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The evolutionary process of interspecific hybridization in cladocerans is reviewed based on ecological and population genetic data. The evolutionary consequences of hybridization, biogeographic patterns and fitness comparisons are analyzed within the conceptual framework of theories on hybridization. Among species of the D. longispina complex no interpopulational transition zones (hybrid zones) have been detected, but rather patchy distributions of hybrids and parentals have been found. Hybrids occur across broad geographic ranges and can be more abundant than parental species. Due to asexual reproduction (ameiotic parthenogenesis), hybrid breakdown can be avoided, and hybrids can even (temporarily) combine advantageous traits of both parental species. Evolutionary consequences may arise from repeated backcrossing, which in some cases results in introgression and patterns of reticulate evolution. [KEYWORDS: Natural interspecific hybridization; introgression; parthenogenesis; life histories; population genetics; daphnia longispina complex Mitochondrial-dna analysis; life-history variation; mosaic hybrid zone; daphnia-pulex; introgressive hybridization;natural hybridization; population-dynamics; gene flow; notropis-cornutus; sunfish lepomis]

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

This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding benthic bivalves under natural conditions. Were this to be so, it would imply that foraging and predator avoidance place conflicting demands on these animals, since an increase in the feeding radius on the surface would be associated with a decrease in the burying depth, and vice versa. The paper presents methods with which to measure siphon length and equations to transform siphon weight to siphon length in two benthic bivalves, Scrobicularia plana and Macoma balthica. Relatively heavy siphons are longer, but also thicker, than lightweight ones. We conclude that most individual bivalves stretch their siphons fully while feeding. However, bivalves with heavy siphons keep part of the siphon in reserve within the shell. Siphon cropping therefore results in an immediate reduction in siphon length, unless the siphon is heavy and a reserve is available. The feeding radius of M. balthica and S. plana is a linear function of shell size. S. plana use half of their siphon length to feed on the surface and the other half to bury themselves, but if the siphon weight is below average, the proportion extended over the surface decreases with siphon size. However, among the animals with a short siphon, those with a good body condition take no risks and live as deeply as possible. A comparison between species reveals that the weight of the extended inhalant siphon per cm amounts, on average, to 0.6% of the total body weight.

A toxicokinetic model for Daphnia magna , which simulates the internal concentration of the insecticide diazinon, its detoxification product 2-isopropyl-6-methyl-4-pyrimidinol, and its active metabolite diazoxon, is presented. During in vivo exposure to diazinon with and without inhibition of cytochrome P450 by piperonyl butoxide, the parent compound as well as its metabolites were quantified with high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) in extracts of D. magna . Rate constants of all relevant toxicokinetic steps were obtained by modeling the time course of the internal concentrations with a multicomponent first-order kinetics model. When cytochrome P450 was inhibited, the kinetic bioconcentration factor (BCF) of diazinon increased from 17.8 to 51.0 mL·g(ww)(-1). This clearly indicates that diazinon is biotransformed to a high degree by cytochrome P450 in D. magna . The dominant elimination step of diazinon was shown to be its oxidative dearylation to pyrimidinol (62% of total elimination) with a corresponding rate constant of 0.16 h(-1). In contrast, oxidative activation to diazoxon with a rate constant of 0.02 h(-1) amounted to only 8% of the total elimination. During exposure to diazinon, the active metabolite diazoxon could be detected only in very low concentrations (approximately 0.5% of the parent compound), presumably due to a very fast reaction with the target site acetylcholinesterase. During the exposure experiments (no feeding of daphnids), an exponential decline of the lipid content in D. magna with a first-order rate constant of 0.013 h(-1) was observed. For short exposure times (≤ 24 h), this had only a minor influence on the determined TK parameters. Such a TK model containing detailed biotransformation processes is an important tool for estimation of the toxic potential of chemicals, particularly, when active metabolites are formed inside an organism.

