Institute for Theoretical Biology, Humboldt-Universität zu Berlin, Berlin, Germany.
J Evol Biol. 2012 Nov;25(11):2149-60. doi: 10.1111/j.1420-9101.2012.02601.x. Epub 2012 Sep 4.
Wolbachia are bacterial endosymbionts that manipulate the reproduction of their arthropod hosts. Although theory suggests that infections are frequently lost within host species due to the evolution of resistance, Wolbachia infect a huge number of species worldwide. This apparent paradox suggests that horizontal transmission between host species has been a key factor in shaping the global Wolbachia pandemic. Because Wolbachia infections are thus acquired and lost like any other infection, we use a standard epidemiological model to analyse Wolbachia horizontal transmission dynamics over evolutionary time. Conceptually modifying the model, we apply it not to transmission between individuals but between species. Because, on evolutionary timescales, infections spread frequently between closely related species and occasionally over large phylogenetic distances, we represent the set of host species as a small-world network that satisfies both requirements. Our model reproduces the effect of basic epidemiological parameters, which demonstrates the validity of our approach. We find that the ratio between transmission rate and recovery rate is crucial for determining the proportion of infected species (incidence) and that, in a given host network, the incidence may still be increasing over evolutionary time. Our results also point to the importance of occasional transmission over long phylogenetic distances for the observed high incidence levels of Wolbachia. In conclusion, we are able to explain why Wolbachia are so abundant among arthropods, although selection for resistance within hosts often leads to infection loss. Furthermore, our unorthodox approach of using epidemiology in evolutionary time can be applied to all symbionts that use horizontal transmission to infect new hosts.
沃尔巴克氏体是一种细菌内共生体，能够操纵其节肢动物宿主的繁殖。尽管理论表明，由于宿主对感染的抵抗力逐渐增强，感染在宿主物种内部经常会丢失，但沃尔巴克氏体仍在全球范围内感染了大量物种。这一明显的悖论表明，种间水平传播已成为塑造全球沃尔巴克氏体流行的关键因素。由于沃尔巴克氏体感染与其他任何感染一样被获得和丢失，我们使用标准的流行病学模型来分析进化时间内沃尔巴克氏体的水平传播动态。我们对模型进行了概念上的修改，将其应用于种间而非个体间的传播。因为在进化时间尺度上，感染在亲缘关系密切的物种之间频繁传播，偶尔也会跨越大的系统发育距离，所以我们将宿主物种集表示为一个小世界网络，该网络同时满足这两个要求。我们的模型再现了基本流行病学参数的效果，证明了我们方法的有效性。我们发现，传播率与恢复率之比对于确定感染物种的比例（发病率）至关重要，并且在给定的宿主网络中，发病率仍可能随着进化时间的推移而增加。我们的研究结果还表明，偶尔跨越大的系统发育距离进行传播对观察到的沃尔巴克氏体高发病率水平也很重要。总之，尽管宿主内部对感染的抵抗力选择常常导致感染丢失，但我们能够解释为什么沃尔巴克氏体在节肢动物中如此丰富。此外，我们在进化时间内使用流行病学的非传统方法可以应用于所有利用水平传播感染新宿主的共生体。
