Transcript jon.fig wasps.ppt

Snail-Schistosome host-parasite system
Eric Stock
Bio. 4800
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Review
Purpose
Methods
◦ Experiments 1 & 2
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Results
◦ Experiments 1 & 2
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Conclusion
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Snail (Biomphalaria glabrata)
Schistosome (Schistosoma mansoni)
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Remember:
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◦ Miracidia infect mollusc – snail
◦ Cercariae infects mammal - human
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Investigate cost of resistance to snail
Artificially breed snails
◦ Resistance (R) and Susceptible (S) to schisto.
Snail fertility (reproductive performance)
◦ # offspring produced
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Snail fecundity (potential reproductive capacity)
◦ # of eggs and embryos formed
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Snails housed in tanks with Styrofoam sheets
to lay eggs on
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6 tanks – 60 sexually mature snails
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4 tanks – exposed each snail to 5 miracidia
◦ 5 miracidia = infection rate of 50% snail population
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2 tanks – controls – no parasites
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After 4-7 weeks screened for cercariae
◦ Screening: Post 48hr dark period – placed in light
for 2hr – looked for shedding of cercariae
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Week 7: divided tanks
◦ separated resistant (R) from susceptible (S) &
control (C)
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10 tanks: 4 R, 4 S, 2 C
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Bred for 15 weeks
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1) # breeding adults remaining (mortality)
2) Total # of egg masses in tank
3) Mean # of eggs/egg mass
◦ embryo within egg visible
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4) # malformed egg masses
5) # offspring/tank
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F6 exposed to 5 miracidia & screened
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To determine if reproductive success was demonstrated in
absence of parasite
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Differences are due to genotype (R & S) not phenotype
(Individual)
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If Phenotypic response:
◦ Parasitized snail offspring more susceptible
◦ Unparasitized offspring more resistant
If Genotypic response:
◦ F6 snails identical between groupings
Experiment 1
Experiment 2
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Resistance and susceptibility to schistosome
infection was heritable
Significant difference between S & R & C fertility
◦ F1 and all subsequent generations
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No R snail shed after 7 weeks = Resistant
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F6 – no significant difference between R & S
◦ Therefore genotype not phenotype
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Significant difference in fertility & mortality
◦ S higher offspring than C and R (lowest)
◦ S higher mortality than C and R (lowest)
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Significant difference in fertility
o
S higher offspring than C and R (lowest)
Week
+ (exposed-infected)
- (unexposed-uninfected)
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Resistant snails produced fewer offspring
◦ = lower fertility
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Wasn’t due to phenotype of snail – genotype
Suggest cost of resistance in absence of
parasite
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Webster, J.P., Woolhouse, M.E.J. (1999) Cost
of resistance : relationship between
reduced fertility and increased resistance
in a snail-dchidtosome host-parasite
system. The Royal Society. 266: 391-39.6