Both host susceptibility and parasite infectivity commonly have a genetic basis, and can therefore be shaped by coevolution. However, these traits are often sensitive to environmental variation, resulting in genotype-by-environment interactions. We tested the influence of temperature on host–parasite genetic specificity in the Daphnia longispina hybrid complex, exposed to the protozoan parasite Caullerya mesnili . Infection rates were higher at low temperature. Furthermore, significant differences between host clones, but not between host taxa, and a host genotype-by-temperature interaction were observed.

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

The competitive ability of hybrids, compared with their parental taxa, can cover a wide fitness range from poor to superior. For example communities of theDaphnia galeata–hyalina–cucullataspecies complex often show hybrid dominance. We tested whether taxa composition of 43 European lakes inhabited by this species complex can be explained by habitat characteristics (e.g. size descriptors, trophy level) or geography. We found thatD. galeataoccurs more frequently south of the Alps, whereasD. hyalinaandD. cucullataare found more in the north. Lakes withD. galeatadominance had higher temperatures whereasD. hyalinadominance could be attributed to low phosphorus loads. The dominance of F1-hybrids, however, was not explainable with current environmental variables. In a subset of 28 lakes, we studied the impact of eutrophication history on F1-hybrid success. Lakes with the highest trophic state in the past tended to be dominated by F1-hybrids. Our data demonstrate that human-mediated habitat disturbance (eutrophication) has facilitated hybrid success and altered theDaphniataxon composition across lakes. At the same time, specific habitat conditions might provide a refuge from hybridization for native genotypes.

To quantify the effect of the filter feeder Daphnia on the aggregation of mineral particles, temporal changes in the particle size distribution of inorganic colloids were experimentally determined both in the presence and in the absence of Daphnia in water samples of Lake Brienz, Switzerland, an oligotrophic lake rich in suspended inorganic colloids. The results obtained show that daphnids favour the aggregation of mineral colloids, but only for particle sizes above the Daphnia filter mesh size. However, the number concentration of particles smaller than the Daphnia filter mesh size simultaneously increases in the presence of the filter feeder, suggesting either the break-down of existing aggregates or the aggregation of particles with initial sizes below the measured size range. The density of daphnids in this lake is currently too low to have any significant effect on the fate of inorganic colloidal particles as compared with aggregation due to physical processes of particle collision. However, in more productive water bodies where Daphnia is more abundant, they may play a significant role.

Over the past two decades, infection levels in hybrids have frequently been compared with those in their progenitor species to find out whether parasites favour or penalize hybridization. Four static infection scenarios are possible, depending on whether hybrid resistance differs from that of one or both parental species. All four alternatives have been supported by some empirical data, but any potential dynamics might have been overlooked because of limited sampling or experimental designs. However, coevolutionary oscillations generated by frequency-dependent selection might provide a better explanation for the observed results than does a static genetic perspective of host resistance.

Aquatic Sciences, 67 (4), ISSN:1015-1621, ISSN:1420-9055

Evidence for extensive interspecific hybridization among species of the genus Daphnia has been accumulating on a global scale. Although there is evidence for limited gene flow between taxa via hybridization, many species still maintain discrete morphological and molecular characteristics. We studied temporal and spatial patterns of sexual reproduction within the Daphnia galeata‐hyalina‐ cucullata hybrid species complex in a lake (Plussee), located in northern Germany. Allozyme electrophoresis allowed us to track seasonal changes in taxon composition as well as the quantification of backcrosses. Sexually-reproducing animals (ephippial females and males) were mainly found in autumn.

The interpretation of low FST values as evidence of high levels of gene flow among habitat fragments may be confounded by population genetic structures that are indicative of historical rather than present-day levels of gene flow. We examined the genetic structure of 23 populations of Baetis alpinus (Insecta: Ephemeroptera) living in alpine streams fragmented by lakes ( approximately 10 000 years old), reservoirs ( approximately 100 years old), and in nonfragmented streams, to examine if lakes act as barriers to gene flow and to investigate the temporal resolution of allozyme markers. Estimates of gene flow indicated little or no genetic divergence along four nonfragmented reference streams and across two lakes and two reservoirs (FST=0.004-0.041), but marked differentiation across four lakes (FST=0.092-0.362) and across one reservoir that was a lake enlarged by a dam (FST=0.075). Differentiation was unrelated to distance between fragments, but occurred only in lakes found in valleys that have been ice-free throughout the Holocene. We suggest that standing water bodies act as barriers to gene flow in B. alpinus and that low FST values observed between fragments separated by reservoirs do not indicate high levels of gene flow but rather show that genetic differentiation was not detectable within the first 100-1000 years of habitat fragmentation.

