Lecture18%20Evolution%20of%20Resistance

Lecture18%20Evolution%20of%20Resistance - Host resistance...

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3/25/2009 1 Evolution of Host Resistance 1 Host resistance evolution • What is the basis of host resistance? • Do pathogens act as agents of selection on host resistance? • What forces maintain variation in host resistance? 2 Points to remember 1. Host resistance is part of an ‘evolutionary arms race’ between host and pathogen Pathogens select for increasing defenses, hosts select for new pathogen variants 3 Points to remember 1. Host resistance is part of an ‘evolutionary arms race’ between hosts and pathogens 2. Host defenses are costly to maintain and subject to life-history trade-offs - Resistance is only beneficial against a pathogen - Most defenses usually come at some cost - Explains in part why hosts are not typically resistant to “novel pathogens” 4 Points to remember 1. Host resistance is part of an ‘evolutionary arms race’ between hosts and pathogens 2. Host defenses are costly to maintain and subject to life history trade offs subject to life-history trade-offs 3. Host defenses vary dynamically with characteristics of the individual and the environment -- Host sex, body condition, stress, food resources, temperature, photoperiod, array of parasites… 5 Evolution of host resistance • What is host resistance? • Ability to avoid or counter parasite infection Inverse of susceptibility “Resistant”= rarely or never infected “Susceptible” = mostly or always infected • Tolerance: performance despite infection (survival, growth, reproduction) 6
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3/25/2009 2 Expression of resistance? • Immune defenses that response to infection • Other barriers to infection: –Biochemical resistance –Key receptors missing –Physical barriers to infection –Behavioral resistance 7 Sickle Cell Anemia and Heterozygote Advantage Sickled cells show loss of normal function and are less able to support malaria parasites 8 Genetics of sickle cell • SS - normal RBCs, high risk of malaria in endemic regions • Ss - partially sickled cells, higher survival in malarial regions • ss - resistant to malaria but “sickle cell syndrome” – Early death Independent origins (at least four times) 9 HIV resistance in humans can result from single mutation in CCR5 co-receptor Frame shift Δ 32 mutation Alters expression of cell surface receptor on CD4+ T-cells Prevents HIV-1 entry 10
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Lecture18%20Evolution%20of%20Resistance - Host resistance...

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