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