Daphnia hybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton

The relative homogeneity of pelagic environments has been regarded as the reason for the absence of hybrid zones for hybridizing planktonic Daphnia (Crustacea: Cladocera); occasional dominance of interspecific hybrids over parental species was explained by their temporal superiority in fluctuating environments. However, water bodies with spatially varying environmental conditions might facilitate the formation of hybrid zones in plankton. We studied the distribution of species and hybrids of the Daphnia longispina complex in 11 canyon-shaped reservoirs, localities characterized by horizontal environmental gradients (particularly of food supply and size-selective predation); we also analysed patterns of carapace size and fecundity among coexisting taxa. Spatial distribution of taxa agreed with their ecological characteristics; those showing different affinities along longitudinal reservoir profiles differed in size according to the presumed fish predation gradient. Only hybrids of Daphnia galeata with Daphnia cucullata and D. longispina (=hyalina) were recorded. The latter two species preferred opposite ends of gradients, such spatial segregation probably explaining the absence of their hybrids. Distributional patterns were relatively stable in two consecutive summers, apart from a substantial decline of D. galeata X cucullata in the second year. The observed pattern of a hybrid-dominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.     

Discrimination of hybrid classes using cross-species amplification of microsatellite loci: methodological challenges and solutions in Daphnia

Microsatellite markers are important tools in population, conservation and forensic studies and are frequently used for species delineation, the detection of hybridization and introgression. Therefore, marker sets that amplify variable DNA regions in two species are required; however, cross-species amplification is often difficult, as genotyping errors such as null alleles may occur. To estimate the level of potential misidentifications based on genotyping errors, we compared the occurrence of parental alleles in laboratory and natural Daphnia hybrids (Daphnia longispina group). We tested a set of 12 microsatellite loci with regard to their suitability for unambiguous species and hybrid class identification using F(1) hybrids bred in the laboratory. Further, a large set of 44 natural populations of D. cucullata, D. galeata and D. longispina (1715 individuals) as well as their interspecific hybrids were genotyped to validate the discriminatory power of different marker combinations. Species delineation using microsatellite multilocus genotypes produced reliable results for all three studied species using assignment tests. Daphnia galeata × cucullata hybrid detection was limited due to three loci exhibiting D. cucullata-specific null alleles, which most likely are caused by differences in primer-binding sites of parental species. Overall, discriminatory power in hybrid detection was improved when a subset of markers was identified that amplifies equally well in both species.     

Interspecific hybridization in Daphnia: distinction and origin of hybrid matrilines

Three coexisting Daphnia species belonging to the D. longispina group (D. galeata, D. hyalina, and D. cucullata) form species-hybrid complexes by producing interspecific hybrids in several lakes in Germany and The Netherlands. To evaluate the genetic consequences of interspecific hybridization, I studied the patterns of mitochondrial DNA (mtDNA) sequence variation. The directionality of interspecific hybridization and divergence of hybrids from parental species was tested, using the DNA sequences of a segment of mtDNA. Via the polymerase chain reaction, it was possible to investigate single animals and even single resting eggs. A species-specific marker was established, using restriction patterns of amplified cytochrome b segments. mtDNA genotypes of hybrids revealed unidirectional mitochondrial gene flow for two hybrids, which were investigated by using multiple clones. No evidence for introgression of mtDNA was found. On the basis of a phylogenetic analysis, the species exhibit considerable distinctness, whereas differences between clones within species and between hybrids and maternal species tend to be very low. These results indicate a recent origin of hybrids and suggest that the radiation of the D. longispina group occurred > 5 Mya.      

Molecular genetic diagnostics of some Daphnia species (Crustacea,Cladocera) from the Volga river

Population genetic structure of the widespread Daphnia species from Kuibyshev and Saratov reservoirs was examined by use of RAPD-PCR technique with four mitochondrial DNA (16S rRNA) specific primers. One of the examined Daphnia clones from the Volga region appeared to be conspecific to the North American Daphnia galeata, while another clone was most likely a hybrid between D. galeata and D. cucullata.     

