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