Dusi Eike, Gougat-Barbera Claire, Berendonk Thomas U, Kaltz Oliver
Institute of Hydrobiology, Technische Universität Dresden, Germany; Institut des Sciences de l'Evolution - UMR 5554, Université de Montpellier, Place Eugène Bataillon, 34095, Montpellier, Cedex 05, France.
Evolution. 2015 Apr;69(4):1069-76. doi: 10.1111/evo.12638. Epub 2015 Apr 9.
Evolutionary transitions from parasitism toward beneficial or mutualistic associations may encompass a change from horizontal transmission to (strict) vertical transmission. Parasites with both vertical and horizontal transmission are amendable to study factors driving such transitions. In a long-term experiment, microcosm populations of the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata were exposed to three growth treatments, manipulating vertical transmission opportunities over ca. 800 host generations. In inoculation tests, horizontal transmission propagules produced by parasites from a "high-growth" treatment, with elevated host division rates increasing levels of parasite vertical transmission, showed a near-complete loss of infectivity. A similar reduction was observed for parasites from a treatment alternating between high growth and low growth (i.e., low levels of population turn-over). Parasites from a low-growth treatment had the highest infectivity on all host genotypes tested. Our results complement previous findings of reduced investment in horizontal transmission and increased vertical transmissibility of high-growth parasites. We explain the loss of horizontal transmissibility by epidemiological feedbacks and resistance evolution, reducing the frequency of susceptible hosts in the population and thereby decreasing the selective advantage of horizontal transmission. This illustrates how environmental conditions may push parasites with a mixed transmission mode toward becoming vertically transmitted nonvirulent symbionts.
从寄生向有益或互利共生关系的进化转变可能包括从水平传播到（严格）垂直传播的变化。具有垂直和水平传播两种方式的寄生虫适合用于研究驱动此类转变的因素。在一项长期实验中，原生动物尾草履虫及其细菌寄生虫波动霍氏孢菌的微观种群接受了三种生长处理，在大约800代宿主中操纵垂直传播机会。在接种试验中，来自“高生长”处理的寄生虫产生的水平传播繁殖体，随着宿主分裂率升高，寄生虫垂直传播水平增加，但其感染力几乎完全丧失。在高生长和低生长交替处理（即种群周转率低）的寄生虫中也观察到了类似的感染力降低。来自低生长处理的寄生虫对所有测试的宿主基因型具有最高的感染力。我们的结果补充了先前关于高生长寄生虫在水平传播方面投资减少和垂直传播能力增加的研究发现。我们通过流行病学反馈和抗性进化来解释水平传播能力的丧失，这降低了种群中易感宿主的频率，从而降低了水平传播的选择优势。这说明了环境条件如何推动具有混合传播模式的寄生虫转变为垂直传播的无毒共生体。
