Diet can alter the cost of resistance to a natural parasite in Caenorhabditis elegans

Resistance to parasites confers a fitness advantage, yet hosts show substantial variation in resistance in natural populations. Evolutionary theory indicates that resistant and susceptible genotypes can coexist if resistance is costly, but there is mixed evidence that resistant individuals have lower fitness in the absence of parasites. One explanation for this discrepancy is that the cost of resistance varies with environmental context. We tested this hypothesis using Caenorhabditis elegans and its natural microsporidian parasite, Nematocida ironsii. We used multiple metrics to compare the fitness of two near-isogenic host genotypes differing at regions associated with resistance to N. ironsii. To quantify the effect of the environment on the cost associated with these known resistance regions, we measured fitness on three microbial diets. We found that the cost of resistance varied with both diet and the measure of fitness. We detected no cost to resistance, irrespective of diet, when..., Fecundity Assay
We conducted two assays to assess different components of fitness across diet conditions. First, to determine if the resistant genotype has lower fecundity than the susceptible genotype, we counted the number of offspring produced by ERT250 and N2 hosts raised on the three bacterial diets. For each genotype, we isolated eggs using a standard bleach wash (Porta-de-la-Riva et al., 2012), then added approximately 100 eggs per plate to DA837-, OP50-, or HB101-seeded 100 mm plates. We allowed hosts to reach the fourth larval (L4) stage at 20°C then moved hosts individually to 35 mm plates seeded with the same bacteria as their original plate (n = 60 hosts, with one host per 35 mm plate; 10-14 hosts per genotype*food combination). We moved each host to a new plate every day for six days, at which point reproduction had finished. Plates with eggs were incubated at 20°C for 24 hours to hatch, then we counted viable offspring.  We censored data from hosts that were lost or suffer...,