Genetic assessment of species boundaries in the North American Daphnia longispina complex (Crustacea: Daphniidae)

Species boundaries in the North American Daphnia longispina group have proved difficult to establish on the basis of morphology alone. This confusion may be due to hydridization, phenotypic plasticity or the existence of sibling species. We therefore used genetic analysis to delineate species boundaries by examining 27 North American populations belonging to the longispina complex for variation at 15–26 allozyme loci. The populations consisted of Daphnia thorata from two western sites and two eastern sites, Daphnia galeata mendotae from its type location and seven sites across its range, and Daphnia rosea from eight temperate and seven arctic sites. Two populations from the Eurasian longispina complex were also included for reference. Populations assigned to D. galeata mendotae formed a genetically cohesive group, whereas a genetic dichotomy was found between temperate and arctic D. rosea , suggesting that this taxon includes two species. Genetic analysis also confirmed the distinctness of western D. thorata from other members of the longispina group. Unexpectedly, eastern populations resembling D. thorata were genetically more similar to temperate D. rosea than to any helmeted species ( D. galeata, Daphnia hyalina or D. thorata ). Our results suggest that the helmet character is a poor indicator of phylogenetic relationships, as the genetic ability to produce this feature has been lost or acquired several times in the evolution of the longispina group.     

Respiratory electron transport system (ETS) activity as an estimator of the thermal tolerance of two Daphnia hybrids

The influence of temperature on the activity of the respiratoryelectron transport system (ETS) was measured in one clone ofDaphnia hyalina x galeata and one of Daphnia cucullata x galeata,isolated from Lake Bled (Slovenia). The ETS activity of ovigerousfemales acclimated to 7, 20 and 25°C, was measured at 5,10, 15, 20, 25 and 30°C. Population growth experiments showedthat D. cucullata x galeata grew better at high rather thanlow temperatures. Daphnia hyalina x galeata, however, grew moresuccessfully at low temperatures than did D. cucullata x galeata.The highest Q₁₀ of ETS activity of D. cucullata x galeata atthe lowest temperature range of 5–15°C indicated theabsence of enzymes that could function sufficiently well atlow temperatures. The ETS activity of the warm-acclimated hybridD. hyalina x galeata reached a maximum at an incubation temperatureof 20°C, while D. cucullata x galeata had maximal ETS activityat 25°C. Thus D. cucullata x galeata has a more efficientenzyme system than D. hyalina x galeata at the higher temperature.The higher Arrhenius activation energy (E_a) for D. cucullatax galeata than for D. hyalina x galeata indicates that enzymesfrom D. cucullata x galeata are more temperature sensitive thanthose from D. hyalina x galeata. In conclusion, the ETS of D.cucullata x galeata is adapted to a higher temperature and tonarrower temperature fluctuations than that of D. hyalina xgaleata.     

Hybridization in the Daphnia galeata complex: are hybrids locally produced?

Within the species complex of Daphnia galeata,D. cucullata and D. hyalina variouscombinations of hybrids and parental taxa occur inlakes throughout Europe. Since daphnids are cyclicparthenogens and mostly reproduce asexually, hybridpopulations can be maintained by asexual reproductionand without recurrent hybridization events. Therefore,it is possible that hybridization events have beenrare, with range expansion occurring by dispersal ofhybrids.Allozyme data from seven European populations wereused to compare genetic variation within and betweenhybrid and parental taxa. An UPGMA cluster analysis ofgenetic distances showed that D. cucullata × galeatahybrids from different lakes grouped indifferent clusters according to the lake from whichthey were isolated, suggesting multiple hybridizationevents. Clonal diversity within hybrid taxa wascomparable to parental taxa. Furthermore, evidence wasfound for introgression of the Pgi-S allele fromD. cucullata to D. galeata in three lakes.These results indicate that multiple hybridizationevents within this species complex are likely, andthat hybrid taxa can reproduce sexually.     

Predator-induced life-history changes and the coexistence of five taxa in a Daphnia species complex

Interspecific hybridization is common in water fleas of the Daphnia galeata species complex (e.g. D. galeata , D. cucullata , D. hyalina and their interspecific hybrids). We studied the effect of fish on the life histories of five taxa of this species complex originating from the Plußsee, northern Germany. Using four clones per taxon, we found that fish kairomones reduce size at birth and size at maturity significantly. For size at maturity larger taxa showed a significantly stronger reaction to fish kairomones than the smaller taxa. With respect to the intrinsic rate of increase, r , we compared two predation regimes (positive size selective and not selective). We found that under the fish predation regime most clones had a higher r when cultured with fish kairomones, leading to a higher r , and a stronger reaction for the smaller taxa. We conclude that fish predation might be an important factor influencing the co-occurrence of Daphnia parental taxa with their hybrids.     

