Genetically divergent cryptic species are frequently detected by molecular methods. These discoveries are often a byproduct of molecular barcoding studies in which fragments of a selected marker are ...used for species identification. Highly divergent mitochondrial lineages and putative cryptic species are even detected in intensively studied animal taxa, such as the crustacean genus Daphnia. Recently, eleven such lineages, exhibiting genetic distances comparable to levels observed among well-defined species, were recorded in the D. longispina species complex, a group that contains several key taxa of freshwater ecosystems. We tested if three of those lineages represent indeed distinct species, by analyzing patterns of variation of ten nuclear microsatellite markers in six populations.
We observed a discordant pattern between mitochondrial and nuclear DNA, as all individuals carrying one of the divergent mitochondrial lineages grouped at the nuclear level with widespread, well-recognized species coexisting at the same localities (Daphnia galeata, D. longispina, and D. cucullata).
A likely explanation for this pattern is the introgression of the mitochondrial genome of undescribed taxa into the common species, either in the distant past or after long-distance dispersal. The occurrence of highly divergent but rare mtDNA lineages in the gene pool of widespread species would suggest that hybridization and introgression in the D. longispina species complex is frequent even across large phylogenetic distances, and that discoveries of such distinct clades must be interpreted with caution. However, maintenance of ancient polymorphisms through selection is another plausible alternative that may cause the observed discordance and cannot be entirely excluded.
Mutations are the ultimate source of heritable variation and therefore the fuel for evolution, but direct estimates of mutation rates exist only for few species. We estimated the spontaneous single ...nucleotide mutation rate among clonal generations in the waterflea Daphnia galeata with a short-term mutation accumulation approach. Individuals from eighteen mutation accumulation lines over five generations were deep sequenced to count de novo mutations that were not present in a pool of F1 individuals, representing the parental genotype. We identified 12 new nucleotide mutations in 90 clonal generational passages. This resulted in an estimated single nucleotide mutation rate of 0.745 x 10-9 (95% c.f. 0.39 x 10-9-1.26 x 10-9), which is slightly lower than recent estimates for other Daphnia species. We discuss the implications for the population genetics of Cladocerans.
Molecular genetic methods continuously uncover cryptic lineages harboured by various species. However, from an applied perspective, it remains unclear whether and to which extent such a genetic ...diversity affects biological traits (e.g. ecological, behavioural and physiological characteristics) and environmental management. We assessed potential deviations regarding the trait ‘environmental stress tolerance’ using individuals from five field populations of each of two cryptic lineages (called A and B) comprised under the nominal species Gammarus fossarum. We used ammonia as a chemical stressor while assessing the feeding rate on leaf discs as a measure of sublethal response. In this context, we established a restriction fragment length polymorphism assay to allow a rapid identification of the lineages. We observed a biologically meaningful and statistically significant twofold higher overall tolerance of one cryptic lineage, lineage B, over the other. Confounding factors that may have the potential to influence the test results, such as life stage, sex, season of collection, parasitism, physiological status of organisms and upstream land‐use patterns of the river catchments, were either controlled for or displayed only minor deviations between lineages. Synthesis and applications. The trait differences observed in the present study seem to be mainly explained by the considerable genetic differentiation between cryptic lineages of one nominal species. Although traits other than tolerance have been minimally investigated in this context, this study indicates implications in the reliability and quality of environmental monitoring and management if cryptic lineage complexes are ignored.
Cyclic parthenogenesis, the alternation of parthenogenetic and sexual reproduction, can lead to a wide scope of population structures, ranging from almost monoclonal to genetically highly diverse ...populations. In addition, sexual reproduction in aquatic cyclic parthenogens is associated with the production of dormant stages, which both enhance potential gene flow among populations as well as impact local evolutionary rates through the formation of dormant egg banks. Members of the cladoceran genus Daphnia are widely distributed key organisms in freshwater habitats, which mostly exhibit this reproduction mode. We assessed patterns of genetic variation within and among populations in the eurytopic and morphologically variable species Daphnia longispina, using data from both nuclear (13 microsatellite loci) and mitochondrial (partial sequencing of the 12S rRNA gene) markers from a set of populations sampled across Europe. Most populations were characterized by very high clonal diversity, reflecting an important impact of sexual reproduction and low levels of clonal selection. Among-population genetic differentiation was very high for both nuclear and mitochondrial markers, and no strong pattern of isolation by distance was observed. We also did not observe any substantial genetic differentiation among traditionally recognized morphotypes of D. longispina. Our findings of high levels of within-population genetic variation combined with high among-population genetic differentiation are in line with predictions of the monopolization hypothesis, which suggests that in species with rapid population growth and potential for local adaptation, strong priority effects due to monopolization of resources lead to reduced levels of gene flow.
