Abstract：
One of the fundamental aims of ecological, epidemiological and evolutionary studies of host-parasite interactions is to unravel which factors affect parasite virulence. Theory predicts that virulence and transmission are correlated by a trade-off, as too much virulence is expected to hamper transmission owing to excessive host damage. Coinfections may affect each of these traits and/or their correlation. Here, we used inbred lines of the spider mite Tetranychus urticae to test how coinfection with T. evansi impacted virulence-transmission relationships at different conspecific densities. The presence of T. evansi on a shared host did not change the relationship between virulence (leaf damage) and the number of transmitting stages (i.e. adult daughters). The relationship between these traits was hump-shaped across densities, both in single and coinfections, which corresponds to a trade-off. Moreover, transmission to adjacent hosts increased in coinfection, but only at low T. urticae densities. Finally, we tested whether virulence and the number of daughters were correlated with measures of transmission to adjacent hosts, in single and coinfections at different conspecific densities. Traits were mostly independent, meaning that interspecific competitors may increase transmission without affecting virulence. Thus, coinfections may impact epidemiology and parasite trait evolution, but not necessarily the virulence-transmission trade-off.This article is part of the theme issue \'Diversity-dependence of dispersal: interspecific interactions determine spatial dynamics\'.
摘要：
生态的基本目标之一，宿主-寄生虫相互作用的流行病学和进化研究旨在揭示影响寄生虫毒力的因素。理论预测，毒力和传播是通过权衡相互关联的，由于过度的宿主损害，预计过多的毒力会阻碍传播。共感染可能会影响这些性状和/或它们的相关性。这里,我们使用蜘蛛螨的自交系来测试与伊万氏T.evansi共感染如何在不同的特定密度下影响毒力传播关系。T.evansi在共享宿主上的存在并没有改变毒力（叶片损伤）与传播阶段（即成年女儿）数量之间的关系。这些性状之间的关系在密度上呈驼峰状，在单一和合并感染中，这对应于一个权衡。此外，向相邻宿主的传播在合并感染中增加，但只有在低T.urticae密度。最后,我们测试了毒力和女儿的数量是否与传播到相邻宿主的措施相关，在不同同种密度的单一和共同感染中。特征大多是独立的，这意味着种间竞争者可能会增加传播而不影响毒力。因此，共感染可能会影响流行病学和寄生虫性状进化，但不一定是毒力-传播权衡。本文是主题问题“扩散的多样性依赖性：种间相互作用决定空间动力学”的一部分。
