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.     

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.     

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.     

Allochronic differentiation among Daphnia species, hybrids and backcrosses: the importance of sexual reproduction for population dynamics and genetic architecture

Seasonal dynamics of the abundance, sexual reproduction and genetic architecture in a Daphnia hyalina-galeata hybrid complex were studied in the large and deep Lake Constance. We found evidence for the occurrence of first and second order hybridization. Our study revealed strong differences between the parental species not only regarding their seasonal dynamics, genetic architecture and diversity, but also their sexual reproductive behaviour. The overwintering D. hyalina showed low genetic diversity, no genetic differentiation during the season, and reproduced sexually in autumn, whereas D. galeata reached higher levels of genetic diversity, reproduced sexually in early summer, and exhibited changes in genetic structure during the season, but was only present from spring to autumn. However, in both species sexual reproduction was a rare event, and daphnids, including hybrids, reproduced predominantly asexually. This allows long-term persistence of hybrids as well without continuing hybridization events. Within all variables studied, F1 and F2 hybrids showed an intermediate pattern, whereas proposed backcross hybrids were more similar to their respective parentals. These differences in phenotype as well as significant differences in pairwise Fst values between parentals suggest that gene flow seems to be relatively low in the Lake Constance hybrid system. We found evidence for unidirectional introgression by backcrossing from D. galeata to D. hyalina and found a decrease in at least one of the proposed introgressed alleles in the hyalina-backcross while the season progressed. Our findings suggest allochronic differentiation within this hybrid population and different microevolutionary trajectories of the parental species, which will be discussed in the light of the ongoing reoligotrophication process of Lake Constance.     

Spatial distribution of the Daphnia longispina species complex and other planktonic crustaceans in the heterogeneous environment of canyon-shaped reservoirs

Canyon-shaped reservoirs are often characterised by longitudinalgradients of environmental factors (including trophic level,phytoplankton and zooplankton biomass and abundance of planktivorousfish) affecting the taxonomic composition of the pelagic community.We tested the hypothesis that the spatial distribution of differentspecies and interspecific hybrids of the Daphnia longispinaspecies complex is non-random under such conditions. Duringthe summer stratification, we sampled crustacean zooplanktonfrom 11 reservoirs, covering both longitudinal (upstream, middle,dam) and vertical (epi-, meta- and hypolimnion) environmentalgradients. Allozyme electrophoresis was used to discriminateamong different Daphnia taxa. All three frequently hybridizingEuropean species of the complex (galeata, cucullata, longispina= hyalina) and hybrids with Daphnia galeata were commonly recorded.Smaller-bodied Daphnia cucullata and its hybrids, when present,preferred mostly the nutrient- and food-rich upstream regions;D. longispina and its hybrids were more commonly found in thedownstream part, and often dominated in the meta- or hypolimnion.Redundancy analyses confirmed significant differences in theDaphnia taxon composition, as well as in spatial distributionof other crustacean species, along both gradients. For the firsttime, we demonstrate regular patterns in the horizontal distributionof Daphnia species and hybrids within a water body, thus acceptingour hypothesis. Such spatial distributional patterns may stronglyimpact local hybridization processes.     

Differences in genetic structure and ecological diversity between parental forms and hybrids in a Daphnia species complex

Hybridization is a common phenomenon in Daphnia species complexes. Hybrids often dominate in Daphnia populations; therefore it is worthwhile to look for principal differences between parental and hybrid populations with respect to their genetic structure and clonal differentiation. We studied natural populations of members of the Daphnia galeata/hyalina/cucullata complex in three lakes. In one of these lakes, one parental species (D. galeata) and one hybrid (D. galeata × cucullata) were investigated more intensively. The frequency of sexual reproduction was higher in parental populations, whereas clonal diversity was higher in hybrid populations. Ecological differentiation among clonal groups was more pronounced in the D. galeata × cucullata hybrid compared to D. galeata, whereas selection intensity was weaker. These results are discussed with respect to stability of clonal groups, multiple hybridizations and selective constraints.     

