Choisy Marc, Brown Sam P, Lafferty Kevin D, Thomas Frédéric
Centre d'Etude sur le Polymorphisme des Microorganismes, Unité Mixte de Recherche, Centre National de la Recherche Scientifique/Institut de Recherche pour le Développement, Centre IRD, 911 Avenue Agropolis, BP 64501, 34394, Montpellier Cedex 5, France.
Am Nat. 2003 Aug;162(2):172-81. doi: 10.1086/375681. Epub 2003 Jul 16.
Although multihost complex life cycles (CLCs) are common in several distantly related groups of parasites, their evolution remains poorly understood. In this article, we argue that under particular circumstances, adding a second host to a single-host life cycle is likely to enhance transmission (i.e., reaching the target host). For instance, in several situations, the propagules of a parasite exploiting a predator species will achieve a higher host-finding success by encysting in a prey of the target predator than by other dispersal modes. In such a case, selection should favor the transition from a single- to a two-host life cycle that includes the prey species as an intermediate host. We use an optimality model to explore this idea, and we discuss it in relation to dispersal strategies known among free-living species, especially animal dispersal. The model found that selection favored a complex life cycle only if intermediate hosts were more abundant than definitive hosts. The selective value of a complex life cycle increased with predation rates by definitive hosts on intermediate hosts. In exploring trade-offs between transmission strategies, we found that more costly trade-offs made it more difficult to evolve a CLC while less costly trade-offs between traits could favor a mixed strategy.
尽管多宿主复杂生命周期（CLCs）在几个亲缘关系较远的寄生虫类群中很常见，但其进化仍知之甚少。在本文中，我们认为在特定情况下，在单宿主生命周期中增加第二个宿主可能会增强传播（即到达目标宿主）。例如，在几种情况下，寄生于捕食者物种的寄生虫的繁殖体通过在目标捕食者的猎物中形成包囊，比通过其他传播方式更有可能成功找到宿主。在这种情况下，选择应有利于从单宿主生命周期向包括猎物物种作为中间宿主的双宿主生命周期转变。我们使用一个最优性模型来探讨这一观点，并结合自由生活物种中已知的传播策略，特别是动物扩散来进行讨论。该模型发现，只有当中间宿主比终末宿主数量更多时，选择才有利于复杂的生命周期。复杂生命周期的选择价值随着终末宿主对中间宿主的捕食率增加而增加。在探索传播策略之间的权衡时，我们发现成本更高的权衡使得进化出复杂生命周期更加困难，而性状之间成本较低的权衡可能有利于混合策略。
