However, barring such complications, we discuss how selection for reduced predator exposure, either temporally or spatially, may have led to the observed changes. This scenario suggests that complexities in ecological interactions represents a challenge when predicting the evolutionary responses of population dynamics to changes in predation pressure in natural systems.

Evolutionary change in metabolic rate of Daphnia pulicaria following invasion by the predator Bythotrephes longimanus.

Metabolic rate is a trait that may evolve in response to the direct and indirect effects of predator‐induced mortality. Predators may indirectly alter selection by lowering prey densities and increasing resource availability or by intensifying resource limitation through changes in prey behavior (e.g., use of less productive areas). In the current study, we quantify the evolution of metabolic rate in the zooplankton Daphnia pulicaria following an invasive event by the predator Bythotrephes longimanus in Lake Mendota, Wisconsin, US. This invasion has been shown to dramatically impact D. pulicaria, causing a ~60% decline in their biomass. Using a resurrection ecology approach, we compared the metabolic rate of D. pulicaria clones originating prior to the Bythotrephes invasion with that of clones having evolved in the presence of Bythotrephes. We observed a 7.4% reduction in metabolic rate among post‐invasive clones compared to pre‐invasive clones and discuss the potential roles of direct and indirect selection in driving this change.

Effect of acute acidic stress on survival and metabolic activity of zooplankton from Lake Biwa, Japan

ABSTRACT Acute acidic stress from sources such as acid rain might impact lake ecosystems in many aspects. Because zooplankton are sensitive to environmental change, they could be a good biotic indicator of acidification effects. We investigated survival and metabolic rates of 3 dominant zooplankton taxa in Lake Biwa, the cladoceran Daphnia pulicaria and the copepods Eodiaptomus japonicus and Cyclopoida spp., when subjected to different pH values (pH 4.0–8.0) at low (10 °C) and high (20 °C) temperatures. Because mortality of D. pulicaria and Cyclopoida spp. exceeded 50% over 24 h of incubation at pH < 4.6, but for E. japonicus at pH < 5.6, E. japonicus may be more vulnerable to acidic stress than the other 2 taxa. This vulnerability to acidification slightly increased at 10 °C for each taxon. Metabolic rates in D. pulicaria remained relatively constant over a wide pH range (4.6–8.0), but those of both copepods decreased at acidic conditions (pH 4.6–7.0). This decrease of metabolic rate might be related to their swimming activity, which decreased in acidic conditions.

Genetic Diversity, Heteroplasmy, and Recombination in Mitochondrial Genomes of Daphnia pulex, Daphnia pulicaria, and Daphnia obtusa.

Genetic variants of mitochondrial DNA at the individual (heteroplasmy) and population (polymorphism) levels provide insight into their roles in multiple cellular and evolutionary processes. However, owing to the paucity of genome-wide data at the within-individual and population levels, the broad patterns of these two forms of variation remain poorly understood. Here, we analyze 1,804 complete mitochondrial genome sequences from Daphnia pulex, Daphnia pulicaria, and Daphnia obtusa. Extensive heteroplasmy is observed in D. obtusa, where the high level of intraclonal divergence must have resulted from a biparental-inheritance event, and recombination in the mitochondrial genome is apparent, although perhaps not widespread. Global samples of D. pulex reveal remarkably low mitochondrial effective population sizes, <3% of those for the nuclear genome. In addition, levels of population diversity in mitochondrial and nuclear genomes are uncorrelated across populations, suggesting an idiosyncratic evolutionary history of mitochondria in D. pulex. These population-genetic features appear to be a consequence of background selection associated with highly deleterious mutations arising in the strongly linked mitochondrial genome, which is consistent with polymorphism and divergence data suggesting a predominance of strong purifying selection. Nonetheless, the fixation of mildly deleterious mutations in the mitochondrial genome also appears to be driving positive selection on genes encoded in the nuclear genome whose products are deployed in the mitochondrion.

The roles of recombination and selection in shaping genomic divergence in an incipient ecological species complex.