A taxonomic reappraisal of the Daphnia hyalina complex (Crustacea: Cladocera): an experimental and ecological approach

The paper is a taxonomic study of the limnetic Daphnia populations of the English Lake District, the populations upon which the most popular taxonomic scheme is based (Johnson, 1952).
Johnson treats these populations as three varieties of the species D. hyalina, namely D. hyalina s. str., D.h. var. galeata and D.h. var. lacustris .
Careful examination of the populations of Esthwaite reveals that what has previously been regarded as a population of a single variety of D. hyalina. includes three distinct forms. Experimental work confirms that these are genetically distinct. One of these forms corresponds to D. hyalina var. galeata , whilst the other two are designated the "toothed" and "round-headed" forms.
The first part of the paper is concerned with the taxonomic status of var. galeata . Brooks (1957) and some later workers treat this as a separate species (D. galeata) on morphological grounds. This is fully supported by the present work. Differences between var. galeata and co-existing forms in incidence of sexual reproduction, in vertical distribution and in seasonal abundance and average brood-size provide further support for the separation of D. galeata.
The second part of the paper is concerned with D. hyalina s.str. which, on the basis of morphology, ecology and reproductive behaviour, is shown to be at least as distinct from other members of the complex as is D. galeata . It is argued that other forms should therefore to be removed from D. hyalina . The taxonomic status and affinities of the remaining "toothed" and "round-headed" forms are discussed.     

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.     

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.     

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.     

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.     

基于16S rDNA和COⅠ基因序列探讨四种溞类的系统关系和分类地位

为了检验已记录的4个溞属种类（(隆线溞Daphnia carinata、透明溞D. hyalina、蚤状溞D. pulex和大型溞D. magna）的系统分类, 用试剂盒法分别提取4种溞类的基因组DNA。利用特异性引物, 通过PCR扩增了4种溞属种类的16S rDNA和COⅠ基因部分序列, 并与来自GenBank中每个种类相似度较高的同种序列进行分析。结果表明, 基于16S rDNA和COⅠ基因, 4个溞属种类的平均种间相似度分别为85.56%和80.67%, 碱基中A+T含量均明显高于G+C含量。基于COⅠ基因, 巢湖隆线溞与来自GenBank上的拟同形溞（(Daphnia similoides AB549199）)相似度为99%, 分歧度为0.4%0.9%。就16S rDNA和COⅠ基因而言, 巢湖透明溞更接近于盔形溞（Daphnia galeata）。基于16S rDNA和COⅠ基因构建的NJ树和贝叶斯树, 两者的主要分枝基本一致。结果暗示, 巢湖隆线溞与拟同形溞为一个种, 透明溞应属于盔形溞。由于缺乏核基因的研究, 有关巢湖溞属的分类地位还需进行更深入的探讨。       

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Resting eggs of lake-Daphania II. In situ observations on the hatching of eggs and their contribution to population and community structure

SUMMARY. 1. To investigate the contribution of hatchlings from lake- Daphnia resting eggs (ephippia) to population and community structure, hatching was monitored in situ from April to November 1986 in two north German lakes. Hatching traps were placed on the sediments and the incidence, genetic and species composition of ephippial hatchlings determined twice-weekly.
2. Hatching began in April after the thawing of ice. and continued for 3–4 weeks until the lakes stratified. There was no obvious relationship between the onset and duration of hatching and the environmental variables recorded, namely Secchi depth, surface temperature, temperature and oxygen concentration at the sediment-water interface.
3. Among the hatchlings were D. galeata, D. hyalina and their interspecific hybrid.
4. In the lake which contained overwintering animals the number of ephippial hatchlings was approximately one third of the total number of juveniles present in the lake.
5. In the second lake, no adults were recorded during winter and the population was probably founded by ephippial hatchlings alone. There was good agreement, in the short term, between the proportion of each species represented among hatchlings and the subsequent species composition in the lake.
6. The hatchlings were genetically diverse, and alleles were representative of those alleles present in contemporary populations.     

