Aquatic Ecology, Eawag, Ueberlandstrasse 133, P.O. Box 611, 8600, Duebendorf, Switzerland,
Microb Ecol. 2014 Oct;68(3):645-56. doi: 10.1007/s00248-014-0434-1. Epub 2014 May 27.
Parasite environments are heterogeneous at different levels. The first level of variability is the host itself. The second level represents the external environment for the hosts, to which parasites may be exposed during part of their life cycle. Both levels are expected to affect parasite fitness traits. We disentangle the main and interaction effects of variation in the immediate host environment, here the diatom Asterionella formosa (variables host cell volume and host condition through herbicide pre-exposure) and variation in the external environment (variables host density and acute herbicide exposure) on three fitness traits (infection success, development time and reproductive output) of a chytrid parasite. Herbicide exposure only decreased infection success in a low host density environment. This result reinforces the hypothesis that chytrid zoospores use photosynthesis-dependent chemical cues to locate its host. At high host densities, chemotaxis becomes less relevant due to increasing chance contact rates between host and parasite, thereby following the mass-action principle in epidemiology. Theoretical support for this finding is provided by an agent-based simulation model. The immediate host environment (cell volume) substantially affected parasite reproductive output and also interacted with the external herbicide exposed environment. On the contrary, changes in the immediate host environment through herbicide pre-exposure did not increase infection success, though it had subtle effects on zoospore development time and reproductive output. This study shows that both immediate host and external environment as well as their interaction have significant effects on parasite fitness. Disentangling these effects improves our understanding of the processes underlying parasite spread and disease dynamics.
寄生虫的生存环境在不同层面上存在异质性。第一个层面的变异性是宿主本身。第二个层面代表了宿主的外部环境，寄生虫在其生命周期的某些阶段可能会暴露于其中。这两个层面都有可能影响寄生虫的适应度特征。我们分别研究了直接宿主环境（此处为通过除草剂预先暴露改变的菱形藻（Asterionella formosa）的宿主细胞体积和宿主状况）和外部环境（宿主密度和急性除草剂暴露）变化对三种寄生虫适应度特征（感染成功率、发育时间和生殖输出）的主要和相互作用影响。除草剂暴露仅降低了低宿主密度环境中的感染成功率。这一结果进一步证实了这样一种假说，即壶菌游动孢子利用依赖光合作用的化学信号来定位其宿主。在高宿主密度下，由于宿主和寄生虫之间的接触率增加，化学趋性的相关性降低，从而遵循传染病学中的质量作用定律。一个基于主体的模拟模型为这一发现提供了理论支持。直接宿主环境（细胞体积）对寄生虫的生殖输出有很大影响，并且与外部暴露于除草剂的环境相互作用。相反，通过除草剂预先暴露改变直接宿主环境并没有增加感染成功率，尽管它对游动孢子的发育时间和生殖输出有细微的影响。本研究表明，直接宿主和外部环境及其相互作用都对寄生虫的适应度有显著影响。分解这些影响可以提高我们对寄生虫传播和疾病动态背后过程的理解。
