Lagrue C, Poulin R
Department of Zoology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand.
Parasitology. 2008 Sep;135(10):1243-51. doi: 10.1017/S0031182008004782. Epub 2008 Jul 29.
Parasites with complex life cycles have developed numerous and very diverse adaptations to increase the likelihood of completing this cycle. For example, some parasites can abbreviate their life cycles by skipping the definitive host and reproducing inside their intermediate host. The resulting shorter life cycle is clearly advantageous when definitive hosts are absent or rare. In species where life-cycle abbreviation is facultative, this strategy should be adopted in response to seasonally variable environmental conditions. The hermaphroditic trematode Coitocaecum parvum is able to mature precociously (progenesis), and produce eggs by selfing while still inside its amphipod second intermediate host. Several environmental factors such as fish definitive host density and water temperature are known to influence the life-history strategy adopted by laboratory raised C. parvum. Here we document the seasonal variation of environmental parameters and its association with the proportion of progenetic individuals in a parasite population in its natural environment. We found obvious seasonal patterns in both water temperature and C. parvum host densities. However, despite being temporally variable, the proportion of progenetic C. parvum individuals was not correlated with any single parameter. The results show that C. parvum life-history strategy is not as flexible as previously thought. It is possible that the parasite's natural environment contains so many layers of heterogeneity that C. parvum does not possess the ability to adjust its life-history strategy to accurately match the current conditions.
具有复杂生命周期的寄生虫已经进化出了众多且极为多样的适应性特征，以提高完成其生命周期的可能性。例如，一些寄生虫可以通过跳过终末宿主并在中间宿主体内繁殖来缩短其生命周期。当终末宿主不存在或很稀少时，由此产生的较短生命周期显然具有优势。在生命周期缩短为兼性的物种中，应根据季节性变化的环境条件采用这种策略。雌雄同体的吸虫微小隐孢子虫能够早熟（幼体生殖），并在其第二中间宿主（一种双足虾类）体内时通过自体受精产生卵。已知一些环境因素，如鱼类终末宿主密度和水温，会影响实验室饲养的微小隐孢子虫所采用的生活史策略。在此，我们记录了自然环境中环境参数的季节性变化及其与寄生虫种群中幼体生殖个体比例的关联。我们发现水温和微小隐孢子虫宿主密度都存在明显的季节性模式。然而，尽管存在时间上的变化，但微小隐孢子虫幼体生殖个体的比例与任何单一参数均无关联。结果表明，微小隐孢子虫的生活史策略并不像之前认为的那么灵活。有可能该寄生虫的自然环境包含如此多层面的异质性，以至于微小隐孢子虫不具备调整其生活史策略以精确匹配当前条件的能力。
