Are they still viable? Physical conditions and abundance of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> resting eggs in sediment cores from lakes in the Tatra Mountains

All species of Daphnia (Cladocera) produce, at some stage in their life cycle, diapausing eggs, which can remain viable for decades or centuries forming a “seed bank” in lake sediments. Because of their often good preservation in lake sediment, they are useful in paleolimnology and microevolutionary studies. The focus of this study was the analysis of cladoceran resting eggs stored in the sediment in order to examine the ephippial eggs bank of Daphnia pulicaria Forbes in six mountain lakes in the High Tatra Mountains, the Western Carpathians (northern Slovakia and southern Poland). Firstly, we analyzed distribution, abundance and physical condition of resting eggs in the sediment for their later used in historical reconstruction of Daphnia populations by genetic methods. To assess changes in the genetic composition of the population through time, we used two microsatellite markers. Although DNA from resting eggs preserved in the High Tatra Mountain lake sediments was extracted by various protocols modified for small amounts of ancient DNA, DNA from eggs was not of sufficient quality for microsatellite analyses. Distribution curves of resting eggs from sediment cores correspond to the environmental changes that have occurred in the High Tatra Mountains area during last two centuries (atmospheric acid deposition, fish introduction) and demonstrate their influence on natural populations. Evaluation of ephippia physical condition (the most common category was empty ephippial covers) suggests that the majority of resting eggs hatched to produce a new generation of Daphnia or may be due to failed deposition of resting eggs by Daphnia to the chitinous case. In conclusion, age, low quantity and poor physical condition of resting eggs from these Tatra lake sediments proved to be unsuitable not just for use in genetic analyses, but also the possibilities of autogenous restoration of Daphnia populations from the resting egg banks in the Tatra sediments are negligible.     

Effects of winter conditions on Daphnia dynamics and genetic diversity in a dimictic temperate reservoir

1. Winter conditions shape plankton dynamics and community composition in temperate regions, but their effect on dynamics and genetic composition of cyclical parthenogens like Daphnia is largely unclear. 2. For 5 years, we studied the dynamics, hatching from resting eggs and genetic structure of a D. galeata × longispina hybrid complex in a dimictic, temperate reservoir. Our main hypothesis was that higher spring densities and an earlier population peak will be observed after warmer winters, with a lower genetic diversity because of a lower contribution of resting eggs to population growth. 3. The study period could clearly be categorised into cold‐winter years (n = 3) and warm‐winter years (n = 2). Daphnia densities at the end of spring overturn were ～10‐fold lower after cold winters than after warm ones, but no pattern emerged concerning the timing and the height of the population peak in early summer. 4. Hatching intensity from resting eggs was higher and contributed up to 8.5% to Daphnia abundance in a cold‐winter year compared to a negligible contribution in a warm‐winter year. Consistent with this finding, new multilocus genotypes (MLGs) adding to the overwintering stock after the end of spring overturn and presumably originating from resting eggs increased genetic diversity and attained high frequencies within the population only after a cold winter. New MLGs were recorded also after warm winters, but they never gained dominance and no shift in genetic diversity was observed. However, genetic diversity was not generally reduced after warm winters. 5. Our results confirm earlier findings that winter conditions have only a limited effect on the main growth phase and the peak of Daphnia during late spring and early summer. However, winter conditions determine the contribution of resting eggs to the population development, which may profoundly alter the genetic composition of the population compared to the previous season.     

New microsatellite primers for Daphnia galeata mendotae

In order to study genetic changes in populations of Daphnia galeata mendotae, I characterized seven polymorphic microsatellite loci. Primers to amplify these loci were tested on individual eggs from the resting egg bank of Onondaga Lake, NY. Levels of polymorphism and cross‐amplification in D. g. galeata indicate that they will be useful markers for ecological genetic studies on both adults and diapausing eggs of these species.     

