Abstract
Insects rely on their innate immune system to successfully mediate complex interactions with their internal microbiota, as well as the microbes present in the environment. Given the variation in microbes across habitats, the challenges to respond to them are likely to result in local adaptations in the immune system. Here we focus upon phagocytosis, a mechanism by which pathogens and foreign particles are engulfed in order to be contained, killed, and processed. We investigated the phenotypic and genetic variation related to phagocytosis in two allopatric populations of the butterfly Pieris napi. Populations were found to differ in their hemocyte composition and overall phagocytic capability, driven by the increased phagocytic propensity of each cell type. Yet, genes annotated to phagocytosis showed no large genomic signal of divergence. However, a gene set enrichment analysis on significantly divergent genes identified loci involved in glutamine metabolism, which recently have been linked to immune cell differentiation in mammals. Together these results suggest that heritable variation in phagocytic capacity arises via a quantitative trait architecture with variation in genes affecting the activation and/or differentiation of phagocytic cells, suggesting them as potential candidate genes underlying these phenotypic differences.
Highlights
Comparisons between populations have long been a powerful tool used by evolutionary biologists to identify both the action and potential targets of natural selection in wild populations (e.g., [1])
Immune performance assays in natural populations often only focus upon phenotypic differences in immune performance without distinguishing between these two different causal routes
There was no significant difference in total hemocyte count between larvae from different populations or sex (Figure 1a; GLM: Ppopulation = 0.43, Psex = 0.45, Table S1)
Summary
Comparisons between populations have long been a powerful tool used by evolutionary biologists to identify both the action and potential targets of natural selection in wild populations (e.g., [1]). Phenotypic differences might arise from changes in the amount, size or performance of cells or organs. Immune performance assays in natural populations often only focus upon phenotypic differences in immune performance without distinguishing between these two different causal routes (e.g., canonical immune genes vs the development of cells involved in immunity). Such insights are needed if we are to understand the microevolutionary dynamics of ecological immunity, as they inform upon whether the immune pathways or the development of immune cells are the target of selection on immune performance
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