During the initial stages of population build-up in new habitats, the time at which immigrants arrive may influence their contribution to the population and thus determine population genetic ...structure. While the numerical advantage associated with founder effects may promote dominance of the offspring of early colonizers, fitness differences associated with ecological differences among genotypes may potentially overwhelm these founder effects. We conducted an outdoor mesocosm experiment to test whether the sequence of arrival determines the relative contribution of genetic lineages (clones) to populations of the water flea Daphnia. A set of D. galeata clones was inoculated pairwise in different time sequences and clonal abundances were monitored over time. In general, we found that, after six weeks (four generations), clones with an initial time advantage of five or fifteen days (1/3 of a generation or one generation) had higher relative abundances than in the treatments with simultaneous inoculations. These differences were in some cases maintained over a longer period (nine months). However, in cases where abundances of genetic lineages varied strongly when inoculated simultaneously, reflecting strong differences in fitness among clones in our experimental containers, initial numerical advantages could be overwhelmed and the competitively strongest clone became dominant after nine months independently of inoculation sequence. Our results highlight the importance of two different processes that determine the fate of clonal lineages and ultimately determine population genetic structure. First, the sequence of arrival is crucial, with an advantage of five days being already sufficient to dominate the population in specific cases. Second, if fitness differences among clones are sufficiently strong, then they may overrule founder effects.
The invasion success of plants into new regions and ecosystems is assumed to depend on certain constellations of species traits. Often, effective sexual reproduction and the generation of high ...performance genotypes allow invaders to rapidly colonize even unfavourable sites by demonstrating high plasticity. Interspecific hybridisation, can produce such high performance genotypes, which may serve as an evolutionary stepping stone for invaders of limited genetic variability.
A very aggressive invader with particularly limited genetic variability is the Japanese Knotweed (Fallopia japonica). Hybridisation with the Giant Knotweed Fallopia sachalinensis in recent years, however, has created the hybrid Bohemian Knotweed (Fallopia×bohemica) presenting new opportunities for the species complex. An accelerated invasion process could be the result.
We assessed whether or not different taxa of the F. japonica species complex would exhibit different growth potential depending on site conditions. After taxon identification for over 100 samples using genetic markers (microsatellite DNA), we combined the data with observations on Fallopia stand growth, horizontal spreading behaviour, flowering and seed set in relation to different environmental site conditions across an ecologically diverse region in Germany (Rhineland Palatinate).
The majority of the samples were interspecific hybrids, which is an outstanding finding compared to earlier studies. The hybrids were genetically diverse exhibiting a high number of multilocus genotypes, suggesting that sexual reproduction is a major strategy for generating new genotypes and for dispersal. Hybrids that were closely related to F. japonica exhibited the highest potential for aggressive growth in terms of horizontal spreading and reduction of competitor plants within stands. Site conditions, especially N-supply, contributed to the aggressive growth found but were less relevant than taxonomic characteristics. Moreover, several hybrids set a lot of seeds that germinated very well.
We demonstrated that hybridisation is able to generate new high performance genotypes and enables the taxa to accelerate and increase the invasion potential. Our study provides new insights into the invasiveness of alien plants in general and for the taxa F. japonica s. l. in particular.
