Data from: Exploitation of the same trophic link favors convergence of larval life-history strategies in complex life cycle helminths

Switching from one host to the next is a critical life history transition in parasites with complex life cycles. Growth and mortality rates are thought to influence the optimal time and size at transmission, but these rates are difficult to measure in parasites. The parasite life cycle, in particular the trophic link along which transmission occurs, may be a reasonable proxy for these rates, leading to the hypothesis that life cycle should shape life history strategy. We compiled data on the size and age at infectivity for trophically-transmitted helminths (i.e. acanthocephalans, cestodes, and nematodes), and then categorized species into trophic links (e.g. planktonic crustaceans to fish, insects to terrestrial vertebrates, etc.). Comparative analyses that explicitly included stabilizing selection within trophic links fit the data significantly better than random walk models, indicating that parasites with different life cycles have different optimal times/sizes for host switching. The major helminth groups have often independently evolved similar life cycles, and we show that this has frequently led to convergent and/or parallel evolution of size and age at infectivity. This suggests that for particular life cycles there are universal optimal transmission strategies, applicable to widely divergent taxa, although the cases of parallelism might indicate that lineage-specific constraints sometimes prevent evolution to a single adaptive peak.对于具有复杂生活史的寄生虫而言，从一个宿主转换至下一个宿主是其生活史中极为关键的转型阶段。尽管生长率与死亡率被认为会影响寄生虫传播的最佳时机与体型，但在寄生虫体内难以对这两类速率进行测量。寄生虫的生活史，尤其是其传播所依托的营养联系（trophic link），或许可以作为这两类速率的合理替代指标，由此提出假说：生活史应当塑造寄生虫的生活史策略。我们收集了营养传播性蠕虫（trophically-transmitted helminths，即棘头动物（acanthocephalans）、绦虫（cestodes）与线虫（nematodes））具感染性时的体型与年龄数据，并将各物种按照营养联系进行分类（例如浮游甲壳动物至鱼类、昆虫至陆生脊椎动物等类别）。相较于随机游走模型（random walk models），纳入营养联系内稳定选择（stabilizing selection）的比较分析能更显著地适配实验数据，这表明拥有不同生活史的寄生虫，其宿主转换的最佳时机与体型也存在差异。主要的蠕虫类群往往会独立演化出相似的生活史，我们的研究表明这常会导致具感染性时的体型与年龄出现趋同演化（convergent evolution）乃至平行演化（parallel evolution）。这意味着针对特定的生活史类型，存在普适性的最佳传播策略，可适用于差异极大的各类群生物（taxa）；不过平行演化的案例也表明，谱系特异性约束（lineage-specific constraints）有时会阻碍演化抵达单一的适应峰（adaptive peak）。