Parker G A, Ball M A, Chubb J C
Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, University of Liverpool, Liverpool, UK.
J Evol Biol. 2015 Feb;28(2):267-91. doi: 10.1111/jeb.12575. Epub 2015 Jan 27.
Links between parasites and food webs are evolutionarily ancient but dynamic: life history theory provides insights into helminth complex life cycle origins. Most adult helminths benefit by sexual reproduction in vertebrates, often high up food chains, but direct infection is commonly constrained by a trophic vacuum between free-living propagules and definitive hosts. Intermediate hosts fill this vacuum, facilitating transmission to definitive hosts. The central question concerns why sexual reproduction, and sometimes even larval growth, is suppressed in intermediate hosts, favouring growth arrest at larval maturity in intermediate hosts and reproductive suppression until transmission to definitive hosts? Increased longevity and higher growth in definitive hosts can generate selection for larger parasite body size and higher fecundity at sexual maturity. Life cycle length is increased by two evolutionary mechanisms, upward and downward incorporation, allowing simple (one-host) cycles to become complex (multihost). In downward incorporation, an intermediate host is added below the definitive host: models suggest that downward incorporation probably evolves only after ecological or evolutionary perturbations create a trophic vacuum. In upward incorporation, a new definitive host is added above the original definitive host, which subsequently becomes an intermediate host, again maintained by the trophic vacuum: theory suggests that this is plausible even under constant ecological/evolutionary conditions. The final cycle is similar irrespective of its origin (upward or downward). Insights about host incorporation are best gained by linking comparative phylogenetic analyses (describing evolutionary history) with evolutionary models (examining selective forces). Ascent of host trophic levels and evolution of optimal host taxa ranges are discussed.
生活史理论为蠕虫复杂生命周期的起源提供了见解。大多数成年蠕虫通过在脊椎动物（通常处于食物链较高位置）中进行有性繁殖而受益，但直接感染通常受到自由生活的繁殖体与终末宿主之间营养空缺的限制。中间宿主填补了这一空缺，促进了向终末宿主的传播。核心问题在于，为什么在中间宿主中有性繁殖甚至有时幼虫生长会受到抑制，从而有利于中间宿主幼虫成熟时的生长停滞以及直到传播到终末宿主之前的繁殖抑制？终末宿主中寿命的延长和更高的生长速度会促使选择更大的寄生虫体型以及性成熟时更高的繁殖力。生命周期长度通过向上和向下纳入这两种进化机制得以增加，使得简单（单宿主）周期变得复杂（多宿主）。在向下纳入中，在终末宿主下方添加一个中间宿主：模型表明，向下纳入可能仅在生态或进化扰动造成营养空缺之后才会进化。在向上纳入中，在原始终末宿主上方添加一个新的终末宿主，原始终末宿主随后成为中间宿主，同样由营养空缺维持：理论表明，即使在恒定的生态/进化条件下这也是合理的。无论其起源（向上或向下）如何，最终的周期都是相似的。通过将比较系统发育分析（描述进化历史）与进化模型（研究选择力）联系起来，能最好地获得关于宿主纳入的见解。还讨论了宿主营养级的上升和最佳宿主分类单元范围 的进化。
