Organisms have evolved internal biological clocks to regulate their activities based on external environmental cues, such as light, temperature, and food. Environmental disruption of these rhythms, such as caused by constant light or frequent light schedule changes, has been shown to impair development, reduce survival, and increase infection susceptibility and disease progression in numerous organisms. However, the precise role of the biological clock in host-parasite interactions is understudied and has focused on unnatural host-parasite combinations in lab-adapted inbred models. Here, we use the natural interaction between monarch butterflies (Danaus plexippus) and their virulent protozoan parasite, Ophryocystis elektroscirrha, to investigate the effects of constant light and frequent light schedule changes on development, survival, and parasite susceptibility. We show that constant light exposure slows the monarchs’ rate of development but does not increase susceptibility to parasitic infection. Furthermore, frequent schedule changes decrease parasite growth, but have no effect on egg-to-adult survival of infected monarchs. Interestingly, these conditions are usually disruptive to the biological clock, but do not significantly impact the clock of monarch larvae. These unexpected findings show that constant light and frequent schedule changes can uncouple host and parasite performance and highlight how natural relationships are needed to expand our understanding of clocks in host-parasite interactions.

生物体已经进化出内部生物钟，以根据外部环境线索（例如光、温度和食物）来调节它们的活动。这些节律的环境破坏，例如由持续光照或频繁的光照时间表变化引起的，已被证明会损害许多生物体的发育、降低存活率并增加感染易感性和疾病进展。然而，生物钟在宿主-寄生虫相互作用中的确切作用尚未得到充分研究，并集中在实验室适应的近交模型中的非自然宿主-寄生虫组合上。在这里，我们利用帝王蝶 ( Danaus plexippus ) 与其毒性原生动物寄生虫Ophryocystis elektroscirrha之间的自然相互作用，研究恒定光照和频繁光照变化对发育、生存和寄生虫易感性的影响。我们表明，持续的光照会减慢君主的发育速度，但不会增加对寄生虫感染的易感性。此外，频繁的时间表变化减少了寄生虫的生长，但对受感染君主的卵到成虫的存活没有影响。有趣的是，这些条件通常会破坏生物钟，但不会显着影响君主幼虫的时钟。这些出乎意料的发现表明，持续的光照和频繁的时间表变化可以解耦宿主和寄生虫的表现，并突出说明如何需要自然关系来扩展我们对宿主-寄生虫相互作用中时钟的理解。