Speciation genomic studies have revealed that genomes of diverging lineages are shaped jointly by the actions of gene flow and selection. These evolutionary forces acting in concert with processes such as recombination and genome features such as gene density shape a mosaic landscape of divergence. We investigated the roles of recombination and gene density in shaping the patterns of differentiation and divergence between the cyclically parthenogenetic ecological sister-taxa, Daphnia pulicaria and Daphnia pulex. First, we assembled a phased chromosome-scale genome assembly using trio-binning for D. pulicaria and constructed a genetic map using an F2-intercross panel to understand sex-specific recombination rate heterogeneity. Finally, we used a ddRADseq data set with broad geographic sampling of D. pulicaria, D. pulex, and their hybrids to understand the patterns of genome-scale divergence and demographic parameters. Our study provides the first sex-specific estimates of recombination rates for a cyclical parthenogen, and unlike other eukaryotic species, we observed male-biased heterochiasmy in D. pulicaria, which may be related to this somewhat unique breeding mode. Additionally, regions of high gene density and recombination are generally more divergent than regions of suppressed recombination. Outlier analysis indicated that divergent genomic regions are probably driven by selection on D. pulicaria, the derived lineage colonizing a novel lake habitat. Together, our study supports a scenario of selection acting on genes related to local adaptation shaping genome-wide patterns of differentiation despite high local recombination rates in this species complex. Finally, we discuss the limitations of our data in light of demographic uncertainty.

Local adaptation mediates direct and indirect effects of multiple stressors on consumer fitness.

Anthropogenic impacts are expected to increase the co-occurrence of stressors that can fundamentally alter ecosystem structure and function. To cope with stress, many organisms locally adapt, but how such adaptations affect the ability of an organism to manage co-occurring stressors is not well understood. In aquatic ecosystems, elevated temperatures and harmful algal blooms are common co-stressors. To better understand the role and potential trade-offs of local adaptations for mitigating the effects of stressors, Daphnia pulicaria genotypes that varied in their ability to consume toxic cyanobacteria prey (i.e., three tolerant and three sensitive) were exposed to five diets that included combinations of toxic cyanobacteria, Microcystis aeruginosa, and agreen alga, Ankistrodesmus falcatus, under two temperatures (20°C vs. 28°C). A path analysis was conducted to understand how local adaptations affect energy allocation to intermediate life history traits (i.e., somatic growth, fecundity, survival) that maximize Daphnia fitness (i.e., population growth rate). Results from the 10-day study show that tolerant Daphnia genotypes had higher fitness than sensitive genotypes regardless of diet or temperature treatment, suggesting toxic cyanobacteria tolerance did not cause a decrease in fitness in the absence of cyanobacteria or under elevated temperatures. Results from the path analysis demonstrated that toxic cyanobacteria had a stronger effect on life history traits than temperature and that population growth rate was mainly constrained by reduced fecundity. These findings suggest that local adaptations to toxic cyanobacteria and elevated temperatures are synergistic, leading to higher survivorship of cyanobacteria-tolerant genotypes during summer cyanobacterial bloom events.

Eutrophication‐driven eco‐evolutionary dynamics indicated by differences in stoichiometric traits among populations of Daphnia pulicaria

Abstract Microevolution can have consequences at higher levels of ecological organization. Although divergence among populations can be rapid and driven by anthropogenic changes to the environment, the ecological relevance of evolution induced by human activities remains poorly understood. A frequent way in which human activities drive microevolution is the increase in supply of nutrients such as phosphorus (P) that are required for fitness‐relevant traits such as growth and reproduction. Because higher P concentrations decrease P‐use efficiency and feeding rate in heterotrophic consumers such as Daphnia, we hypothesized that such adjustments should alter consumer–resource dynamics. We examined how cultural eutrophication in temperate lakes causes trait variation in the grazer Daphnia pulicaria. We tested for variation in Daphnia traits and genetic variation in the metabolic enzyme phosphoglucose isomerase (Pgi) which are each known to respond to eutrophication. We then examined the impact of this variation on consumer–resource dynamics using a combination of experiments and a multi‐lake survey. We found that Daphnia from hypereutrophic lakes responded to experimental hypereutrophic conditions with increased growth rates and fecundity when raised on P‐fertilized seston, but had reduced performance on P‐poor seston relative to eutrophic source Daphnia. These results suggest that Daphnia may face a trade‐off in performance at low versus excess P that may be mediated in part by genetic variation at the Pgi locus. The variation observed in laboratory growth experiments scaled up to Daphnia populations in both mesocosm experiments and among lakes. In both the mesocosm experiment and in the lake survey, Daphnia from hypereutrophic source lakes reached high biomass while phytoplankton biomass also remained high. Given the prevalence and rapid eutrophication of freshwater ecosystems worldwide, these results indicate that considering the potential effects of evolutionary change in ecosystem models could be useful in forecasting the effects of anthropogenic environmental change on pivotal ecosystem services.

