Jensen Knut Helge, Little Tom J, Skorping Arne, Ebert Dieter
Department of Biology, University of Fribourg, Fribourg, Switzerland.
PLoS Biol. 2006 Jul;4(7):e197. doi: 10.1371/journal.pbio.0040197.
The trade-off hypothesis for the evolution of virulence predicts that parasite transmission stage production and host exploitation are balanced such that lifetime transmission success (LTS) is maximised. However, the experimental evidence for this prediction is weak, mainly because LTS, which indicates parasite fitness, has been difficult to measure. For castrating parasites, this simple model has been modified to take into account that parasites convert host reproductive resources into transmission stages. Parasites that kill the host too early will hardly benefit from these resources, while postponing the killing of the host results in diminished returns. As predicted from optimality models, a parasite inducing castration should therefore castrate early, but show intermediate levels of virulence, where virulence is measured as time to host killing. We studied virulence in an experimental system where a bacterial parasite castrates its host and produces spores that are not released until after host death. This permits estimating the LTS of the parasite, which can then be related to its virulence. We exposed replicate individual Daphnia magna (Crustacea) of one host clone to the same amount of bacterial spores and followed individuals until their death. We found that the parasite shows strong variation in the time to kill its host and that transmission stage production peaks at an intermediate level of virulence. A further experiment tested for the genetic basis of variation in virulence by comparing survival curves of daphniids infected with parasite spores obtained from early killing versus late killing infections. Hosts infected with early killer spores had a significantly higher death rate as compared to those infected with late killers, indicating that variation in time to death was at least in part caused by genetic differences among parasites. We speculate that the clear peak in lifetime reproductive success at intermediate killing times may be caused by the exceptionally strong physiological trade-off between host and parasite reproduction. This is the first experimental study to demonstrate that the production of propagules is highest at intermediate levels of virulence and that parasite genetic variability is available to drive the evolution of virulence in this system.
毒力进化的权衡假说预测，寄生虫传播阶段的产生与宿主利用之间存在平衡，从而使终生传播成功率（LTS）最大化。然而，这一预测的实验证据并不充分，主要原因是LTS（表明寄生虫适应性）难以测量。对于具有去势作用的寄生虫，该简单模型已进行修正，以考虑寄生虫将宿主繁殖资源转化为传播阶段这一因素。过早杀死宿主的寄生虫几乎无法从这些资源中获益，而推迟杀死宿主则会导致收益递减。因此，正如最优模型所预测的那样，诱导去势的寄生虫应尽早去势，但表现出中等水平的毒力，其中毒力以宿主死亡时间来衡量。我们在一个实验系统中研究了毒力，在该系统中，一种细菌寄生虫使宿主去势，并产生孢子，这些孢子直到宿主死亡后才释放。这使得能够估计寄生虫的LTS，进而将其与毒力联系起来。我们将一个宿主克隆的多个大型溞（甲壳纲）个体暴露于等量的细菌孢子中，并跟踪这些个体直至死亡。我们发现，寄生虫在杀死宿主的时间上表现出很大差异，并且传播阶段的产生在中等毒力水平时达到峰值。进一步的实验通过比较感染从早期致死感染与晚期致死感染中获得的寄生虫孢子的溞类的存活曲线，来测试毒力变异的遗传基础。与感染晚期致死孢子的宿主相比，感染早期致死孢子的宿主死亡率显著更高，这表明死亡时间的差异至少部分是由寄生虫之间的遗传差异引起的。我们推测，在中等致死时间时终生繁殖成功率出现明显峰值，可能是由于宿主与寄生虫繁殖之间存在异常强烈的生理权衡。这是第一项实验研究，证明繁殖体的产生在中等毒力水平时最高，并且寄生虫遗传变异性可推动该系统中毒力的进化。
