The trophic vacuum and the evolution of complex life cycles in trophically-transmitted helminths

Parasitic worms (helminths) frequently have complex life cycles in which they are transmitted trophically between two or more successive hosts. Sexual reproduction often takes place in high trophic-level (TL) vertebrates, where parasites can grow to large sizes with high fecundity. Direct infection of high TL hosts, while advantageous, may be unachievable for parasites constrained to transmit trophically, because helminth propagules are unlikely to be ingested by large predators. Lack of niche overlap between propagule and definitive host (the trophic transmission vacuum) may explain the origin and/or maintenance of intermediate hosts, which overcome this transmission barrier. We show that nematodes infecting high TL definitive hosts tend to have more successive hosts in their life cycles. This relationship was modest, though, driven mainly by the minimum TL of hosts, suggesting that the shortest trophic chains leading to a host define the boundaries of the transmission vacuum. We also ...

寄生蠕虫（helminths）通常具有复杂的生活史，其可通过营养级传递在两个或多个连续宿主间传播。有性生殖常发生于高营养级（TL）脊椎动物体内，此时寄生虫可发育至较大体型并具备极高的繁殖力。直接感染高营养级宿主虽具备演化优势，但对于依赖营养级传递的寄生虫而言却难以实现，原因是蠕虫传播体（propagules）几乎不可能被大型捕食者摄食。传播体与终末宿主（definitive host）之间的生态位缺失，即营养级传递真空（trophic transmission vacuum），或可解释中间宿主（intermediate host）的起源与维持机制——中间宿主正是为了克服这一传播障碍而演化产生的。本研究显示，寄生于高营养级终末宿主的线虫，其生活史中包含的连续宿主数量往往更多。不过这一相关性并不显著，其驱动因素主要为宿主的最低营养级，这提示指向某一宿主的最短营养链，界定了营养级传递真空的边界。我们还……