Individual spores of Caullerya mesnili. Copyright: Lohr et al. 2010, Journal of Eukaryotic Microbiology.
Host-parasite interactions
Host-parasite interactions in hybridizing Daphnia, from correlations to experiments Project Objectives
How does hybridization affect ecological processes? There are over 200 species of Daphnia (waterfleas), a type of crustacean found in a variety of aquatic ecosystems, some of which can hybridize with each other. We spoke to Dr Piet Spaak and Dr Justyna Wolinska about their work in investigating the extent to which hybridization and parasitism influence major ecological processes. Daphnia are a type of small crustacean that live in the open water and are found in many aquatic ecosystems, including lakes and ponds across Europe. They eat algae and are themselves food for fish, which makes them crucial in aquatic food webs. There are over 200 different species of Daphnia, including Daphnia longispina and Daphnia galeata, and these species can hybridize with each other, a topic central to Dr Piet Spaak’s research. “I’ve been interested in hybridization between different species since 1990. Around 20 years ago there was a lot of interest among the research community in the role of parasites in ecosystems. I thought: Is this something that influences the success of parental species in hybrids?” he outlines. This question is at the core of a research project backed by the Swiss and German National Science Foundations, in which Dr Spaak and Dr Wolinska are additionally investigating the impact of eutrophication on aquatic ecosystems. “We found that eutrophication influenced the Daphnia populations in lakes. We found hybrids in lakes, between two species,” he says. These crustaceans hold a great deal of interest for scientists, partly because they are able to reproduce very rapidly through both sexual and asexual reproduction. A female Daphnia is essentially able to clone herself and produce genetically identical babies. “You can very rapidly have thousands of genetically identical Daphnia. On top of that, Daphnia can also reproduce sexually, where genetic material from two lineages is combined,” says Dr Spaak. These fertilized sexual eggs are resting eggs that can survive in the sediment for many years. Researchers in the project are analysing Daphnia that
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for example in eutrophication level, or in water temperature,” outlines Dr Wolinska. The next step would be to investigate how these environmental conditions affect the frequency and extent of infection by parasites. “We are studying Greifensee in detail over time. We can also test to what extent the level of infection depends on seasonally changing environmental conditions,” continues Dr Wolinska. “If we see something interesting, we then want to verify that in the laboratory.” Researchers are able to precisely control the conditions in a lab environment for Daphnia clones, which helps scientists to investigate the impact of any specific environmental variable. Observations from the field can be tested in the lab, from which researchers can then look to build a deeper understanding. “For example, we
and they are not selective, and when they consume filamentous structures their feeding apparatus gets clogged. Cyanobacteria are also less nutritious than green algae,” outlines Dr Wolinska. The wider context here is ongoing concern about the health of aquatic ecosystems and the long-term consequences for biodiversity. “There are other factors that influence lakes too, such as climate change, so it’s not only about the amount of nutrients,” says Dr Spaak. The general aim in the project is to study the extent to which hybridization and parasitism influence major ecological processes however, rather than more specific objectives around lake management. A lot of progress has been made on the molecular side of this research. “We have sequenced the genome of Daphnia galeata. We’ve also tried to sequence the genome of the parasite, yet that’s a lot
We are studying Daphnia populations over time. We can test to what extent the level of infection depends on
This SNF-DFG funded project aims to understand the influence of environmental factors on hybridization rate and host-parasite interactions in lake plankton. Specifically, we investigate how eutrophication affects hybridization rate and parasitic infections in Daphnia (water fleas). To do so, we combine field observations and experimental studies. This project, which results from a close collaboration between Swiss and German science teams, will further our understanding of the environmental conditions that drive the evolutionary processes in natural systems.
Project Funding
This project is funded by the Swiss National Science Foundation (SNF 166628) and the German Research Foundation (DFG WO 1587/6-1).
