Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel.
J Evol Biol. 2011 Jun;24(6):1307-16. doi: 10.1111/j.1420-9101.2011.02264.x. Epub 2011 Apr 11.
In many natural populations, hosts are found to be infected by more than one parasite species. When these parasites have different host exploitation strategies and transmission modes, a conflict among them may arise. Such a conflict may reduce the success of both parasites, but could work to the benefit of the host. For example, the less-virulent parasite may protect the host against the more-virulent competitor. We examine this conflict using the waterflea Daphnia magna and two of its sympatric parasites: the blood-infecting bacterium Pasteuria ramosa that transmits horizontally and the intracellular microsporidium Octosporea bayeri that can concurrently transmit horizontally and vertically after infecting ovaries and fat tissues of the host. We quantified host and parasite fitness after exposing Daphnia to one or both parasites, both simultaneously and sequentially. Under conditions of strict horizontal transmission, Pasteuria competitively excluded Octosporea in both simultaneous and sequential double infections, regardless of the order of exposure. Host lifespan, host reproduction and parasite spore production in double infections resembled those of single infection by Pasteuria. When hosts became first vertically (transovarilly) infected with O. bayeri, Octosporea was able to withstand competition with P. ramosa to some degree, but both parasites produced less transmission stages than they did in single infections. At the same time, the host suffered from reduced fecundity and longevity. Our study demonstrates that even when competing parasite species utilize different host tissues to proliferate, double infections lead to the expression of higher virulence and ultimately may select for higher virulence. Furthermore, we found no evidence that the less-virulent and vertically transmitting O. bayeri protects its host against the highly virulent P. ramosa.
在许多自然种群中，宿主被发现感染了不止一种寄生虫。当这些寄生虫具有不同的宿主利用策略和传播模式时，它们之间可能会发生冲突。这种冲突可能会降低两种寄生虫的成功率，但也可能对宿主有利。例如，毒性较低的寄生虫可能会保护宿主免受毒性较强的竞争者的侵害。我们使用水生甲壳动物大型水蚤及其两种共生寄生虫来检验这种冲突：感染血液的细菌波氏菌通过水平传播，而细胞内微孢子虫 Octosporea bayeri 在感染宿主的卵巢和脂肪组织后可以同时进行水平和垂直传播。我们在宿主暴露于一种或两种寄生虫、同时暴露和先后暴露的情况下，量化了宿主和寄生虫的适应性。在严格的水平传播条件下，波氏菌在同时和先后双重感染中都竞争性地排斥 Octosporea，无论暴露的顺序如何。在双重感染中，宿主的寿命、繁殖和寄生虫孢子的产生与单独感染波氏菌相似。当宿主首先垂直（经卵）感染 Octosporea bayeri 时，Octosporea 在一定程度上能够抵抗与 P. ramosa 的竞争，但两种寄生虫产生的传播阶段都少于单独感染时。与此同时，宿主的繁殖力和寿命都有所下降。我们的研究表明，即使竞争的寄生虫物种利用不同的宿主组织进行增殖，双重感染也会导致更高的毒力表达，最终可能会选择更高的毒力。此外，我们没有发现毒性较低且垂直传播的 Octosporea bayeri 会保护其宿主免受高度毒性的 P. ramosa 的侵害。