Mechanisms regulating zooplankton populations in a high-mountain lake

SUMMARY 1. We studied the seasonal succession of phyto- and zooplankton and the potential impact of predation by salmonids on zooplankton population dynamics in a high-mountain Swiss lake.
2. A comparison of patterns in the abundance, body length, fecundity and age structure in the Daphnia galeata population strongly suggests that trout predation had little impact on the population and was not the cause for a decline in summer.
3. The dominance in the lake of adult trout that feed mainly on benthic prey may buffer the effect of predation on the larger zooplankton. Further, the relatively high amount of phytoplankton after spring thaw could be important for sustaining the Daphnia population under moderate fish predation.
4. Partial correlation analyses proved circumstantial evidence for both exploitative and interference competition between some zooplankton taxa. D. galeata depressed performance of other plankton species through exploitative competition.
5. Our study shows that the impact of fish on zooplankton in high-mountain lakes depends strongly on food web structure and trophic state of the lake. Where fish predation is weak, invertebrate predation combined with competition for food may be responsible for the dominance of large-bodied zooplankton species.     

Clonal diversity,clonal persistence and rapid taxon replacement in natural populations of species and hybrids of the Daphnia longispina complex

Hybridization is common among cyclical parthenogens, especially in zooplankton species assemblages of the genus Daphnia. To explore hybridization dynamics and the extent of clonal diversity in the Daphnia longispina complex, we analysed population structure in eight permanent lakes. Based on 15 microsatellite loci, three major taxonomic units emerged: two species, D. galeata and D. longispina and their F1 hybrids, supported by factorial correspondence analysis and two Bayesian methods. At the same time, the detection of backcross classes differed between methods. Mean clonal diversity was lowest in the F1 hybrids, as expected from the high rate of asexual reproduction. Within taxa, replicated genotypes were of clonal origin, but clonal lineages persisted in subsequent years in only one of three resampled lakes. In another lake, the taxon composition changed from being dominated by hybrids to complete dominance by one parental taxon. Such a year‐to‐year taxon replacement has not been reported for the D. longispina complex before. Our data on this hybrid complex illustrate that high‐resolution genotyping is essential for the understanding of ecological and evolutionary outcomes of hybridization in partially clonal taxa.     

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.      

The edibility of Staurastrum chaetoceras and Cosmarium abbreviatum (desmidiaceae) for Daphnia galeata/hyalina and the role of desmids in the aquatic food web

A laboratory clone of Daphnia galeata/hyalina was fed with two different planktic desmid taxa: Staurastrum chaetoceras and Cosmarium abbreviatum var. planctonicum, being about equal in cell size. Whereas Staurastrum chaetoceras was readily ingested and assimilated to a high degree, Cosmarium was hardly incorporated. This could be partly due to the presence of an extracellular mucilaginous envelope in the latter species. When decapsulated by mild sonification, Cosmarium cells were significantly better ingested but digestion was still inferior to that of Staurastrum, presumably because of the more compact cell shape of Cosmarium.From literature it appears that small-sized planktic desmid species occasionally may constitute a main food source for zooplankton, especially in eutrophic lakes. Most likely however, desmids, particularly large-sized species, play a much more important role in the food chain in the benthic compartment of shallow, oligotrophic water bodies where they serve as a food source for various macro-invertebrate taxa.     

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.     

Interspecific hybridization betweenDaphnia hyalina,D. galeata,andD. cucullata and seasonal abundances of these species and their hybrids

The three cladoceran speciesDaphnia hyalina, D. galeata, andD. cucullata frequently coexist in the lakes of northern Germany. Although there are some problems in distinguishing them morphologically, they are easily determined by gelelectrophoresis: each species carries a different allele at the glutamate oxaloacetate transaminase (GOT) locus. Animals morphologically intermediate between two species are heterozygous for the alleles carried by the species they resemble. This pattern is in agreement with the findings at other loci, where also diagnostic alleles exist. These findings are most easily explained by interspecific hybridization between the three species. No evidence is found for backcrosses involving hybrids ofD. cucullata, whereas some backcrosses betweenD. hyalina, D. galeata, and their hybrids are found in some lakes. In four lakes the seasonal abundances of the three species and their hybrids are determined.