Parasite-susceptibility phenotypes of F 1 Biomphalaria glabrata progeny derived from interbreeding Schistosoma mansoni -resistant and -susceptible snails

Abstract

In an effort to investigate the 'flow' of parasite-resistance genes through laboratory snail populations, we determined the susceptibility of progeny snails from freely interbreeding parasite-susceptible and parasite-resistant parents. Five parental populations of Biomphalaria glabrata were used to generate the progeny snails. Three of them contained different proportions of Schistosoma mansoni-susceptible albino snails (NMRI stock) and S. mansoni-resistant pigmented snails (BS-90), while single stock controls comprised the other two parental populations. F(1) snails from each parental population were exposed to S. mansoni miracidia. Some of the progeny snails were exposed as juveniles, others as adults. According to Hardy-Weinberg principle predictions, the F(1) generation from the three pigmented/albino parental populations displayed higher than expected numbers of pigmented (resistant) snails and lower than expected numbers of albino (susceptible) snails. Among the assumptions of the Hardy-Weinberg principle that were not met within these populations could include non-random mating, unequal fecundity, different hatching and survival rates of different genotypes, or other life-history differences between snail stocks. It is clear, though, that for these two laboratory snail stocks there is no fitness cost attached to genetic resistance to the parasite.

Inheritance of Schistosoma mansoni infection incompatibility in Biomphalaria alexandrina snails

“…Under the above described conditions, we transferred groups of roughly 25 snails to separate containers, each containing five litres of DTW. They were kept in darkness for about four weeks (Smithers & Terry 1965, Frandsen & Christensen 1984, Haroun 1996, Lewis et al 2002.…”

“…Snails of each group were reared together in a separate aquarium containing about 4-5 L of DTW (Azim & Watson 1948, Kogan et al 1954, Smithers & Terry 1965, Frandsen & Christensen 1984, Dettman et al 1989, Lewis et al 2002. To avoid fertilised eggs before the beginning of the experiment, we discarded egg batches from each of the five groups during the 1st four weeks.…”

“…They were individually exposed to S. mansoni miracidia (Kogan et al 1954, El-Gindy et al 1978, Shoukry et al 1997. Four weeks later, these juveniles were individually tested for cercarial shedding, returned to a separate container containing 250 mL of DTW and maintained in the same conditions described previously (Azim & Watson 1948, Kogan et al 1954, Smithers & Terry 1965, Frandsen & Christensen 1984, Dettman et al 1989, Lewis et al 2002.…”

“…During winter, they were kept at 26 o C and maintained at room temperature in the summer (Haroun 1996). Fresh lettuce leaves were supplied as food every couple of days; dead snails were regularly removed (Smithers & Terry 1965, Frandsen & Christensen 1984, Lewis et al 2002.…”

“…This study would enable workers to predict the flow of resistance genes in the population and assess the feasibility of this snail control method (Lewis et al 2002). It has been suggested that incompatibility to S. mansoni infection, a refractory characteristic of Biomphalaria alexandrina, could be hereditary, similar to susceptibility (Abdel-Hamid et al 2006).…”