Redman Elizabeth M, Wilson Kenneth, Cory Jenny S
Molecular Ecology and Biocontrol Group, NERC Centre for Ecology and Hydrology, Mansfield Road, Oxford, OX1 3SR, UK.
Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK.
J Anim Ecol. 2016 Sep;85(5):1200-9. doi: 10.1111/1365-2656.12547. Epub 2016 Jun 13.
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 (WT) 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 WT 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 WT virus over any single variant. Spodoptera exempta larvae infected with WT 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 intervariant 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菌株组成。理解是什么维持了这种多样性仍然是疾病生态学研究的一个关键重点。此外，宿主体内病原体动态对感染结果和病原体毒力的进化都可能产生强烈影响，因此，了解病原体多样性的影响对于疾病管理很重要。我们比较了从非洲黏虫草地贪夜蛾中分离出的草地贪夜蛾核多角体病毒（SpexNPV）的八个基因可区分变体。NPV是专性杀手，绝大多数传播阶段直到宿主死亡后才会释放。NPV变体在毒力上有显著差异，根据它们的剂量反应曲线可分为两组。它们在致死速度和对草地贪夜蛾的繁殖力（传播潜力）方面也有所不同。从中分离出每个变体的混合基因型野生型（WT）SpexNPV比任何单个变体的毒力都显著更强，其平均死亡率处于单个变体中最快的一组。然而，WT病毒产生的新感染阶段比任何单个变体都少，这可能反映了变体之间的竞争。一项结合死亡率和致死速度数据的生存分析证实了基因混合的WT病毒优于任何单个变体。感染WT SpexNPV的草地贪夜蛾幼虫预计比感染来自两个基因型簇中任何一个簇的分离株的昆虫死亡速度快2.7倍和1.9倍。理论表明，病原体适应性特征之间可能存在权衡。在所有幼虫中，病毒产量与致死速度之间存在负线性关系，即宿主死亡越快，产生的传播阶段就越少。我们还在变体间水平上发现了相同趋势的近乎显著的关系。然而，没有证据表明诱导的死亡率水平与传播潜力（病毒产量）或致死速度之间存在显著关系。
