R<i>eadme</i> file for S6 Data.

Evolutionary expectations about the virulence of parasites (i.e., the parasite-induced mortality rate of the host) often focus solely on the within-host transmission stage, overlooking the time spent between hosts and variations in transmission cycles. Moreover, the parasite growth rate within the host is closely linked to virulence. Here, we suggest that a simplified view of transmission and parasite evolution makes predicting how virulence will evolve difficult. We illustrate our ideas with a parasite with a simple life cycle, the microsporidian Vavraia culicis, which infects the mosquito Anopheles gambiae. We selected the parasite over six host generations for early or late host transmission, corresponding to shorter or longer time within the host. Selecting for late transmission increased their exploitation of the host, resulting in higher host mortality and a shorter life cycle with rapid infective spore production, comparatively to selection for early transmission. In response, hosts infected with late-selected spores shortened their life cycle and shifted to earlier reproduction. Using different host harm metrics, we demonstrate and discuss the pros and cons of using different measures of virulence. These and other findings emphasize the importance of considering the entire transmission cycle in studies of parasite evolution and raise concerns about how host density and social settings might influence virulence evolution.

现有关于寄生虫毒力（即寄生虫诱导的宿主死亡率）的演化预期，通常仅聚焦于宿主内传播阶段，却忽略了宿主间的间隔时长与传播周期的变异。此外，宿主体内的寄生虫生长速率与毒力紧密相关。本文提出，对传播过程与寄生虫演化的简化认知，会使得毒力演化方向的预测变得困难。我们以感染冈比亚按蚊（Anopheles gambiae）的微孢子虫（microsporidian）Vavraia culicis——一种具有简单生活史的寄生虫——为例，阐释了上述观点。我们通过6个宿主世代的选育，获得了分别对应宿主内停留时长更短的早期传播、以及时长更长的晚期传播的寄生虫株系。与早期传播选育组相比，晚期传播选育组的寄生虫对宿主的利用程度更高，导致宿主死亡率升高，且生活史更短、感染性孢子的产生速度更快。作为响应，感染晚期选育组孢子的宿主会缩短自身生活史，并提前进行繁殖。我们采用不同的宿主损伤衡量指标，论证并讨论了使用不同毒力衡量方法的优劣。本研究及相关发现均强调，在寄生虫演化研究中考虑完整传播周期的重要性，同时也引发了关于宿主密度与社会环境如何影响毒力演化的探讨。