Lecture33_Parasitism - Parasitism and mutualism I...

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Unformatted text preview: Parasitism and mutualism I. Host-parasite interactions as coevolutionary arms races Malaria as a model of an arms race: parasitism by Plasmodium A. Impact kills ~ million people per year million affected per year B. Basic biology life cycle: note that Plasmodium lives in two hosts C. Arms race I: Malaria and public health professionals use of DDT: chloroquine and other quinine-derived drugs: D. Arms Race II: Evolutionary responses in humans In areas where malaria is endemic, does natural selection favor individuals with resistant alleles? 1. Certain immune system molecules provide protection against certain Plasmodium strains e.g. HLA-B53 allele in West African people: older strains Plasmodium falciparum (cp26 and cp29) versus newly evolved strains (cp27 and cp28) Predict: Resistance in people with HLA-B53 cp26 cp29 cp27 cp28 2. Selection in favor of new alleles? Hemoglobin (Hb) is a protein that carries oxygen in red blood cells The sickling allele (HbS) helps eliminate Plasmodium when it is heterozygous; causes sickle cell disease when homozygous NN NS SS New data: study Hb alleles in healthy and Plasmodium-infected people from Burkina Faso Note: relatively high frequency of HbC allele Two key observations: 1) In healthy people, HbC genotypes are in 2) Among Plasmodium-infected people, observe Conclude: III. Can parasites manipulate their hosts? Does natural selection favor alleles that allow parasites to manipulate host behavior in a way that Host manipulation by Plasmodium? 19 human volunteers sleep in collect Predict: (under host manipulation hypothesis) What % of uninfected mosquitoes bit more than one person? What % of infected mosquitoes bit more than one person? Difference was even more pronounced for Claim: malaria parasite is manipulating mosquito hosts in a way that make them IV. Mutualisms as dynamic interactions A. Examples of mutualisms: B. Treehoppers and ants as a case study treehoppers are ectoparasites of plants: they excrete "honeydew" Question: Do treehoppers gain anything from this association? Experiment: --1000 m2 study plot --in each of 3 years, randomly assign host plants to ants present versus no ants Plot data for plants with and without ants treehopper offspring per plant Time (over summer) NOTE: in one year, see Why? Hypothesis: ants Punchline: nature of association is ...
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