Fitness consequences of altered feeding behavior in immune-challenged mosquitoes

Background: Malaria-infected mosquitoes have been reported to be more likely to take a blood meal when parasites are infectious than when non-infectious. This change in feeding behavior increases the likelihood of malaria transmission, and has been considered an example of parasite manipulation of host behavior. However, immune challenge with heat-killed Escherichia coli induces the same behavior, suggesting that altered feeding behavior may be driven by adaptive responses of hosts to cope with an immune response, rather than by parasite-specific factors. Here we tested the alternative hypothesis that down-regulated feeding behavior prior to infectiousness is a mosquito adaptation that increases fitness during infection. Methods: We measured the impact of immune challenge and blood feeding on the fitness of individual mosquitoes. After an initial blood meal, Anopheles stephensi Liston mosquitoes were experimentally challenged with heat-killed E. coli at a dose known to mimic the same temporal changes in mosquito feeding behavior as active malaria infection. We then tracked daily egg production and survivorship of females maintained on blood-feeding regimes that either mimicked down-regulated feeding behaviors observed during early malaria infection, or were fed on a four-day feeding cycle typically associated with uninfected mosquitoes. Results: Restricting access to blood meals enhanced mosquito survival but lowered lifetime reproduction. Immune- challenge did not impact either fitness component. Combining fecundity and survival to estimate the population- scale intrinsic rate of increase (r), we found that, contrary to the mosquito adaptation hypothesis, mosquito fitness decreased if blood feeding was delayed following an immune challenge. Conclusions: Our data provide no support for the idea that malaria-induced suppression of blood feeding is an adaptation by mosquitoes to reduce the impact of immune challenge. Alternatively, the behavioral alterations may be neither host nor parasite adaptations, but rather a consequence of constraints imposed on feeding by activation of the mosquito immune response, i.e. non-adaptive illness-induced anorexia. Future work incorporating field conditions and different immune challenges could further clarify the effect of altered feeding on mosquito and parasite fitness.

研究背景：已有研究证实，当疟原虫具备感染性时，感染疟原虫的蚊子更易吸食血液，这一现象相较于疟原虫无感染性时更为显著。这种取食行为的改变会提升疟疾传播风险，此前被视为寄生虫操纵宿主行为的典型例证。然而，使用热灭活大肠杆菌（Escherichia coli）进行免疫激发可引发相同的行为变化，这提示取食行为的改变或许是宿主为应对免疫应答而产生的适应性反应，而非寄生虫特异性因子介导的结果。本研究旨在验证另一项假说：在疟原虫具备感染性之前下调取食行为，是蚊子在感染过程中提升自身适合度的适应性策略。
研究方法：本研究量化了免疫激发与血液取食对单只蚊子适合度的影响。在完成初始血液取食后，研究人员对斯氏按蚊（Anopheles stephensi Liston）实施实验性免疫激发，所用热灭活大肠杆菌的剂量可模拟活性疟原虫感染时蚊子取食行为的同期变化。随后，研究人员追踪了两种不同取食模式下雌蚊的每日产卵量与存活情况：一种取食模式模拟了疟疾感染早期观察到的取食行为下调状态，另一种则采用通常对应未感染蚊子的四天取食周期。
研究结果：限制血液取食可提升蚊子的存活率，但会降低其终身繁殖能力。免疫激发并未对这两项适合度指标产生显著影响。结合繁殖力与存活率数据估算种群层面的内禀增长率（intrinsic rate of increase, r）后，研究人员发现：与蚊子适应性假说相悖的是，若免疫激发后延迟血液取食，蚊子的适合度会出现下降。
研究结论：本研究数据不支持"疟疾诱导的血液取食抑制是蚊子为减轻免疫应答影响而产生的适应性行为"这一观点。与之相对，这种行为改变既可能并非宿主的适应性反应，也非寄生虫的适应性策略，而是蚊子免疫应答激活后对取食行为造成限制的结果，即非适应性的疾病诱导性厌食。未来可结合野外环境与不同免疫激发类型开展研究，以进一步阐明取食行为改变对蚊子与寄生虫适合度的影响。