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The ups and downs of life: population expansion and bottlenecks of helminth parasites through their complex life cycle

机译：生活的起伏：复杂的生命周期中蠕虫寄生虫的种群扩展和瓶颈

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The fundamental assumption underpinning the evolution of numerous adaptations shown by parasites with complex life cycles is that huge losses are incurred by infective stages during certain transmission steps. However, the magnitude of transmission losses or changes in the standing crop of parasites passing from upstream (source) to downstream (target) hosts have never been quantified in nature. Here, using data from 100 pairs of successive upstream-downstream life stages, from distinct populations representing 10 parasite species, we calculated the total density per m(2) of successive life stages. We show that clonal amplification of trematodes in their first intermediate host leads to an average 4-fold expansion of numbers of individuals at the next life stage, when differences in the longevity of successive life stages are taken into account. In contrast, trophic transmission to the definitive host results in almost no numerical change for trematodes, but possibly in large decreases for acanthocephalans and nematodes, though a correction for longevity was not possible for the latter groups. Also, we only found a positive association between upstream and downstream stage densities for transmission involving free-swimming cercariae in trematodes, suggesting a simple output-recruitment process. For trophic transmission, there was no coupling between downstream and upstream parasite densities. These first quantitative estimates of ontogenetic rises and falls in numbers under natural conditions provide new insights into the selective pressures acting on parasites with complex cycles.
机译：生命周期复杂的寄生虫所表现出的多种适应进化的基本假设是，在某些传播步骤中，感染阶段会造成巨大的损失。但是，从本质上从未量化过传播损失的大小或从上游（源）到下游（目标）宿主的寄生虫现存作物的变化。在这里，我们使用来自100对连续上游-下游生命阶段的数据，这些数据来自代表10种寄生虫物种的不同种群，我们计算了每m（2）连续生命阶段的总密度。我们显示，当考虑到连续生命期的寿命差异时，在其第一个中间宿主中吸虫的克隆扩增会导致下一生命期个体数量的平均4倍扩展。相比之下，营养传递给确定的宿主，对于吸虫而言几乎没有数值变化，但是对于棘头动物和线虫而言，可能没有大的减少，尽管对于后一组而言，寿命的校正是不可能的。此外，我们仅发现吸虫在尾吸虫中自由游动尾cer的传播的上游和下游密度之间存在正相关，这表明一个简单的产量招募过程。对于营养传递，下游和上游寄生虫密度之间没有耦合。这些在自然条件下对个体发育上升和下降的数量的第一个定量估计为对作用于具有复杂周期的寄生虫的选择性压力提供了新的见解。

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