Browsing by Subject "Daphnia pulex"

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  • Cornetti, Luca; Fields, Peter D.; Van Damme, Kay; Ebert, Dieter (2019)
    In the post-genomic era, much of phylogenetic analyses still relies on mitochondrial DNA, either alone or in combination with few nuclear genes. Although this approach often makes it possible to construct well-supported trees, it is limited because mtDNA describes the history of a single locus, and nuclear phylogenies based on a few loci may be biased, leading to inaccurate tree topologies and biased estimations of species divergence time. In this study, we perform a phylogenomic analysis of the Daphniidae family (Crustacea: Branchiopoda: Anomopoda) including some of the most frequently studied model organisms (Daphnia magna and D. pulex) whose phylogenetic relationships have been based primarily on an assessment of a few mtDNA genes. Using high-throughput sequencing, we were able to assemble 38 whole mitochondrial genomes and draft nuclear genomes for 18 species, including at least one species for each known genus of the family Daphniidae. Here we present phylogenies based on 636 nuclear single-copy genes shared among all sampled taxa and based on whole mtDNA genomes. The phylogenies we obtained were highly supported and showed some discrepancies between nuclear and mtDNA based trees at deeper nodes. We also identified a new candidate sister lineage of Daphnia magna. Our time-calibrated genomic trees, which we constructed using both fossil records and substitution rates, yielded very different estimates of branching event times compared to those based on mtDNA. By providing multi-locus, fossil-calibrated trees of the Daphniidae, our study contributes to an improved phylogenetic framework for ecological and evolutionary studies that use water fleas as a model system.
  • Lakka, Hanna-Kaisa (Helsingfors universitet, 2013)
    Lepidurus arcticus (Pallas, 1793) is a keystone species in High Arctic ponds, which are exposed to a wide range of environmental stressors. This thesis provides information on the ecology of this little studied species by paying particular focus on the sensitivity of L. arcticus to acidification and climate change. Respiration, reproduction, olfaction, morphology, salinity and pH tolerance of the species were studied in the laboratory and several environmental parameters were measured in its natural habitats in Arctic ponds. Current global circulation models predict 2–2.4 °C increase in summer temperatures on Spitsbergen, Svalbard, Norway. The L. arcticus respiration activity was tested at different temperatures (3.5, 10, 16.5, 20, 25 and 30 °C). The results show that L. arcticus is clearly adapted to live in cold water and have a temperature optimum at +10 °C. This species should be considered as stenothermal, because it seems to be able to live only within a narrow temperature range. L. arcticus populations seem to have the capacity to respond to the ongoing climate change on Spitsbergen. Changes can be seen in the species' reproductive capacity and in the individuals' body size when comparing results with previous studies on Spitsbergen and in other Arctic areas. Effective reproduction capacity was a unique feature of the L. arcticus populations on Spitsbergen. L. arcticus females reached sexual maturity at a smaller body size and sexual dimorphism appeared in smaller animals on Spitsbergen than anywhere else in the subarctic or Arctic regions. L. arcticus females were able to carry more eggs (up to 12 eggs per female) than has been observed in previous studies. Another interesting feature of L. arcticus on Spitsbergen was their potential to grow large, up to 39.4 mm in total length. Also cannibalistic behaviour seemed to be common on Spitsbergen L. arcticus populations. The existence of different colour morphs and the population-level differences in morphology of L. arcticus were unknown, but fascinating characteristic of this species. Spitsbergen populations consisted of two major (i.e. monochrome and marbled) and several combined colour morphs. Third interesting finding was a new disease for science which activated when the water temperature rose. I named this disease to Red Carapace Disease (RCD). This High Arctic crustacean lives in ponds between the Arctic Ocean and glaciers, where the marine environment has a strong impact on the terrestrial and freshwater ecosystems. The tolerance of L. arcticius to increased water salinity was determined by a LC50 -test. No mortality occurred during the 23 day exposure at low 1–2 ‰ water salinity. A slight increase in water salinity (to 1 ‰) speeded up the L. arcticus shell replacement. The observations from natural populations supported the hypothesis that the size of the animals increases considerably in low 1.5 ‰ salt concentrations. Thus, a small increase in water salinity seems to have a positive impact on the growth of this short-lived species. Acidification has been a big problem for many crustaceans, invertebrates and fishes for several decades. L. arcricus does not make an exception. Strong acid stress in pH 4 caused a high mortality of mature L. arcticus females. The critical lower limit of pH was 6.1 for the survival of this acid sensitive species. Thus, L. arcticus populations are probably in danger of extinction due to acidification of three ponds on Spitsbergen. A slight drop (0.1–1.0) in pH values can wipe out these L. arcticus populations. The survival of L. arcticus was strongly related to: (1) the water pH, (2) total organic carbon (TOC) and pH interaction, (3) the water temperature and (4) the water salinity. Water pH and TOC values should be monitored in these ponds and the input of acidifying substances in ponds should be prevented.