Lagrue C, Poulin R
Department of Zoology, University of Otago, Dunedin, New Zealand.
J Evol Biol. 2009 Aug;22(8):1727-38. doi: 10.1111/j.1420-9101.2009.01787.x. Epub 2009 Jun 23.
The typical multi-host life cycle of many parasites, although conferring several advantages, presents the parasites with a highly hazardous transmission route. As a consequence, parasites have evolved various adaptations increasing their chances of transmission between the different hosts of the life cycle. Some trematode species like the opecoelid Coitocaecum parvum have adopted a more drastic alternative strategy whereby the definitive host is facultatively dropped from the cycle, resulting in a shorter, hence easier to complete, life cycle. Like other species capable of abbreviating their life cycle, C. parvum does so through progenetic development within its intermediate host. Laboratory-reared C. parvum can modulate their developmental strategy inside the second intermediate host according to current transmission opportunities, though this ability is not apparent in natural C. parvum populations. Here we show that this difference is likely due to the time C. parvum individuals spend in their intermediate hosts in the natural environment. Although transmission opportunities, i.e. chemical cues of the presence of definitive hosts, promoted the adoption of a truncated life cycle in the early stages of infection, individuals that remained in their amphipod host for a relatively long time had a similar probability of adopting progenesis and the abbreviated cycle, regardless of the presence or absence of chemical cues from the predator definitive host. These results support the developmental time hypothesis which states that parasites capable of facultative life cycle abbreviation should eventually adopt progenesis regardless of transmission opportunities, and provide further evidence of the adaptive plasticity of parasite transmission strategies.
许多寄生虫典型的多宿主生命周期虽然带来了一些优势，但也给寄生虫带来了一条高度危险的传播途径。因此，寄生虫进化出了各种适应性机制，以增加它们在生命周期中不同宿主之间传播的机会。一些吸虫物种，如卵圆科的微小柯托吸虫，采取了一种更为极端的替代策略，即终末宿主可选择性地从生命周期中去除，从而形成一个更短、因而更容易完成的生命周期。与其他能够缩短其生命周期的物种一样，微小柯托吸虫通过在中间宿主体内的幼体发育来实现这一点。实验室饲养的微小柯托吸虫可以根据当前的传播机会在第二中间宿主体内调节其发育策略，不过这种能力在自然的微小柯托吸虫种群中并不明显。在这里，我们表明这种差异可能是由于微小柯托吸虫个体在自然环境中的中间宿主体内停留的时间所致。虽然传播机会，即终末宿主存在的化学信号，在感染早期促进了缩短生命周期的采用，但在其双足类宿主中停留相对较长时间的个体，无论是否存在来自捕食性终末宿主的化学信号，都有相似的采用幼体发育和缩短生命周期的概率。这些结果支持了发育时间假说，该假说认为能够兼性缩短生命周期的寄生虫最终应采用幼体发育，而不论传播机会如何，并为寄生虫传播策略的适应性可塑性提供了进一步的证据。