Molecular genetic analysis of Daphnia in the English Lake District: species identity, hybridisation and resting egg banks

• 1 Taxonomic issues in many Daphnia species complexes are often confused by a high degree of phenotypic plasticity and by interspecific hybridisation. Here, we employ molecular genetic tools to confirm the species composition and incidence of hybridisation in extant and resting egg populations of Daphnia from Windermere and Esthwaite Water in the English Lake District.
• 2 A combination of species‐diagnostic allozymes and mtDNA, confirms that contemporary populations are dominated by a single species, D. galeata.
• 3 We present the first account of genetic characterisation of dated ‘resting’ eggs using microsatellites and mtDNA, employing PCR‐based DNA recovery, thus providing a temporal dimension to taxonomic patterns. Thirty years ago, two species were present in Esthwaite, D. galeata and D. hyalina, but Windermere populations were dominated by D. galeata only.
• 4 The use of PCR‐based mtDNA RFLP analysis as a species‐diagnostic tool, and microsatellites to monitor clonal diversity, provide a valuable approach for long‐term studies, especially in populations free from the complicating effect of frequent hybridisation. The detailed limnological records available for many large lakes, and associated changes in land‐use, pollutants and climate, combined with long‐term ephippial molecular genetic data, provide opportunities for exploring natural and anthropogenic impacts on genetic and community structure.

     

Life history strategies of cladocerans: comparisons of tropical and temperate taxa

We review recent works on different life history variables of cladoceran taxa in tropical and temperate freshwater bodies, comparing the strategies that cladocerans have evolved to adapt to contrasting environmental conditions in the two geographical regions. These life-history parameters relate to age and size at maturity, survival, fecundity, life-expectancy at birth, lifespan, gross, and net reproductive rates, generation time, the rate of population increase, peak population density and day of peak abundance. We also discuss the role of photoperiod and temperature on some of these life history parameters. We found a general paucity of experimental work and field data in tropics on cladocerans. There is very limited information on the few Daphnia species found in the tropics. The misconception of low species diversity of cladocerans in the tropics arose due to several reasons including lack of extensive and intensive field collections. Higher water temperatures apparently promote permanent infestation of tropical waters with toxic cyanobacteria, which reduce the zooplankton diversity. In addition to higher temperatures in the tropics, the year-round high predation pressure of planktivorous fish probably causes the tropical species, particularly in pelagic habitats, to reach maturity earlier (< 3 days) than in temperate regions. Species of Daphnia in temperate regions are particularly adapted to living at food concentrations that are much lower and seasonably more variable than those for tropical genera such as Diaphanosoma. This is further corroborated by the more than an order of magnitude higher threshold food concentration (TFC) for tropical Cladocera than for their temperate counterparts. Fecundity patterns differ between tropical and temperate cladoceran taxa: cultured under optimal temperature regimes, tropical taxa have fewer eggs than temperate species of a comparable body size. Predation pressure may act differently depending on the size of the cladoceran neonates and thus on their population size structure. Global warming and climate changes seem to affect the behaviour (migration), distribution, and abundance of cladocerans. Apparently, in direct response to these changes, the possibility of encountering the tropical cladocerans in the northern, temperate hemisphere (bioinvasions) is on the rise.     

Amplified Fragment Length Polymorphism (AFLP) Reveals Species-Specific Markers in the Daphnia Galeata–HyalinaSpecies Complex

Daphnia often occur in species complexes that consist of two or more co-occurring species and their hybrids. Hybrid individuals are often capable of sexual reproduction and so backcrossing with introgression occurs. To better understand hybridization and backcrossing frequency, we sought to develop PCR-based, species-specific markers in the Daphnia galeata–hyalina species complex using amplified fragment length polymorphism (AFLP). This technique produces large numbers of reproducible markers for assessing diversity across the nuclear genome and provides several advantages over mtDNA and microsatellite approaches. We examined 28 clones of D. galeata, D. hyalina, and their hybrids isolated from Lake Constance on the Swiss-German border. Using a single AFLP primer combination we found five potential species-specific markers, defined as bands that occurred in >80% of one parental species and <20% of the other. Two bands appeared to be co-dominant and were present (homozygous) in D. galeata, absent in D. hyalina, and heterozygous in the hybrid. We conclude AFLP could provide enough PCR-based, species-specific markers to identify species, hybrids, and backcrosses from even small amounts of tissue (i.e. resting eggs).     

