Snail-Schistosome host-parasite system
Eric StockBio. 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 fertilityo 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
Top Related