Contrasting study designs reveal nuance in heterozygosity-parasite associations in the wild

Heterozygosity-fitness correlations (HFCs) are widely used to explore the effects of inbreeding in wild populations. However, the biological significance of HFCs has been the subject of intense debate, and it has been suggested that the magnitude and direction of these correlations may be context-dependent (e.g., vary with different host characteristics or environmental conditions). We tested this hypothesis in a free-ranging population of Grant's gazelles (Nanger granti). Specifically, we tested for associations between standardized multilocus heterozygosity (sMLH) and endoparasite infections, and examined how these relationships varied with animal age, sex and environmental context (e.g., seasonality). We used three approaches: a cross-sectional approach focusing on 103 individuals sampled at a single time point, a longitudinal approach focusing on 25 naturally infected individuals sampled over 12 months, and an experimental approach in which 15 individuals were cleared of their parasites and parasite re-accumulation was tracked over 12 months. We found that the presence of heterozygosity-parasite associations varied with study design and context. Cross-sectional patterns varied with environmental context, whereas the longitudinal analysis revealed host trait-specific HFCs, and the experiment established a causal link between heterozygosity and parasitism. Overall, our longer-term study approaches indicated that higher levels of heterozygosity are associated with lower parasite burdens, underscoring the value of longitudinal and experimental approaches for detecting HFCs in wild populations.