page_title: Host–parasite coevolution
section_title: Model systems for research
hierarchical_section_title: Host–parasite coevolution / Model systems for research
caption_attribution_description: Deutsch: Daphnia spec., Frankfurt/Main, Deutschland Français&#160;: Daphnia spec., Frankfurt/Main, Allemagne English: Daphnia spec., Frankfurt/Main, Germany
context_page_description: Host–parasite coevolution is a special case of coevolution, the reciprocal adaptive genetic change of a host and a parasite through reciprocal selective pressures.
  It is characterized by reciprocal genetic change and thus changes in allele frequencies within populations. These are determined by three main types of selection dynamics: negative frequency-dependent selection when a rare allele has a selective advantage; overdominance caused by heterozygote advantage; and directional selective sweeps near an advantageous mutation.
  Theories of host–parasite coevolution include the geographic mosaic theory, which assumes a selection mosaic, coevolutionary hotspots, and geographic mixing; the Red Queen hypothesis, which proposes that parasitism favours sexual reproduction in the host; and an evolutionary trade-off between transmission and virulence, since if the parasite kills its host too quickly, the parasite will not be able to reproduce.
  Model systems include the nematode Caenorhabditis elegans with the bacterium Bacillus thuringiensis; the crustacean Daphnia and its numerous parasites; and Escherichia coli and the mammals whose intestines it inhabits.
context_section_description: The nematode Caenorhabditis elegans and the bacterium Bacillus thuringiensis were only recently established as a model system for studying host–parasite coevolution. Laboratory evolution experiments provided evidence for many of the basic predictions of these coevolutionary interactions, including reciprocal genetic change, and increases in the rate of evolution and genetic diversity.
  The crustacean Daphnia and its numerous parasites have become one of the main model systems for studying coevolution. 
  The host can be asexual as well as sexual (induced by changes in the external environment), so sexual reproduction can be stimulated in the laboratory. Decades of coevolution between Daphnia magna and the bacterium Pasteuria ramosa have been reconstructed, reanimating resting stages of both species from laminated pond sediments and exposing hosts from each layer to parasites from the past, the same and the future layers. The study demonstrated that parasites were on average most infective with their contemporary hosts, consistent with negative frequency dependent selection.
  Escherichia coli, a Gram-negative proteobacterium, is a common model in biological research, for which comprehensive data on various aspects of its life-history is available. It has been used extensively for evolution experiments, including those related to coevolution with phages. These studies revealed – among others – that coevolutionary adaptation may be influenced by pleiotropic effects of the involved genes. In particular, binding of the bacteriophage to E. coli surface receptor is the crucial step in the virus infection cycle. A mutation in the receptor's binding site may cause resistance. Such mutations often show pleiotropic effects and may cause a cost of resistance. In the presence of phages, such pleiotropy may lead to polymorphisms in the bacterial population and thus enhance biodiversity in the community.
  Another model system consists of the plant- and animal-colonizing bacterium Pseudomonas and its bacteriophages. This system provided new insights into the dynamics of coevolutionary change. It demonstrated that coevolution may proceed via recurrent selective sweeps, favouring generalists for both antagonists. Furthermore, coevolution with phages may promote allopatric diversity, potentially enhancing biodiversity and possibly speciation. Host-parasite coevolution may also affect the underlying genetics, for example by favouring increased mutation rates in the host.
  Tropical tree and liana interactions have also been the subject of study. Here lianas have been viewed as hyper-diverse generalist macro-parasites that affect host survival by parasitising on the host's structural support for access to canopy light, while usurping resources that would otherwise be available to their host. Host trees have widely varying levels of tolerance to infestation of their crowns by lianas.
caption: Daphnia magna
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