Data from: Genotype and diet affect resistance, survival, and fecundity but not fecundity tolerance

Insects are exposed to a variety of potential pathogens in their environment, many of which can severely impact fitness and health. Consequently, hosts have evolved resistance and tolerance strategies to suppress or cope with infections. Hosts utilizing resistance improve fitness by clearing or reducing pathogen loads, and hosts utilizing tolerance reduce harmful fitness effects per pathogen load. To understand variation in, and selective pressures on, resistance and tolerance, we asked to what degree they are shaped by host genetic background, whether plasticity in these responses depends upon dietary environment, and whether there are interactions between these two factors. Females from ten wild-type Drosophila melanogaster genotypes were kept on high- or low-protein (yeast) diets and infected with one of two opportunistic bacterial pathogens, Lactococcus lactis or Pseudomonas entomophila. We measured host resistance as the inverse of bacterial load in the early infection phase. The relationship (slope) between fly fecundity and individual-level bacteria load provided our fecundity tolerance measure. Genotype and dietary yeast determined host fecundity and strongly affected survival after infection with pathogenic P. entomophila. There was considerable genetic variation in host resistance, a commonly found phenomenon resulting from for example varying resistance costs or frequency-dependent selection. Despite this variation and the reproductive cost of higher P. entomophila loads, fecundity tolerance did not vary across genotypes. The absence of genetic variation in tolerance may suggest that at this early infection stage, fecundity tolerance is fixed or that any evolved tolerance mechanisms are not expressed under these infection conditions.

昆虫在生存环境中会接触到多种潜在病原体，其中许多可严重影响其适合度与健康。为此，宿主演化出了抗性与耐受两类策略，以抑制或应对感染。采用抗性策略的宿主可通过清除或降低病原体负荷来提升自身适合度；而采用耐受策略的宿主，则可通过降低每单位病原体负荷所带来的有害适合度效应，来维持自身适合度。为解析抗性与耐受的变异模式及其所受的选择压力，我们旨在明确：其一，这两类策略在多大程度上受宿主遗传背景调控；其二，此类免疫响应的可塑性是否依赖于饮食环境；其三，这两个因素之间是否存在交互作用。本实验选取10种野生型黑腹果蝇（Drosophila melanogaster）基因型的雌蝇，将其饲养在高或低蛋白（酵母）饮食环境中，随后分别感染两种机会性细菌病原体——乳酸乳球菌（Lactococcus lactis）或昆虫假单胞菌（Pseudomonas entomophila）。我们将宿主抗性定义为感染早期阶段细菌负荷的倒数，并以果蝇繁殖力与个体细菌负荷之间的关系（斜率）作为繁殖力耐受的衡量指标。实验结果显示，果蝇基因型与饮食酵母含量共同决定了宿主的繁殖力，并显著影响了感染致病性昆虫假单胞菌后的存活率。宿主抗性存在显著的遗传变异，这一常见现象可由抗性成本差异、频率依赖选择等多种因素导致。尽管存在上述遗传变异，且高昆虫假单胞菌负荷会带来繁殖代价，但不同基因型间的繁殖力耐受并未出现显著差异。耐受性状不存在遗传变异这一结果，或许意味着在该早期感染阶段，繁殖力耐受是固定不变的，或是在本次实验的感染条件下，任何演化而来的耐受机制均未得到表达。