Recent studies on the life histories of Daphnia hybrids and their parental species have revealed that hybrids can combine an intermediate size with a relatively high reproductive rate, which might explain their success in many European lakes. Based on this information, we formulated the temporal hybrid superiority hypothesis to explain the coexistence of parental species and their hybrids. Our hypothesis states that in temporally changing environments, such as lakes, hybrids can coexist with their parental species because hybrid individuals have a higher fitness compared to the parental individuals during specific periods of the year. This is in contrast to the hypothesis that hybrids have a lower fitness than the parentals, which should be compensated by recurrent hybrid production. To test this hypothesis we studied population dynamics and seasonal changes in body size of two Daphnia species (D. galeata and D. cucullata) and their interspecific hybrid D. cucullata X D. galeata in Tjeukemeer, a shallow eutrophic lake in the Netherlands. In 1989, the year with the highest fish predation pressure, the larger D. galeata was first replaced by the smaller hybrid that was soon succeeded by the even smaller D. cucullata. In 1990 the Daphnia community was dominated by the hybrid and in 1991 by D. galeata. Deviations of the instantaneous rate of increase, averaged over the three species, showed a positive relationship with fish predation risk for D. cucullata, a negative relationship for D. galeata, and no relationship for the hybrid. This suggests a higher influence of fish predation on D. galeata. We conclude that the selective advantage of the hybrid over D. galeata under fish predation, expected from life-history experiments, is in agreement with our field data. [KEYWORDS: Shallow eutrophic lake; life-history variation; interspecific hybridization; fresh-water; body-size; tjeukemeer; zooplankton; cladocerans; growth; food]

During the first few weeks of a recurring seasonal period of diel vertical migration in Lake Maarsseveen (The Netherlands), part of the hybrid Daphnia galeata × hyalina population migrated, while another part remained in the epilimnion. In the epilimnion, 0+ perch prey upon daphnids during daytime. Gradually, the number of adult Daphnia in the epilimnion decrease until the epilimnion is nearly devoid of daphnids. The population as a whole may decrease, as in 1991, or may increase as in 1992. Genotype composition, as determined by allozyme analysis, changed substantially within a fortnight in 1992, and one genotype became dominant. Our data are in agreement with the hypothesis that predation on different genotypes (clones) occurs during the beginning of a seasonal period of diel vertical migration, though our data do not allow to exclude alternative explanations.

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.

(2005). Parasite — host preference in hybrid Daphnia populations. SIL Proceedings, 1922-2010: Vol. 29, No. 1, pp. 340-341.

Temporal genetic variation was studied within the cyclic parthenogenetic Daphnia galeata-cucullata species complex in Lake Tjeukemeer (The Netherlands). During three successive years, three allozyme loci were studied in order to compare the level of genetic variation in D. galeata (G), D. cucullata (C) and their interspecific hybrid (C × G). This data set was used to investigate: whether C × G hybrids are regularly produced by the parental species and to what extent backcrossing takes place in this species complex. G and C × G were the most abundant taxa, whereas C was only abundant in late summer and autumn. Genetic diversity was highest in C, lowest in G, and intermediate in C × G. pgm and pgi allele frequencies of C × G were not intermediate to those of the parental species but were more similar to G than to C. In addition, almost no significant deviations from Hardy-Weinberg were observed. An UPGMA analysis of Nei's genetic distances showed that C × G hybrids are more closely related to G (difference