Population genetic structure of three pond-inhabiting Daphnia species on a regional scale (Flanders, Belgium)

1. Only a few studies have compared patterns of genetic variation among populations of different Daphnia species on a regional scale. The present study addresses this gap and examines the relationship between diversity as revealed by allozyme variation and habitat size for populations of Daphnia pulex, D. obtusa and D. curvirostris in Flanders (Belgium). In addition, we examined whether patterns of isolation‐by‐distance could be observed in each of these three Daphnia species. 2. The relationship between genetic diversity and habitat size varied among Daphnia species that occur in the same region. In D. pulex and D. obtusa populations, a positive relationship between local genetic diversity and habitat size was found, whereas the relationship was negative in D. curvirostris populations. 3. Regional genetic diversity was lower than expected from patterns of local genetic diversity in D. pulex and D. obtusa populations in Flanders. This suggests that the subdivision of local Daphnia populations in a region did not obviously increase genetic diversity. 4. Genetic differentiation among populations of these three species in Flanders was moderate and comparable with values observed in other Daphnia species. Patterns of isolation‐by‐distance could be observed, but the scatter was high (D. pulex) or the slope was very low (D. obtusa).     

Daphnia community analysis in shallow Kenyan lakes and ponds using dormant eggs in surface sediments

1. Water fleas of the genus Daphnia are considered rare in tropical regions, and information on species distribution and community ecology is scarce and anecdotal. This study presents the results of a survey of Daphnia species distribution and community composition in 40 standing waterbodies in southern Kenya. The study sites cover a wide range of tropical standing aquatic habitats, from small ephemeral pools to large permanent lakes between approximately 700 and 2800 m a.s.l. Our analysis combines data on Daphnia distribution and abundance from zooplankton samples and dormant eggs in surface sediments. 2. Nearly 70% (27 of 40) of the sampled waterbodies were inhabited by Daphnia. Although their abundance in the active community was often very low, this high incidence shows that Daphnia can be equally widespread in tropical regions as in temperate regions. 3. Analysis of local species assemblages from dormant eggs in surface sediments was more productive than snapshot sampling of zooplankton communities. Surface‐sediment samples yielded eight Daphnia species in total, and allowed the detection of Daphnia in 25 waterbodies; zooplankton samples revealed the presence of only four Daphnia species in 16 waterbodies. 4. Daphnia barbata, D. laevis, and D. pulex were the most frequently recorded and most abundant Daphnia species. Canonical correspondence analysis of species–environment relationships indicates that variation in the Daphnia community composition of Kenyan waters was best explained by fish presence, temperature, macrophyte cover and altitude. Daphnia barbata and D. pulex tended to co‐occur with each other and with fish. Both species tended to occur in relatively large (>10 ha) and deeper (>2 m) alkaline waters (pH 8.5). Daphnia laevis mainly occurred in cool and clear, macrophyte‐dominated lakes at high altitudes.     

High predation is of key importance for dominance of small-bodied zooplankton in warm shallow lakes: evidence from lakes, fish exclosures and surface sediments

The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. Daphnia and Simocephalus) appeared in fishless mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of small-sized zooplankton in warm lakes.     

<Emphasis Type="Italic">Daphnia</Emphasis>species diversity in Kenya,and a key to the identification of their ephippia

The distribution of Daphniaspecies in tropical Africa is poorly known and understood. Daphniaare assumed rare in tropical regions, but systematic studies covering large areas are sparse. We sampled the active community (live zooplankton) and/or the dormant community (diapausing egg banks in the sediment) of 41 standing water bodies in Kenya in search for Daphnia.Overall the dormant communities yielded 11 species of Daphnia, a species richness more than twice the species richness found in the active communities. Dormant community species diversity better reflects the spatial, and particularly the temporal (multi-annual) variation in environmental conditions available to Daphniain these tropical standing waters. Hence, we suggest that the dormant community be taken into account when assessing local zooplankton diversity, especially in fluctuating tropical lake ecosystems, where the presence of each local Daphniaspecies in the active community may be strongly seasonal or erratic. Geographic distribution data from this study are supplemented with previous records of Daphniain East Africa to provide an overview of the known distribution of Daphniain Kenya and neighbouring countries. We also present a detailed key for morphological identification of the ephippia of the 11 Daphniaspecies encountered, complemented with photographs and drawings of diagnostic characters.     

