Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, FIN-40014, Jyväskylä, Finland.
J Anim Ecol. 2013 Jan;82(1):191-200. doi: 10.1111/j.1365-2656.2012.02028.x. Epub 2012 Sep 17.
In nature, parasites commonly share hosts with other conspecific parasite genotypes. While adult parasites typically show aggregated distribution in their final hosts, aggregation of clonal parasite genotypes in intermediate hosts, such as those of trematodes in molluscs, is not generally known. However, infection of a host by multiple parasite genotypes has significant implications for evolution of virulence and host-parasite coevolution. Aggregated distribution of the clonal stages can increase host mortality and reduce larval output of each infecting genotype through interclonal competition, and therefore have significant implications for parasite epidemiology. The aim of this study was (i) to find out how common multiple genotype infections (MGIs) are in aquatic snails serving as intermediate hosts for different trematode species; (ii) to find out whether the prevalence of infection could be used to predict MGI frequencies and (iii) to use the relationship to infer whether MGIs aggregate in molluscan hosts. We determined the prevalence of trematode (Diplostomum pseudospathaceum) infections and the frequency of MGIs in snail (Lymnaea stagnalis) host populations as well as compiled corresponding literature data from a range of snail-trematode systems. We used Bayesian simulations to explore the relationship between prevalence of infection and MGI frequency in these data, and tested whether genotypes aggregate in snails by comparing the simulated relations with null model (Poisson and demographic Poisson) expectations. Our results show that MGIs are common in aquatic snails with up to 90% of the infected snails carrying MGIs. Parasite prevalence is a good predictor of MGI frequencies at a range of prevailing natural prevalences of infection (0-50%). The frequency of MGIs was higher than expected by both null models, indicating parasite aggregation at genotype level. These findings are in sharp contrast with the absence of multiple infections in snails at level of trematode species, suggesting that co-infections by multiple species and multiple genotypes of one species are controlled by different biological processes. Aggregation of MGIs in snail hosts appears to be widespread across different snail-trematode systems.
在自然界中，寄生虫通常与其他同种寄生虫基因型共享宿主。虽然成年寄生虫在其最终宿主中通常表现出聚集分布，但在中间宿主（如软体动物中的吸虫）中克隆寄生虫基因型的聚集分布通常不为人知。然而，宿主被多种寄生虫基因型感染对毒力进化和宿主-寄生虫协同进化有重要意义。克隆阶段的聚集分布可以通过种间竞争增加宿主死亡率并减少每个感染基因型的幼虫产量，因此对寄生虫流行病学有重要意义。本研究的目的是：（i）了解不同吸虫物种的水生螺类作为中间宿主的多重基因型感染（MGI）有多常见；（ii）了解感染的流行率是否可用于预测 MGI 频率；（iii）利用这种关系推断 MGI 在软体动物宿主中是否聚集。我们确定了螺类（Diplostomum pseudospathaceum）感染的流行率和 MGI 在螺类（Lymnaea stagnalis）宿主种群中的频率，并从一系列螺类-吸虫系统中汇编了相应的文献数据。我们使用贝叶斯模拟来探索这些数据中感染流行率与 MGI 频率之间的关系，并通过将模拟关系与零模型（泊松和人口泊松）预期进行比较来测试基因型是否在螺类中聚集。我们的结果表明，MGI 在水生螺类中很常见，高达 90%的感染螺类携带 MGI。寄生虫流行率是在一系列流行的自然感染流行率（0-50%）下预测 MGI 频率的良好指标。MGI 的频率高于零模型的预期，表明在基因型水平上存在寄生虫聚集。这些发现与在螺类水平上吸虫种不存在多重感染形成鲜明对比，表明多重感染和一种物种的多种基因型的共同感染受不同的生物学过程控制。MGI 在螺类宿主中的聚集似乎在不同的螺类-吸虫系统中广泛存在。