We report on a discovery of six divergent lineages within the European Daphnia longispina complex from various localities in central to northeast Europe. The levels of divergence from well-known ...species of the complex suggest that they represent as yet unrecognized distinct taxa. These newly recognized lineages always coexisted in syntopy with widespread species of the complex. Particularly rich in cryptic diversity (with four of the six lineages present) was the delta of the River Pechora in northern European Russia, a region not covered by an ice sheet during most of the last glacial period. We suggest that ice-free regions of northeastern Europe may have been important refugia for planktonic species, and still are overlooked hot spots of diversity. Our findings confirm that the real diversity within widespread crustacean planktonic taxa is much higher than presently recognized. The potential presence of cryptic species should be considered in ecological studies.
•Microsatellite markers for five surface and groundwater species.•Populations of surface and groundwater species are highly variable and structured.•Neighboring populations share low levels of gene ...flow.•Genetic methods reveal hidden natural or man-made groundwater hydrological patterns.•The StygotracingⓇapproach represents a promising tool for drinking water management.
Groundwater is one of our most important resources, however groundwater ecosystems are among the most understudied habitats of the planet earth. Studies on groundwater organisms are hampered by the difficult accessibility of species, the lack of morphological differentiation and the limitation for laboratory cultures. One important approach to overcome these shortcomings is to provide sensitive genetic methods to unravel patterns of biodiversity, population structure and gene flow in natural populations. In this study we present five sets of microsatellite markers developed for the isopods Asellus aquaticus and Proasellus slavus, the cyclopoides Paracyclops fimbriatus and Acanthocyclops sensitivus and the harpacticoide Bryocamptus echinatus (Crustacea). Two of these species were subjected to detailed population genetic analyses: We studied 501 specimens of Asellus aquaticus from four different regions in Northern Germany using nine microsatellite markers and 70 specimens of Bryocamptus echinatus using nine microsatellite markers from three different sampling sites in South-Western Germany. Our results show that genetic diversity is high (A. aquaticus: 10 to 20 and B. echinatus: 4 to 18 alleles per locus) among populations of aquatic invertebrates, populations are highly differentiated (FST > 0.2) and genetic differentiation was associated with geographic patterns. Applications of molecular genetic methods and their use for the detection of hydrological exchange processes relevant for drinking water suppliers are demonstrated and discussed.
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•Daphnia was not susceptible to the crayfish plague disease agent Aphanomyces astaci.•Aphanomyces astaci zoospores represent a valuable food source to Daphnia.•Zoospore consumption by ...Daphnia reduced the Ap. astaci load in susceptible crayfish.•Daphnia could act as buffer against zoospore-transmitted diseases in lentic waters.
Pathogen spores have been recognized as prey with implications for resource dynamics, energy transfer and disease transmission. In aquatic ecosystems, filter-feeders are able to consume such motile forms of pathogens that can cause severe disease in susceptible hosts. The interactions between European crayfish and the crayfish plague pathogen Aphanomyces astaci are of particular conservation interest. In this study, we aim to evaluate the ecological interactions between Ap. astaci, its host Astacus astacus and individuals of the genus Daphnia, filter-feeding planktonic crustaceans. Our focus was on the consumption of the motile zoospores by Daphnia individuals, but we also considered the potential of Daphnia as non-target hosts. We conducted a series of infection and life-history experiments with Ap. astaci, three Daphnia species (D. magna, D. galeata, and D. pulex) and the noble crayfish As. astacus. We did not observe any lethal effects in the infection experiments involving Ap. astaci and Daphnia. Only D. pulex showed differences in some life-history traits. The feeding experiment using the motile zoospores of Ap. astaci as alternative food source or as supplement to different amounts of algal food revealed their nutritional value: D. magna individuals survived, grew, and reproduced on a zoospore diet alone. When zoospores were supplemented to the regular algal diet, all life-history parameters have been significantly improved. However, this successful consumption of zoospores did not result in a reduced mortality of the susceptible crayfish As. astacus during the infection experiment. Nevertheless, the pathogen load of Ap. astaci in the tissues of As. astacus was significantly reduced as a consequence of the feeding activity of Daphnia. Our results indicate that an abundant filter-feeding community can reduce the amount of infective zoospores in the water body and thus be beneficial to susceptible crayfish hosts, potentially acting as a general buffer against zoospore-transmitted diseases in lentic waters.
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