Genetic diversity and phylogeography of Daphnia similoides sinensis located in the middle and lower reaches of the Yangtze River

Geographical patterns, climate, and environmental change have important influences on the distribution and spread of aquatic organisms. However, the relationships between the geographical pattern and phylogenetics of Daphnia as well as environmental change are not well known. The genetic diversity and phylogeography of seven D. similoides sinensis populations located in the middle and lower reaches of the Yangtze River were investigated based on the combination of mitochondrial (COI gene) and nuclear (14 microsatellite primers) markers. Based on the mitochondrial gene markers, D. similoides sinensis from the middle and lower reaches of the Yangtze River had one ancestral haplotype and two evolutionary clades. In addition, D. similoides sinensis population deviated from neutral evolution, showing signs of a bottleneck effect followed by population expansion. Based on the microsatellite markers, the seven D. similoides sinensis populations formed three main groups. The dendrogram (NJ/ME) showed that D. similoides sinensis based on the mitochondrial genes marker were obviously clustered two main clades, whereas there were three clades based on the microsatellite markers. Our results suggested that the habitat fragmentation due to the barrier of the dams and sluices promoted the genetic differentiation and phylogeography of D. similoides sinensis populations in the middle and lower reaches of the Yangtze River.     

Analysis of Genetic Diversity of North Eurasian Populations Using Autosomal Microsatellite Loci

The paper presents the results of analysis of the gene pools of several North Eurasian ethnic groups (Buryats, Evenks, Altaians, Russians, Kyrgyzes, Tuvinians, Tatars, and Ukrainians) examined using a panel of autosomal microsatellite markers (D4S397,D5S393, D7S640, D8S514, D9S161, D10S197, D11S1358, D12S364 and D13S173) mapped on different chromosomes and represented by the (CA) _n dinucleotide repeats. In the group of populations examined the proportion of genetic variability at microsatellite loci explained by interpopulation differences was about 2.5%, while genetic differences between the individuals within a population accounted for 97.5% of this variability. Analysis of genetic relationships among the populations revealed substantial differences between the populations belonging to the Indo-European and Altaic linguistic families in gene diversity at microsatellite loci.     

DNA extraction and amplification methods for ephippial cases of Daphnia resting eggs in lake sediments: a novel approach for reconstructing zooplankton population structure from the past

This study describes a method of DNA extraction and amplification for ephippial cases of Daphnia resting eggs from lake sediment. Recent studies have reconstructed succession records of Daphnia species by genetically analyzing Daphnia resting eggs stored in lake sediments and revealed changes in dominant Daphnia species that correspond well with environmental changes. However, this approach is not applicable to lakes where most of the resting eggs in the sediment have already hatched out. We modified conventional methods for DNA extraction and amplification to enable genetic analyses of the ephippial case that envelops and protects the resting eggs, and we compared the performance of the modified method to the conventional one. We confirmed that we could efficiently analyze the sequences of the ephippial cases collected in a sediment core using our modified method. It enables us to reconstruct changes in genetic structure of Daphnia populations regardless of hatching rates of the resting eggs.     

Status of Daphnia Resting Egg Banks in Bohemian Forest Lakes Affected by Acidification

We studied the egg banks of Daphnia gr. longispina in four acidified lakes in the Bohemian Forest (Czech Republic, Europe). Daphnia had become extinct in three of the studied lakes due to anthropogenic acidification and we assessed the possibility of autochthonous recovery of the population after recent amelioration of the lake environment. We determined the ephippia distribution in eleven cores 10–30 cm long, and the state of Daphnia resting eggs in over 13 000 ephippia from eight cores. Apparently well-preserved eggs were used for hatching experiments and for DNA amplification. Vertical profiles of ephippia densities in several cores did not agree with historical data on the Daphnia presence in the lakes, as the sediment had been repeatedly disturbed by human activities in the past. Ephippia are present near the surface of the sediment in all lakes, and they might therefore receive hatching cues. We were not able, however, to prove that viable eggs are present in the ephippia banks of the lakes. The percentage of the well-preserved eggs in all but one core was below 1%. In addition, no egg hatched in the hatching experiments and we were not able to amplify DNA from the preserved eggs of lakes where Daphnia is extinct, although amplification from relatively young eggs from the fourth lake was successful in 60% of the cases. We conclude that the recovery of Daphnia populations in studied lakes from autochthonous sources is unlikely.     

