page_title: Host–parasite coevolution
section_title: Trade-off between transmission and virulence
hierarchical_section_title: Host–parasite coevolution / Theories / Trade-off between transmission and virulence
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: Tribolium castaneum, the red flour beetle, is a host for the microsporidian Nosema whitei. This parasitoid kills its host for transmission, so the host's lifespan is important for the parasite's success. In turn, parasite fitness most likely depends on a trade-off between transmission (spore load) and virulence. A higher virulence would increase the potential for the production of more offspring, but a higher spore load would affect the host's lifespan and therefore the transmission rate. This trade-off is supported by coevolutionary experiments, which revealed the decrease of virulence, a constant transmission potential and an increase in the host's lifespan over a period of time.
  Further experiments demonstrated a higher recombination rate in the host during coevolutionary interactions, which may be selectively advantageous because it should increase diversity of host genotypes.
  Resources are generally limited. Therefore, investment in one trait (e.g. virulence or immunity) limits investment in other life-history traits (e.g. reproductive rate). Moreover, genes are often pleiotropic, having multiple effects. Thus, a change in a pleiotropic immunity or virulence gene can automatically affect other traits. There is thus a trade-off between benefits and costs of the adaptive changes that may prevent the host population from becoming fully resistant or the parasite population from being highly pathogenic. The costs of gene pleiotropy have been investigated in coevolving Escherichia coli and bacteriophages. To inject their genetic material, phages need to bind to a specific bacterial cell surface receptor. The bacterium may prevent injection by altering the relevant binding site, e.g. in response to point mutations or deletion of the receptor. However, these receptors have important functions in bacterial metabolism. Their loss would thus decrease fitness (i.e. population growth rate). As a consequence, there is a trade-off between the advantages and disadvantages of a mutated receptor, leading to polymorphism at this locus.
caption: Tribolium castaneum
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