Department of Biological Sciences, Western Michigan University, 1903 West Michigan Avenue, Kalamazoo, MI 49008-5410, USA.
Int J Parasitol. 2010 Dec;40(14):1665-73. doi: 10.1016/j.ijpara.2010.07.004. Epub 2010 Aug 8.
Recent work has suggested that the outcomes of host-symbiont interactions can shift between positive, neutral and negative depending on both biotic and abiotic conditions. Even organisms traditionally defined as parasites can have positive effects on hosts under some conditions. For a given host-parasite system, the effects of infection on host fitness can depend on host vigour, route of transmission and environmental conditions. We monitored sublethal microsporidian infections in populations of Gammarus pseudolimnaeus (Amphipoda: Gammaridae) from four cool water streams in southwestern Michigan, USA. Our objectives were to: (i) infer the mechanism of transmission (horizontal, vertical or mixed) from observed effects of infection on host fitness, (ii) determine if the magnitude of the effects on host fitness is a function of parasite load (infection intensity) compared with simple presence or absence of infection, and (iii) determine if there is variation in parasite effects on host fitness in isolated populations. PCR and DNA sequence analyses revealed that there were two microsporidia present among the four host populations: Dictyocoela sp. and Microsporidium sp. PCR screening of a subset of infected hosts showed that Dictyocoela sp. accounted for 90% of infections and was present in all four G. pseudolimnaeus populations, while Microsporidium sp. was found in two populations but was only relatively common in one. We found very low prevalence in males (∼5%), but high prevalence in females (range: 37-85%). Female fitness was positively associated with infection in two streams, resulting from either higher fecundity or more reproductive bouts. Infection had a negative effect on the number of reproductive bouts in a third population, and no effect on fecundity in a fourth population. Infection intensity explained additional variation in fecundity in one population; females with intermediate infection intensity had higher fecundity than females with either light or heavy infection intensity. Given the high prevalence of infection in females compared with males and the generally weak negative fitness effects coupled with some positive fitness effects, it is likely that both Dictyocoela sp. and Microsporidium sp. are primarily vertically transmitted, feminizing microsporidia. Our results suggest that microsporidian effects on G. pseudolimnaeus fitness were context-dependent and varied with host sex and local environment.
最近的研究表明，宿主-共生体相互作用的结果可以根据生物和非生物条件在积极、中性和消极之间转变。即使是传统上被定义为寄生虫的生物，在某些条件下也可能对宿主产生积极影响。对于给定的宿主-寄生虫系统，感染对宿主适应性的影响取决于宿主活力、传播途径和环境条件。我们监测了来自美国密歇根州西南部四个冷水溪流的 Gammarus pseudolimnaeus（甲壳纲：Gammaridae）种群中的亚致死微孢子虫感染。我们的目标是：（i）从感染对宿主适应性的影响推断出传播机制（水平、垂直或混合），（ii）确定对宿主适应性的影响的幅度是否与寄生虫负荷（感染强度）有关，而不是简单的感染存在与否，以及（iii）确定寄生虫对宿主适应性的影响在隔离种群中是否存在差异。PCR 和 DNA 序列分析表明，在四个宿主种群中存在两种微孢子虫：Dictyocoela sp. 和 Microsporidium sp.。对一组感染宿主的 PCR 筛选显示，Dictyocoela sp. 占 90%的感染，存在于四个 G. pseudolimnaeus 种群中，而 Microsporidium sp. 存在于两个种群中，但在一个种群中相对常见。我们发现雄性的流行率非常低（约 5%），但雌性的流行率很高（范围：37-85%）。在两个溪流中，雌性适应性与感染呈正相关，这是由于繁殖力更高或繁殖回合更多。在第三个种群中，感染对繁殖回合的数量有负面影响，而在第四个种群中对繁殖力没有影响。感染强度在一个种群中解释了繁殖力的额外变化；感染强度中等的雌性比感染强度较轻或较重的雌性繁殖力更高。鉴于与雄性相比，雌性感染的高流行率以及与某些积极的适应性效应相关的普遍较弱的负适应性效应，Dictyocoela sp. 和 Microsporidium sp. 很可能主要是垂直传播的，使雌性化的微孢子虫。我们的研究结果表明，微孢子虫对 G. pseudolimnaeus 适应性的影响是上下文相关的，并且随宿主性别和当地环境而异。
