Bedhomme Stephanie, Agnew Philip, Vital Yuri, Sidobre Christine, Michalakis Yannis
Department of Biology, Queen's University, Kingston, Ontario, Canada.
PLoS Biol. 2005 Aug;3(8):e262. doi: 10.1371/journal.pbio.0030262. Epub 2005 Jul 19.
Costs of parasitism are commonly measured by comparing the performance of infected groups of individuals to that of uninfected control groups. This measure potentially underestimates the cost of parasitism because it ignores indirect costs, which may result from the modification of the competitiveness of the hosts by the parasite. In this context, we used the host-parasite system consisting of the yellow fever mosquito Aedes aegypti and the microsporidian parasite Vavraia culicis to address this question: Do infected individuals exert a more or less intense intraspecific competition than uninfected individuals? Our experimental results show that, indeed, infected hosts incur a direct cost of parasitism: It takes them longer to become adults than uninfected individuals. They also incur an indirect cost, however, which is actually larger than the direct cost: When grown in competition with uninfected individuals they develop even slower. The consequence of this modification of competitiveness is that, in our system, the cost of parasitism is underestimated by the traditional measure. Moreover, because the indirect cost depends on the frequency of interactions between infected and uninfected individuals, our results suggest that the real cost of parasitism, i.e., virulence, is negatively correlated with the prevalence of the parasite. This link between prevalence and virulence may have dynamical consequences, such as reducing the invasion threshold of the parasite, and evolutionary consequences, such as creating a selection pressure maintaining the host's constitutive resistance to the parasite.
寄生成本通常通过比较受感染个体组与未受感染对照组的表现来衡量。这种衡量方法可能低估了寄生成本，因为它忽略了间接成本，而间接成本可能是由寄生虫改变宿主的竞争力所导致的。在此背景下，我们利用由黄热病蚊子埃及伊蚊和微孢子虫寄生虫库蚊瓦夫拉虫组成的宿主 - 寄生虫系统来解决这个问题：受感染个体与未受感染个体相比，种内竞争是更强还是更弱？我们的实验结果表明，受感染宿主确实会产生寄生的直接成本：它们比未受感染个体需要更长时间才能发育为成虫。然而，它们也会产生间接成本，而间接成本实际上比直接成本更大：当与未受感染个体竞争生长时，它们发育得更慢。这种竞争力改变的结果是，在我们的系统中，寄生成本被传统衡量方法低估了。此外，由于间接成本取决于受感染个体与未受感染个体之间的相互作用频率，我们的结果表明，寄生的实际成本，即毒力，与寄生虫的流行率呈负相关。流行率与毒力之间的这种联系可能会产生动态后果，例如降低寄生虫的入侵阈值，以及进化后果，例如产生一种选择压力，维持宿主对寄生虫的固有抗性。