EVOLUTIONARY POTENTIAL AND LOCAL GENETIC DIFFERENTIATION IN A PHENOTYPICALLY PLASTIC TRAIT OF A CYCLICAL PARTHENOGEN,DAPHNIA MAGNA

The frequent use of neutral markers to quantify genetic variation in natural populations emphasizes the role of stochastic events in explaining genetic differentiation between populations. Complementary studies on ecologically relevant traits are needed to assess the role of natural selection acting on adaptive variation in the development of local genetic differentiation. To test the hypothesis of local adaptation in the cyclical parthenogenetic species Daphnia magna, the phototactic behavior in the absence and presence of fish kairomone was assayed for 30 clones derived from resting eggs isolated from three habitats characterized by a different predation pressure by fish. Clones derived from populations in which fish are present were, on average, more negatively phototactic in and more responsive to the presence of fish kairomone than clones derived from a fishless habitat. In addition, the results show a high genetic diversity for the traits studied in all three gene pools investigated, indicating a high potential for microevolutionary changes in behavior of these Daphnia populations in the face of changes in predation pressure. The results of the present study indicate that working with large samples at the expense of having less precise estimates of genotypic values for specific genotypes may result in a loss of information with regard to the evolutionary potential of local populations.     

Local human pressures influence gene flow in a hybridizing Daphnia species complex

Anthropogenic environmental changes are considered critical drivers of the genetic structure of populations and communities through, for example, the facilitation of introgressive hybridization between syntopic species. However, the mechanisms by which environmental perturbations trigger changes in the genetic structure of populations and communities, such as the processes that determine the directionality of hybridization and patterns of mitochondrial introgression over many generations, remain largely unexplored. In this study, the changes in genetic structure of hybridizing members of the Daphnia longispina species complex were reconstructed over the last 100 years for three large temperate lakes under strong anthropogenic pressures via palaeogenetic analyses of resting egg banks. Drastic changes in the genetic structure of the Daphnia community, associated with hybridization events between D. longispina and D. galeata and subsequent introgression, were detected in Lakes Geneva and Bourget. In Lake Bourget, these changes were induced by the successful establishment of D. galeata with rising phosphorus levels and reinforced by the sensitivity of D. longispina to fish predation pressure. In Lake Geneva, the pattern of hybridization during eutrophication is more likely a function of the original taxonomic composition of the species complex in this lake. Lakes seem to require at least a meso‐oligotrophic status to allow D. galeata populations to establish and accordingly no D. galeata genotypes were found in the egg bank of oligotrophic Lake Annecy. In contrast to the generally assumed pattern of unidirectional hybridization in this species complex, bidirectional hybridization was recorded in Lakes Geneva and Bourget. Our results also demonstrate complex genetic trajectories within this species complex and highlight the irreversibility of changes in the genotypic architecture of populations driven by local human pressures. Finally, we show that extensive hybridization and introgression do not necessarily result in a large and homogenous hybrid swarm.     

Prey selection by Chaoborus punctipennis under laboratory conditions

Equal numbers of Diaphanosoma leuchtenbergianum, Daphnia parvula, and Diaptomus pallidus were offered to individual, fourth-instar larvae of Chaoborus punctipennis. When the prey species were presented separately to the larvae, 100% of the Diaphanosoma, 67% of the Daphnia, and 57% of the Diaptomus were consumed. However, when the three species were offered to the predator concurrently, 90% of the Diaphanosoma, 10% of the Daphnia and < 1% of the Diaptomus were eaten. This strong selection for Diaphanosoma was consistent at three prey densities.     

Diploid male production correlates with genetic diversity in the parasitoid wasp Venturia canescens: a genetic approach with new microsatellite markers

Sex determination is ruled by haplodiploidy in Hymenoptera, with haploid males arising from unfertilized eggs and diploid females from fertilized eggs. However, diploid males with null fitness are produced under complementary sex determination (CSD), when individuals are homozygous for this locus. Diploid males are expected to be more frequent in genetically eroded populations (such as islands and captive populations), as genetic diversity at the csd locus should be low. However, only a few studies have focused on the relation between population size, genetic diversity, and the proportion of diploid males in the field. Here, we developed new microsatellite markers in order to assess and compare genetic diversity and diploid male proportion (DMP) in populations from three distinct habitat types – mainland, island, or captive –, in the parasitoid wasp Venturia canescens. Eroded genetic diversity and higher DMP were found in island and captive populations, and habitat type had large effect on genetic diversity. Therefore, DMP reflects the decreasing genetic diversity in small and isolated populations. Thus, Hymenopteran populations can be at high extinction risk due to habitat destruction or fragmentation.     

Clonal diversity and turnover in an overwintering Daphnia pulex population,and the effect of fish predation

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