Modes and mechanisms of a Daphnia invasion

Whether exotic species invade new habitats successfully depends on (i) a change in the invaded habitat that makes it suitable for the invader and (ii) a genetic change in the invading taxon that enhances its fitness in the new habitat, or both. We dissect the causes of invasions of Swiss lakes, north of the Alps, by Daphnia galeata (a zooplankter typical of eutrophic lakes, e.g. those south of the Alps, which are also warmer) by comparing the fitness performance of eight geographically distributed clones that were fed algal-food typical of oligotrophic versus eutrophic conditions at two temperatures. Daphnia longispina, native to oligotrophic Swiss lakes, served as a reference. Daphnia galeata requires eutrophic food to persist, whereas D. longispina survives and grows on oligotrophic food but does even better on eutrophic food. Invasion by D. galeata is further explained because invading clones from the north perform better on eutrophic food and at cooler temperatures than native clones from the south, suggesting a local response to countergradient selection. Our data support the hypothesis that populations of the invader in northern lakes are dominated by well-adapted genotypes. Our results illustrate how environmental change (i.e. eutrophication) and local adaptation can act together to drive a successful invasion.     

Analysis of reticulate relationships within the Daphnia longispina species complex. Allozyme phenotype and morphology

A two-step method is proposed to get reliable associations between morphology and genotype in clonal assemblages in which more than two predominantly parthenogenetic species are thought to coexist with hybrids. In dataset 1, the genetic relationships among clones of the Daphnia longispina hybrid complex from seven prealpine lakes in southern Germany were studied based on the variation at 21 enzyme loci. The spatial arrangement in the multidimensional scaling plot revealed a reticulate pattern among three presumably parental species, D. cucullata, D. galeata and D. hyalina, and three hybrid groups, D. cucullata/galeata, D. cucullata/hyalina and D. galeata/hyalina. The Got1 locus was believed to be a discriminating factor between species and hybrids (cf. Wolf and Mort, 1986). However, this locus is more variable, and 57% of the clones would have been misidentified using it. Moreover, the morphological variation within genetically defined groups is also higher than previously assumed. In dataset 2, the revision of morphological and genetic markers greatly improved the association between morphology and genotype in newly collected animals. The spatial arrangement of clones in multidimensional scaling plots and morphological asymmetries to parents suggest both, different degrees of introgression and bidirectional hybridization. Most unexpected genotypes were found in the cxh hybrid group, suggesting that F1-hybrids are fertile. The results showed (1) that the clonal diversity was very high (2) that detailed analyses of multiple morphological and allozyme markers are necessary to resolve taxonomic relationships within clonal assemblages consisting of multiple species, hybrids and differently introgressed backcrosses, and (3) that the three original species seem to have sufficient within-species recombination and a low enough rate of backcrossing to allow taxonomic identification. It must remain undecided if the present situation is locally restricted, if it is stable or represents a transient situation which could lead to either a consolidation of the three species by gradual elimination of the hybrids, to a taxonomic breakdown, or to hybrid speciation.     

Calcium content of crustacean zooplankton and its potential role in species distribution

SUMMARY 1. The occurrence and species composition of crustacean zooplankton in Norwegian lakes was related to ambient Ca concentrations, pH, total phosphorus and chlorophyll a , and to the presence/absence of other zooplankters and a planktivorous fish (roach: Rutilus rutilus ).
2. Ambient Ca concentrations appeared to influence the distribution of Daphnia species: Daphnia longispina often occurred in Ca-rich lakes with low fish-predation pressure while the smaller Daphnia cristata often occurred in opposite conditions.
3. Body Ca contents were measured in zooplankters from a wide range of localities, to examine Ca requirements and thus the potential for Ca-limitation of common species.
4. All Daphnia species had relatively high specific Ca contents [ranging from 0.8 to 4.4% Ca dry weight (DW)⁻¹] compared with other cladocerans and also copepods (ranging from 0.1 to 0.4% Ca DW⁻¹). Within the Daphnia genus, the specific Ca content increased with increasing body size of the species, and thus the large-bodied species had especially high Ca demands.
5. Because of their high Ca demands, species of Daphnia could be competitively disadvantaged in softwater lakes relative to less Ca-demanding species.     

Genetic markers, genealogies and biogeographic patterns in the cladocera

Cladoceran crustaceans are an important component of zooplankton in a wide range of freshwater habitats. Although the ecological characteristics of several cladoceran species have been well studied, biogeographical studies have been hampered by problematic taxonomic affiliations. However, recently developed molecular techniques, provide a powerful tool to subject aquatic taxa to comparative analyses. Here we highlight recent molecular approaches in aquatic ecology by presenting a simple method of DNA preparation and PCR amplification of the mitochondrial DNA (16S rDNA) in species from nine different families within the cladocera. On a broad taxonomic scale, sequence analysis of this mtDNA fragment has been used to produce the first molecular based phylogeny of the cladocera. This analysis clustered the cladoceran families in a fashion similar to that suggested by previous systematic classifications. In a more detailed analysis of the family Daphniidae, nuclear randomly amplified polymorphic DNA (RAPD), mitochondrial restriction fragment length polymorphism (RFLP) and morphological analyses were combined to identify species and interspecific hybrids within the Daphnia galeata species complex across 50 lakes in 13 European countries and one lake in Africa. The study revealed interspecific hybridization and backcrossing between some taxa (D. cucullata and D. galeata) to be widespread, and species and hybrids to frequently occur in sympatry. Genetic, as well as morphological information, suggests the occurrence of D. hyalina outside the Holarctic.     