Project Partners
• Prof. Iñaki Ruiz-Trillo, Institut de Biologia Evolutiva (CSIC-Universitat Pompeu Fabra), Barcelona, Catalonia, Spain • Prof. Michael Monaghan, The Berlin Center for Genomics in Biodiversity Research (BeGenDiv), Berlin, Germany
Contact Details
Dr Piet Spaak E: Piet.Spaak@eawag.ch W: https://www.eawag.ch/de/abteilung/ eco/schwerpunkte/zooplankton-oekologieund-evolution/ Dr Justyna Wolinska E: wolinska@igb-berlin.de W: https://www.igb-berlin.de/en/wolinska Literature Lu et al. 2020 Mol. Phylogenet. Evol. 151, 106891. Turko et al. 2018 Evolution 72, 619-629. Dr Piet Spaak
Dr Justyna Wolinska
changing environmental conditions. Daphnia galeata infected with protozoan parasite Caullerya mesnili. Multiple spore clusters are visible in the Daphnia gut (green). This parasite is very virulent to its host; it shuts down Daphnia reproduction and causes its earlier death. Picture: Kirstin Bittner, Eawag.
hatched from resting eggs of different ages. “We have Daphnia in our lab that hatched from 60 year-old resting eggs and therefore are genetically the same as the Daphnia that have been in the lake during those times. That gives us some very interesting research opportunities,” continues Dr Spaak.
Impact of eutrophication This research is being conducted against a backdrop of continued concern about the impact of eutrophication, where aquatic ecosystems become excessively rich in nutrients, which then affects the ecological balance. Greifensee, as well as almost all lakes in Western and Central Europe, went through a eutrophication peak around the early ‘80s, since when the level of nutrients has decreased, a major point of interest in the project. “We can use time-series data
Caullerya spore clusters
to look at how eutrophication levels affect biological processes,” explains Dr Justyna Wolinska, Research Group Leader at IGB in Berlin and Professor at the Freie Universität Berlin. “We can study a very large number of Daphnia generations in short timescales. We can look at changes in the same lake over time as it goes through different phases of eutrophication,” says Dr Wolinska. The project team is also studying the interactions of Daphnia with its parasites such as Caullerya mesnili, a highly virulent parasite that has caused disease in lakes across Europe. Researchers are taking a twopronged approach to this work, combining observations in the field with laboratory work. “First of all we observe patterns in nature. Here we use spatial studies, where we investigate several different lakes, which differ in environmental conditions,
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observed in the field that if there were more cyanobacteria, there were more infections. From that you can formulate a hypothesis that higher levels of cyanobacteria make Daphnia more susceptible to infection. We then fed some Daphnia in the lab cyanobacteria, while we fed other Daphnia normal algae,” explains Dr Spaak. While cyanobacteria and algae both appear as green cells on the surface of a lake to the naked eye, Dr Spaak says they in fact are distinct from each other. “Cyanobacteria are not eukaryotes, they are really bacteria, that’s the difference. These cyanobacteria can also produce toxins,” he continues.
Cyanobacteria This is not the only way in which cyanobacteria have a negative impact on aquatic ecosystems. Alongside producing toxins, they also have quite difficult, filamentous morphologies, which means they are not an ideal food source for zooplankton. “Daphnia are filter-feeders
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more complex, because the parasite is intercellular,” says Dr Spaak. There is enormous scope for further research, and in future Dr Wolinska hopes to build a deeper mechanistic understanding of the processes that have been observed in the field. “For example, what kinds of molecular mechanisms are responsible for the processes we’ve observed? We would also like to look at the genomic side of host-parasite adaptations,” she says. A number of studies have also shown that the microbiome of an organism plays a role in the interaction with diseases, representing another potential avenue of investigation. A Daphnia aquires its microbiome in the gut from the environment. When they feed bacteria come in and a population develops in their gut. “The question then is, does the diversity of the bacteria play a role in the success of the parasite?” asks Dr Spaak. “These are the types of mechanistic things, in terms of genes, genetics, and also biology, that we are looking at.”
Dr Piet Spaak is a senior scientist at Swiss Federal Institute of Aquatic Science and Technology (Eawag) and teaches at ETH-Zürich. His research focuses on understanding the interaction between aquatic organisms and their changing environment. He uses Daphnia as a model system for field and experimental work. Dr Justyna Wolinska is group leader at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) and professor at the Freie Universität Berlin. She studies evolutionary and ecological processes mediated by parasitism in aquatic ecosystems. For example, she investigates how parasitism contributes to the maintenance of genetic diversity.
German Research Foundation
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