Evolutionary Ecology and Genetics, Zoological Institute, Christian-Albrechts-Universität zu Kiel, Am Botanischen Garten 1-9, Kiel, 24118, Germany.
Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford, OX1 3PS, UK.
Environ Microbiol. 2017 May;19(5):2090-2100. doi: 10.1111/1462-2920.13736.
Virulence is often under selection during host-parasite coevolution. In order to increase fitness, parasites are predicted to circumvent and overcome host immunity. A particular challenge for pathogens are external immune systems, chemical defence systems comprised of potent antimicrobial compounds released by prospective hosts into the environment. We carried out an evolution experiment, allowing for coevolution to occur, with the entomopathogenic fungus, Beauveria bassiana, and the red flour beetle, Tribolium castaneum, which has a well-documented external immune system with strong inhibitory effects against B. bassiana. After just seven transfers of experimental evolution we saw a significant increase in parasite induced host mortality, a proxy for virulence, in all B. bassiana lines. This apparent virulence increase was mainly the result of the B. bassiana lines evolving resistance to the beetles' external immune defences, not due to increased production of toxins or other harmful substances. Transcriptomic analyses of evolved B. bassiana implicated the up-regulation of oxidative stress resistance genes in the observed resistance to external immunity. It was concluded that external immunity acts as a powerful selective force for virulence evolution, with an increase in virulence being achieved apparently entirely by overcoming these defences, most likely due to elevated oxidative stress resistance.
毒力在宿主-寄生虫的共同进化中经常受到选择。为了提高适应性，寄生虫被预测会规避和克服宿主的免疫。对于病原体来说，一个特别的挑战是外部免疫系统，即由潜在宿主释放到环境中的具有强大抗菌化合物组成的化学防御系统。我们进行了一项进化实验，允许与昆虫病原真菌球孢白僵菌和赤拟谷盗（Tribolium castaneum）共同进化，赤拟谷盗具有有案可查的外部免疫系统，对球孢白僵菌具有强烈的抑制作用。仅仅经过 7 次实验进化的传递，我们就看到了所有球孢白僵菌系中寄生虫引起的宿主死亡率的显著增加，这是毒力的一个代表。这种明显的毒力增加主要是由于球孢白僵菌系进化出了对甲虫外部免疫防御的抗性，而不是由于毒素或其他有害物质的产生增加。对进化后的球孢白僵菌的转录组分析表明，氧化应激抗性基因的上调与观察到的对外界免疫的抗性有关。结论是，外部免疫是毒力进化的强大选择力，毒力的增加显然完全是通过克服这些防御机制实现的，最有可能是由于氧化应激抗性的提高。