西藏湖泊长刺溞复合种的系统进化关系

本文通过对西藏湖泊长刺溞复合种(Daphnia longispina complex)中分布最广的3个物种, 即长刺溞(D. longispina)、盔形溞(D. galeata)和颈齿溞(D. dentifera)线粒体COI基因序列以及GenBank中欧洲的长刺溞、加拿大的颈齿溞和我国东部低海拔地区的盔型溞COI基因序列的比较分析, 研究了西藏湖泊长刺溞复合种的系统进化关系, 发现西藏地区的盔型溞、颈齿溞和长刺溞均已出现较大分化。颈齿溞种群内遗传差异度为0.33-2.32%, 盔型溞为0.33-2.74%, 长刺溞的遗传差异度最高, 为1.31-5.50%。基于COI基因序列构建的最大似然树和贝叶斯系统树均表明, 长刺溞复合种由3个进化分支组成, 分别对应长刺溞、盔型溞和颈齿溞, 三者之间的遗传差异度为9.40-16.98%(Kimura 2-parameter双参数模型)。基于COI基因单倍型(haplotype)所构建的网络关系也支持上述3个分支的存在。早期记录虽然显示长刺溞在我国分布较广, 但本次调查只在班公错有发现, 相比之下, 盔形溞和颈齿溞则分布更广。我们的研究表明, 由于形态学鉴定上的局限性, 早期的长刺溞记录很可能混杂了容易引起混淆的盔型溞或颈齿溞。     

Life-history variation in a hybrid species complex ofDaphnia

Summary Life-history variation was examined among members of theDaphnia longispina group, which consists ofD. galeata, D. hyalina, D. cucullata, and hybrids. Factorial experiments were conducted at two temperatures (14° and 20° C) and two food concentrations (0.2 and 1.0 mg Cl⁻¹). Differences in life-history features (size at maturity, age at first reproduction, size of first clutch, offspring size in first clutch) under the different environmental conditions were assessed among eightDaphnia clones, which represented members of this species complex. Significant differences between parentals and hybrids for most life-history features were observed under various treatments; generally, hybrid clones showed intermediate life-history traits when compared with parentals. When comparisons were made among clones within a given species (i.e.D. galeata, D. galeata xcucullata, D. cucullata), clonal differences were also noted for certain life-history traits. The data are discussed with reference to the formation and maintenance of hybrid species complexes in nature. This paper is dedicated to the memory of Hans Georg Wolf, who died suddenly in May 1990 at the age of 39 years.     

Fine-scale genetic analysis of Daphnia host populations infected by two virulent parasites - strong fluctuations in clonal structure at small temporal and spatial scales

Numerous theoretical studies suggest that parasites impose a strong selection pressure on their host, driving genetic changes within host populations. Yet evidence of this process in the wild is scarce. In the present study we surveyed, using high resolution microsatellite markers, the genetic structure of cyclically parthenogenetic Daphnia hosts within two different Daphnia communities belonging to the Daphnia longispina hybrid complex. One community, consisting of a single host species, was infected with the protozoan parasite Caullerya mesnili. The second community consisted of two parental Daphnia spp. and their hybrids, and was infected with the yeast parasite Metschnikowia. Significant differences in the clonal composition between random and infected sub-samples of Daphnia were detected on several occasions within both communities, indicating that host genotypes differ in resistance to both parasites. In addition, one parental species in the multi-taxon community was consistently under-infected, compared with the other taxa. Overall, our field data confirm that infection patterns are strongly affected by host genetic composition in various Daphnia-microparasite systems. Thus, parasite-driven selection operates in natural Daphnia populations and microparasites influence the clonal structure of host populations.     

