Abstract
Summary 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.
Highlights
Pathogens are ubiquitous in animal and plant populations and often cause acute infections that can result in high levels of mortality and devastating pathogen diversity in natural populations and the mechanisms that act to maintain it (Hodgson et al 2001; Lively et al 2014)
Spodoptera exempta larvae infected with WT Spodoptera exempta nucleopolyhedrovirus (SpexNPV) were predicted to die 2Á7 and 1Á9 times faster than insects infected with isolates from either of the two clusters of genotypes
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
Summary
Pathogens (micro-parasites) are ubiquitous in animal and plant populations and often cause acute infections that can result in high levels of mortality and devastating pathogen diversity in natural populations and the mechanisms that act to maintain it (Hodgson et al 2001; Lively et al 2014). There are additional mechanisms for promoting diversity that could come about through beneficial interactions between genotypes during co-infection, and the presence of deletion mutants or defective interfering particles, which may act as cheating genotypes (Hodgson et al 2003). Some of these evolutionary mechanisms have been supported through studies on a limited number of host– parasite systems How they apply to a broader range of species, their relative importance and the conditions under which they are most prevalent are still not clear (Lively et al 2014)
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