Abstract
BackgroundThe population structure of cyclical parthenogens such as water fleas is strongly influenced by the frequency of alternations between sexual and asexual (parthenogenetic) reproduction, which may differ among populations and species. We studied genetic variation within six populations of two closely related species of water fleas of the genus Daphnia (Crustacea, Cladocera). D. galeata and D. longispina both occur in lakes in the Tatra Mountains (Central Europe), but their populations show distinct life history strategies in that region. In three studied lakes inhabited by D. galeata, daphnids overwinter under the ice as adult females. In contrast, in lakes inhabited by D. longispina, populations apparently disappear from the water column and overwinter as dormant eggs in lake sediments. We investigated to what extent these different strategies lead to differences in the clonal composition of late summer populations.ResultsAnalysis of genetic variation at nine microsatellite loci revealed that clonal richness (expressed as the proportion of different multilocus genotypes, MLGs, in the whole analysed sample) consistently differed between the two studied species. In the three D. longispina populations, very high clonal richness was found (MLG/N ranging from 0.97 to 1.00), whereas in D. galeata it was much lower (0.05 to 0.50). The dominant MLGs in all D. galeata populations were heterozygous at five or more loci, suggesting that such individuals all represented the same clonal lineages rather than insufficiently resolved groups of different clones.ConclusionsThe low clonal diversities and significant deviations from Hardy-Weinberg equilibrium in D. galeata populations were likely a consequence of strong clonal erosion over extended periods of time (several years or even decades) and the limited influence of sexual reproduction. Our data reveal that populations of closely related Daphnia species living in relatively similar habitats (permanent, oligotrophic mountain lakes) within the same region may show strikingly different genetic structures, which most likely depend on their reproductive strategy during unfavourable periods. We assume that similar impacts of life history on population structures are also relevant for other cyclical parthenogen groups. In extreme cases, prolonged clonal erosion may result in the dominance of a single clone within a population, which might limit its microevolutionary potential if selection pressures suddenly change.
Highlights
The population structure of cyclical parthenogens such as water fleas is strongly influenced by the frequency of alternations between sexual and asexual reproduction, which may differ among populations and species
Clonal richness, calculated as the number of multilocus genotypes (MLGs) detected in the sample divided by the total number of studied individuals in each population (MLG/N), ranged from 0.97 to 1.00; clonal diversity, calculated as the complement of the maximum likelihood estimator of the Simpson’s index, (1-D), reached the maximum possible value of 1.00 in all three populations
None of the D. longispina populations significantly deviated from expectations of the Hardy-Weinberg equilibrium (HWE)
Summary
The population structure of cyclical parthenogens such as water fleas is strongly influenced by the frequency of alternations between sexual and asexual (parthenogenetic) reproduction, which may differ among populations and species. In aquatic environments that undergo regular seasonal changes, including cold temperate lakes or temporary ponds, many zooplankton groups such as water fleas (Cladocera) usually survive unfavourable periods of winter or drought as dormant stages, and hatch at the onset of more favourable conditions. The typical cladoceran life cycle (reviewed in [2]) is cyclical parthenogenesis: females reproduce clonally under favourable conditions, and switch to male production and sexual reproduction when conditions deteriorate. Each hatchling from a dormant egg of a cyclically parthenogenetic cladoceran has a unique genotype [4,5] Under this scenario, a high number of genotypes may be expected at the beginning of the growing season in such populations. Apart from perceivable reductions in clonal and genetic diversity, this typically leads to deviations from the Hardy-Weinberg equilibrium, and increases among-population genetic differentiation as a result of enhanced genetic drift [4,5,6]
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