To investigate whether reproductive variation in the Daphnia galeata-cucullata-hyalina hybrid species complex can explain the occurrence of hybrids and backcrosses, a total of 43 clones from three north German lakes were tested for allocation to sexual reproduction under equal stress conditions. Six replicates per clone were followed until the seventh adult instar. I used the following cues to promote sexual reproduction: short photoperiod, water from a crowded Daphnia culture, fish-conditioned water and low food concentration. For each animal, clutch size and clutch sex were recorded. Ephippia, which were empty since the animals were cultured individually, were only produced by D. cucullata and D. cucullata x hyalina (26% and 6% of the broods, respectively), whereas almost all taxa produced males (the range was 2–15%). Intraspecific variation for male and ephippial production was also found. The fact that the taxa tested show different responses to the stimuli of-fered may indicate that there is reproductive isolation between them. This is supported by field data from the Tjeukemeer (The Netherlands) which shows that D. galeata mostly produces ephippia in spring and D. cucullata in autumn. The presence of hybrids with ephippia in both seasons, however, shows that backcrossing is also possible.

Recent studies have shown that Daphnia hybrids are intermediate to their parental species for several life history traits, and that they combine advantageous traits of the parental species, thereby decreasing their risk of being preyed upon by planktivorous fish. In this study D. galeata, D. cucullata and their interspecific hybrid are compared with respect to cyclomorphosis and juvenile growth as a potential mechanism to avoid invertebrate predation. Helmets and tail spines are known to reduce predation risk, whereas fast juvenile growth reduces the period that Daphnia are vulnerable to predation. In laboratory cultures, I tested the hypothesis that, with regard to cyclomorphosis, juvenile hybrids are more comparable to the juveniles of D. cucullata, and with respect to growth rate, they are more comparable to D. galeata. The hybrid did not differ significantly from D. galeata in the time needed to reach 0.9 mm body length. In contrast, the relative spine and helmet length of juvenile hybrids were more comparable with those of D. cucullata, whereas in adult instars these characters were more similar to D. galeata. Thus during the first juvenile instars, the hybrids combine the relative long helmet and spine of D. cucullata, with the faster growth of D. galeata. This combination of characteristics might reduce hybrid vulnerability to invertebrate predation in field situations.

The seasonal dominance of cyanobacteria in the phytoplankton community of lake ecosystems can have severe implications for higher trophic levels. For herbivorous zooplankton such as Daphnia, cyanobacteria have poor nutritional value and some species can produce toxins affecting zooplankton survival and reproduction. Here we present another, hitherto largely unexplored aspect of cyanobacteria, namely that they can increase Daphnia susceptibility to parasites. In a 12-yr monthly time-series analysis of the Daphnia community in Greifensee (Switzerland), we observed that cyanobacteria density correlated significantly with the epidemics of a common gut parasite of Daphnia, Caullerya mesnili, regardless of what cyanobacteria species was present or whether it was colonial or filamentous. The temperature from the previous month also affected the occurrence of Caullerya epidemics, either directly or indirectly by the promotion of cyanobacterial growth. A laboratory experiment confirmed that cyanobacteria increase the susceptibility of Daphnia to Caullerya, and suggested a possible involvement of cyanotoxins or other chemical traits of cyanobacteria in this process. These findings expand our understanding of the consequences of toxic cyanobacterial blooms for lake ecosystems and might be relevant for epidemics experienced by other aquatic species.

We asked whether pre- (e.g., assortative mating, temporal isolation) or postzygotic (e.g., hybrid inviability, infertility) barriers are more likely to affect the hybridization between Daphnia galeata and Daphnia hyalina .W e compared the taxonomic composition of different reproductive stages in the life cycle of D. galeata, D. hyalina, and their hybrids in Greifensee (Switzerland) by using molecular genetic methods. We found evidence for reproductive isolation between taxa and that hybrids in particular, have reduced sexual fitness. The results provide one potential mechanism for parental taxa to remain distinct. F1 hybrid dominance in Greifensee could be explained by an increased asexual reproduction of hybrids that results in a higher proportion of gravid females compared with the parental D. galeata. The low sexual fitness of hybrids limits the abilities of hybrids to take advantage of diapausing eggs. The lower dispersal ability, including colonization of new habitats, and survival probability during harsh environmental conditions, could, therefore, lead to underestimates of historical hybrid occurrence by using diapausing egg bank reconstructions.