Data from: Trade-offs and mixed infections in an obligate-killing insect pathogen

Natural populations of pathogens are frequently composed of numerous interacting strains. Understanding what maintains this diversity remains a key focus of research in disease ecology. In addition, within-host pathogen dynamics can have a strong impact on both infection outcome and the evolution of pathogen virulence and thus understanding the impact of pathogen diversity is important for disease management. We compared eight genetically distinguishable variants from Spodoptera exempta nucleopolyhedrovirus (SpexNPV) isolated from the African armyworm, Spodoptera exempta. NPVs are obligate killers and the vast majority of transmission-stages are not released until after the host has died. The NPV variants differed significantly in their virulence and could be clustered into two groups based on their dose-response curves. They also differed in their speed of kill and productivity (transmission potential) for S. exempta. The mixed-genotype wild-type SpexNPV, from which each variant was isolated, was significantly more virulent than any individual variant and its mean mortality rate was within the fastest group of individual variants. However, the wild-type virus produced fewer new infectious stages than any single variant, which might reflect competition among the variants. A survival analysis, combining the mortality and speed of kill data, confirmed the superiority of the genetically-mixed wild-type virus over any single variant. S. exempta larvae infected with wild-type SpexNPV were predicted to die 2.7 and 1.9 times faster than insects infected with isolates from either of the two clusters of genotypes. Theory suggests that there are likely to be trade-offs between pathogen fitness traits. Across all larvae, there was a negative linear relationship between virus yield and speed of kill, such that more rapid host death carried the cost of producing fewer transmission stages. We also found a near-significant relationship for the same trend at the inter-variant level. However, there was no evidence for a significant relationship between the induced level of mortality and transmission potential (virus yield) or speed of kill.病原体的自然种群通常由众多相互作用的毒株构成，阐明维持这种多样性的机制仍是疾病生态学研究的核心议题之一。此外，宿主体内的病原体动态会对感染结局以及病原体毒力的演化产生显著影响，因此解析病原体多样性的作用机制对疾病防控至关重要。 本研究对分离自非洲粘虫（Spodoptera exempta）的斜纹夜蛾核型多角体病毒（Spodoptera exempta nucleopolyhedrovirus，简称SpexNPV）的8个遗传可区分毒株进行了比较分析。核型多角体病毒（nucleopolyhedrovirus，简称NPV）是专性致死性病原体，绝大多数传播阶段的病毒颗粒仅在宿主死亡后才会被释放。 这些NPV毒株在毒力上存在显著差异，可根据剂量反应曲线划分为两个类群。同时，它们在致死宿主的速度以及对非洲粘虫的病毒增殖量（传播潜力）方面也存在显著差异。从其中分离得到各毒株的混合基因型野生型SpexNPV，其毒力显著强于任何单个毒株，且其平均致死率处于单个毒株中最快的类群之列。但该野生型病毒产生的新感染性病毒颗粒数量少于任何单个毒株，这一现象可能反映了毒株间的竞争作用。 结合致死率与致死速度数据开展的生存分析证实，混合基因型野生型病毒的致病效果优于任何单个毒株。研究预测，感染野生型SpexNPV的非洲粘虫幼虫的死亡速度，分别是感染两个基因型类群分离毒株的幼虫的2.7倍与1.9倍。 理论研究表明，病原体的适合度性状之间大概率存在权衡关系。在所有受试幼虫中，病毒增殖量与致死速度之间存在显著负线性相关关系，即宿主死亡越快，其产生的传播阶段病毒颗粒数量越少，这是快速致死宿主的适应性代价。在毒株间水平上，我们也观察到了趋于该趋势的近显著相关关系。但并未发现宿主诱导死亡率与传播潜力（病毒增殖量）或致死速度之间存在显著相关关系的证据。