Gill raker structure and selective predation on zooplankton by particulate feeding fish

The relationship between the gill raker structure of planktivorous fish (number, distance between gill rakers and length) and selective feeding on different species and size classes of zooplankton was studied. Gill raker structure was measured for brown trout Salmo trutta , Arctic charr Salvelinus alpinus , whitefish Coregonus lavaretus , roach Rutilus rutilus , bleak Alburnus alburnus , and three-spined stickleback Gasterosteus aculeatus . All species are facultative planktivorous fish and occur commonly in Scandinavian lakes. The effect of gill raker structure was studied by comparing prey found in fish stomachs with the availability of zooplankton from several lakes. Gill raker length and distance were significantly correlated with fish length. Although gill raker structure differed among species, all fish species selected the larger zooplankters. The minimum size of cladoceran species found in fish stomachs was much smaller than the distance between gill rakers. Despite great differences in gill raker spacing, the minimum size ingested of Daphnia galeata and Bosmina longispina was similar for all predators. The hypothesis that small zooplankton are strained and retained by the gill rakers in particulate feeding planktivorous fish, particularly in salmonids and roach, is rejected.     

三峡水库蓄水后期香溪河库湾浮游甲壳动物群落结构

为了解三峡水库蓄水后期香溪河库湾浮游甲壳动物群落结构及其时空变化, 于2015年1月至2017年12月对香溪河库湾浮游甲壳动物进行了逐月采样分析。共鉴定出浮游甲壳动物23种, 其中枝角类10种, 桡足类13种。出现频率较高的物种均为长江流域常见种, 生物量和密度峰值分别出现在5月和6月, 在1月和2月则未能采集到标本, 生物量和密度在年际间差异显著, 但群落组成差异不显著, 群落季节变化规律表现为春季-夏季以蚤状溞(Daphnia pulex)和僧帽溞(Daphnia cucullata)大型枝角类占优势, 秋季以简弧象鼻溞(Bosmina coregoni)小型种占优势。库中生物多样性指数高于库首和库尾, 库首和库尾生物多样性指数差异不明显, 生物多样性在季节上同样具有一定差异。CCA分析表明水温、叶绿素浓度和水深是解释浮游甲壳动物群落变化重要因素。我们的研究结果表明尽管香溪河库湾营养盐、水温、浮游甲壳动物生物量和密度均未达到稳定状态, 但群落组成在年际间无显著性差异, 浮游甲壳动物物种丰富度也高于蓄水初期, 生物多样性指数存在一定的时空差异, 适当程度的干扰有利于维持浮游甲壳动物较高的生物多样性。     

Natural distribution and co-infection patterns of microsporidia parasites in the Daphnia longispina complex

Microsporidia are intracellular parasites, frequently infecting the planktonic crustacean Daphnia. Questioning the ability to detect and identify microsporidia with conventional microscopic techniques, we applied molecular methods in order to investigate the distribution and co-infection patterns of this parasite among 8 communities of the Daphnia longispina hybrid complex. Eight microsporidian taxa were detected, including 3 that previously had not been characterized genetically. Microsporidian communities from nearby lakes were found to be more similar to each other, apparently due to short distance dispersal via secondary hosts. Moreover, we detected seasonal (but not interannual) changes in microsporidian community structure. With some microsporidia being host-specific, these changes might have resulted from seasonal changes in host taxon and clonal composition. The 2 dominant and closely related parasite species were found mainly in single infections, whereas another pair of related microsporidians was found predominantly in co-infections; suggesting species-level differences in the ability to colonize infected hosts. By applying molecular methods, we were not only able to unambiguously identify parasite taxa but also to reveal multiple infections that otherwise would have remained undetected. Given the increased level of accuracy and sensitivity, we highly recommend molecular approaches in future parasite surveys of Daphnia infections.     

Long-term responses of zooplankton to invasion by a planktivorous fish in a subarctic watercourse

1. Introduced or invading predators may have strong impacts on prey populations of the recipient community mediated by direct and indirect interactions. The long-term progression of predation effects, covering the invasion and establishment phase of alien predators, however, has rarely been documented.
2. This paper documents the impact of an invasive, specialized planktivorous fish on its prey in a subarctic watercourse. Potential predation effects on the crustacean plankton, at the community, population and individual levels, were explored in a long-term study following the invasion by vendace ( Coregonus albula ).
3. Over the 12-year period, the density and species richness of zooplankton decreased, smaller species became more abundant and Daphnia longispina , one of the largest cladocerans, was eliminated from the zooplankton community.
4. Within the dominant cladocerans, including Daphnia spp., Bosmina longispina and Bosmina longirostris , the body size of ovigerous females and the size at first reproduction decreased after the arrival of the new predator. The clutch sizes of Daphnia spp. and B. longirostris also increased.
5. Increased predation pressure following the vendace invasion induced many effects on the crustacean zooplankton, and we document comprehensive and strong direct and indirect long-term impacts of an introduced non-native predator on the native prey community.