Effects of Cu/Zn superoxide dismutase (sod1) genotype and genetic background on growth, reproduction and defense in Biomphalaria glabrata

Abstract

Resistance of the snail Biomphalaria glabrata to the trematode Schistosoma mansoni is correlated with allelic variation at copper-zinc superoxide dismutase (sod1). We tested whether there is a fitness cost associated with carrying the most resistant allele in three outbred laboratory populations of snails. These three populations were derived from the same base population, but differed in average resistance. Under controlled laboratory conditions we found no cost of carrying the most resistant allele in terms of fecundity, and a possible advantage in terms of growth and mortality. These results suggest that it might be possible to drive resistant alleles of sod1 into natural populations of the snail vector for the purpose of controlling transmission of S. mansoni. However, we did observe a strong effect of genetic background on the association between sod1 genotype and resistance. sod1 genotype explained substantial variance in resistance among individuals in the most resistant genetic background, but had little effect in the least resistant genetic background. Thus, epistatic interactions with other loci may be as important a consideration as costs of resistance in the use of sod1 for vector manipulation.

Figure 1. Breeding scheme for generating F2 populations with different genetic backgrounds.

We created F2 populations by crossing inbred lines (3 generations of selfing) that were fixed for BB or CC genotypes. F1 offspring from unique inbred line crosses (fixed for the BC genotype) were then bred to generate outbred F2 populations that were segregating for BB, BC, CC genotypes in the expected Mendelian ratio. This was done three times to generate three different genetic backgrounds that differ in average resistance.

Figure 2. Resistance of genetic background as a function of average resistance of grandparental inbred lines.

Mid-grandparent resistance was estimated by averaging the resistance of the four inbred, grandparental lines (determined previously). The resistance of each genetic background (Grand-offspring resistance) was estimated by parasite challenges done in triplicate (n = 24×3) for each background (• genetic background 1, ▴ genetic background 2, and ▪ genetic background 3).

Figure 3. Effects of genetic background and sod1 genotype on resistance to infection.

Graphs illustrate the average resistance of each genotypic class within each background after a challenge with five PR-1 S. mansoni miracidia. Resistance means are the averages of three replicates (tubs starting with n = 24 snails each). Error bars represent 1±SE (background 1: n = 55 (BB = 10, BC = 36, CC = 9), background 2: n = 63 (BB = 8, BC = 39, CC = 16), background 3: n = 63 (BB = 14, BC = 35, CC = 14)). There were significant main effects of genetic background, genotype, and interaction between genetic background and sod1 genotype. (See text for statistical analyses.)

Figure 4. Effect of genetic background and sod1 genotype on life-history traits.

(A) Average size by genotypic class within each lineage at 12 weeks after egg masses were deposited. The points represent the averages of the mean size of individuals of each genotype within each of 3–4 cups (containing 13–17 F2 snails per cup). Error bars represent 1±SE (background 1: n = 45 (BB = 16, BC = 20, CC = 9), background 2: n = 57 (BB = 16, BC = 28, CC = 13) background 3: n = 58 (BB = 16, BC = 28, CC = 14)). Snails with the CC genotype grew significantly more slowly than those with BC and BB genotypes. (B) Average size at 32 weeks of each genotypic class within each lineage. Means are the average of all snails within the genotypic class, and error bars represent 1±SE (background 1: n = 27 (BB = 9, BC = 10, CC = 8), background 2: n = 27 (BB = 9, BC = 10, CC = 8), background 3: n = 30 (BB = 10, BC = 10, CC = 10)). Again, snails with the CC genotype grew significantly more slowly than those with BC and BB genotypes. (C) Average total egg production for five weeks per snail (each raised individually) by genotypic class within each lineage. Means are the average of all snails within the genotypic class, and error bars represent 1±SE (background 1: n = 25 (BB = 9, BC = 9,CC = 7), background 2: n = 27 (BB = 9, BC = 10, CC = 8), background 3: n = 30 (BB = 10, BC = 10,CC = 10)). (D) Mortality at 8-month census of each genotypic class within each lineage. Data points are estimates of the percent mortality in each genotypic class and error bars represent 1±SE on the proportion (for all backgrounds n = 30 (BB = 10, BC = 10, CC = 10)). Snails with the CC genotype exhibited significantly greater mortality than those with the BB or BC genotype. (E) Mortality at 12-month census of each genotypic class within each lineage. Data points are estimates of percent mortality in each genotypic class and error bars represent 1±SE on the proportion (for all backgrounds n = 30 (BB = 10, BC = 10, CC = 10)).