Genome-Wide Allele-Specific Expression in Obligately Asexual Daphnia pulex and the Implications for the Genetic Basis of Asexuality.

Although obligately asexual lineages are thought to experience selective disadvantages associated with reduced efficiency of fixing beneficial mutations and purging deleterious mutations, such lineages are phylogenetically and geographically widespread. However, despite several genome-wide association studies, little is known about the genetic elements underlying the origin of obligate asexuality and how they spread. Because many obligately asexual lineages have hybrid origins, it has been suggested that asexuality is caused by the unbalanced expression of alleles from the hybridizing species. Here, we investigate this idea by identifying genes with allele-specific expression (ASE) in a Daphnia pulex population, in which obligate parthenogens (OP) and cyclical parthenogens (CP) coexist, with the OP clones having been originally derived from hybridization between CP D. pulex and its sister species, Daphnia pulicaria. OP D. pulex have significantly more ASE genes (ASEGs) than do CP D. pulex. Whole-genomic comparison of OP and CP clones revealed ∼15,000 OP-specific markers and 42 consistent ASEGs enriched in marker-defined regions. Ten of the 42 ASEGs have alleles coding for different protein sequences, suggesting functional differences between the products of the two parental alleles. At least three of these ten genes appear to be directly involved in meiosis-related processes, for example, RanBP2 can cause abnormal chromosome segregation in anaphase I, and the presence of Wee1 in immature oocytes leads to failure to enter meiosis II. These results provide a guide for future molecular resolution of the genetic basis of the transition to ameiotic parthenogenesis.

Draft genomes for one Microcystis-resistant and one Microcystis-sensitive strain of the water flea, Daphnia pulicaria.

Daphnia species are well-suited for studying local adaptation and evolutionary responses to stress(ors) including those caused by algal blooms. Algal blooms, characterized by an overgrowth (bloom) of cyanobacteria, are detrimental to the health of aquatic and terrestrial members of freshwater ecosystems. Some strains of Daphnia pulicaria have demonstrated resistance to toxic algae and the ability to mitigate toxic algal blooms. Understanding the genetic mechanism associated with this toxin resistance requires adequate genomic resources. Using whole-genome sequence data mapped to the Daphnia pulex reference genome (PA42), we present reference-guided draft assemblies from one tolerant and one sensitive strain of D. pulicaria, Wintergreen-6 (WI-6), and Bassett-411 (BA-411), respectively. Assessment of the draft assemblies reveal low contamination levels, and high levels (95%) of genic content. Reference scaffolds had coverage breadths of 98.9–99.4%, and average depths of 33X and 29X for BA-411 and WI-6, respectively. Within, we discuss caveats and suggestions for improving these draft assemblies. These genomic resources are presented with a goal of contributing to the resources necessary to understand the genetic mechanisms and associations of toxic prey resistance observed in this species.

Calcium stress in Daphnia pulicaria and exposure to predator-derived cues: Making a bad situation worse?

Calcium concentrations ([Ca]) in some boreal lakes on the Canadian Shield have been declining over the previous couple of decades. With [Ca] in many lakes below 2 mg Ca/L, there are concerns about possible ecological effects of sustained Ca limitation on zooplankton assemblages. Here we examine whether short- and long-term effects of Ca stress on a keystone herbivore, Daphnia pulicaria, change with the presence of an invertebrate predator, Chaoborus flavicans. We did so by testing whether effects of low Ca on Daphnia pulicaria differ in the presence and absence of kairomones derived from C. flavicans. We compared mass-specific growth rates, body %Ca content, life-history traits, and survival of Daphnia exposed to a range of [Ca] with or without Chaoborus kairomones. We found a mix of individual and interactive effects of kairomones and low Ca supply on Daphnia responses. With longer and chronic exposure to both stressors, the effects of low Ca were intensified by the presence of kairomones, particularly for several reproductive measures (interbrood duration, mean brood size, and number of broods). Given the ubiquity of Chaoborus in softwater boreal lakes, our results show the need to consider their presence when assessing possible effects of low [Ca] on zooplankton assemblages.

Outcome of competition between two Daphnia species in the absence of predators: laboratory experiment findings support field observations

Zooplankton communities are typically comprised of smaller-bodied species when size-selective fish predators are abundant, but become dominated by large-bodied species when fish predators are scarce. Superiority by larger-bodied grazers over smaller-bodied species in competition for algal resources has been proposed to be the mechanism responsible for this observed pattern. To investigate this mechanism, we performed a laboratory experiment with two freshwater zooplankton species, the larger-bodied Daphnia pulicaria and smaller-bodied D. mendotae, obtained from Square Lake (Washington County, Minnesota). The Daphnia species were grown in monoculture and in combination over a 24-day period to assess the outcome of competition between the two species and their effect on algae cell densities. We hypothesized that the larger-bodied D. pulicaria species would outcompete D. mendotae, and that D. pulicaria would exert greater control on algae levels than would D. mendotae. Results of the experiment strongly supported these hypotheses, and were consistent with findings of a recently completed field study of Square Lake that discovered that terminating the program of stocking rainbow trout (a zooplanktivorous predator) in the lake resulted in D. pulicaria replacing D. mendotae as the dominant Daphnia species and in the reduction of algae levels in the lake's surface waters.

The rapid, mass invasion of New Zealand by North American Daphnia "pulex".

Nonindigenous members of the Daphnia pulex complex have been found in many lakes in New Zealand (NZ) in the past 20 years, suggesting a recent invasion. However, very little is known about the precise phylogenetic origin of invasive Daphnia, whether each lake is invaded by a single clone or multiple clones, the lineage of the invasive clones, and whether they are obligately asexual clones. Furthermore, the source and time of arrival of the invasive genotype(s) are unclear. We address these questions by genomic sequencing of Daphnia populations from 13 lakes in the South Island and 1 lake in the North Island, NZ. All biallelic sites in these NZ populations have similar numbers of reads for the two parental alleles, suggesting each NZ population originates from a single asexual clone. Based on 25,643 monomorphic lineage-specific markers, the invasive Daphnia in the South Island were found to be Daphnia pulicaria Forbes, while those in the North Island are hybrids of D. pulicaria Forbes and D. cf. pulex sensu Hebert. Both the South and North Island Daphnia are phylogenetically clustered with North American Daphnia, thereby suggesting their North American origins. We found also that all South Island clones contain identical mitochondrial genomes, suggesting the origin and proliferation from a single founder clone, which we experimentally verified to be an obligate asexual. Estimates from molecular data imply a colonization time for the South Island clones of ~60 years ago, with a likely invasion route associated with the introduction of salmonids from North America.

Quantitative genetics of phosphorus content in the freshwater herbivore, Daphnia pulicaria.

Phosphorus (P) is essential for growth of all organisms, and P content is correlated with growth in most taxa. Although P content was initially considered to be a trait fixed at the species level, there is growing evidence for considerable intraspecific variation. Selection on such variation can thus alter the rates at which P fluxes through food webs. Nevertheless, prior work describing the sources and extent of intraspecific variation in P content were not genetically explicit, confounded by unknown genetic background and evolutionary history. We constructed an F2 recombinant population of the dominant freshwater grazer, Daphnia pulicaria to mitigate such issues. F2 recombinants exhibited considerable variation in growth rate, P content (0.49%-1.97%), P use efficiency (PUE; 51-208mg biomass/mg P), and correlated traits such as hatching time of resting eggs, in common garden conditions. These results clearly demonstrate the scope of genetic recombination in generating variation in ecologically relevant traits. The absence of environmental selection is a likely component driving such variation not observed in natural settings. Although phosphoglucose isomerase (PGI) genotype was significantly associated with variation in hatching time of resting eggs, contrary to prior work with less rigorous designs, and allelic variation at the PGI locus did not explain variation in P content and PUE of Daphnia, indicating that such quantitative traits are under polygenic control. Together, these results suggest that although there is considerable genetic scope for variation in key ecologically relevant traits, such as P content and efficiency of P use, these traits are likely under strong stabilizing selection, most likely due to selection on growth rate and size. Importantly, our observations suggest that anthropogenic alterations to P supply due to eutrophication could alter selection on these traits, thereby rapidly altering the role Daphnia plays in the P cycle of lakes.