Data from: Experimental evolution of infectious behaviour in a facultative ectoparasite

Parasitic lifestyles have evolved many times in animals, but how such life-history strategies evolved from free-living ancestors remains a great puzzle. Transitional symbiotic strategies, such as facultative parasitism, are hypothesized evolutionary stepping-stones towards obligate parasitism. However, to consider this hypothesis, heritable genetic variation in infectious behaviour of transitional symbiotic strategies must exist. In this study, we experimentally evolved infectivity and estimated the additive genetic variation in a facultative parasite. We performed artificial selection experiments in which we selected for either increased or decreased propensity to infect in a facultatively parasitic mite (Macrocheles muscaedomesticae). Here, infectiousness was expressed in terms of mite attachment to a host (Drosophila hydei) and modeled as a threshold trait. Mites responded positively to selection for increased infectivity; realized heritability of infectious behaviour was significantly different from zero and estimated to be 16.6% (±4.4% SE). Further, infection prevalence was monitored for 20 generations post-selection. Selected lines continued to display relatively high levels of infection demonstrating a degree of genetic stability in infectiousness. Our study is the first to provide an estimate of heritability and additive genetic variation for infectious behaviour in a facultative parasite, which suggests natural selection can act upon facultative strategies with important implications for the evolution of parasitism.

寄生生活方式在动物界已多次独立演化，但这类生活史策略如何从自由生活的祖先类群演化而来，仍是学界一大未解难题。兼性寄生（facultative parasitism）等过渡型共生策略，被假说为通向专性寄生（obligate parasitism）的进化跳板。然而，要验证这一假说，必须存在过渡型共生策略感染行为的可遗传加性遗传变异（additive genetic variation）。本研究通过实验演化手段探究了感染性，并对兼性寄生物的加性遗传变异进行了定量估算。我们以兼性寄生螨——家蝇巨螯螨（Macrocheles muscaedomesticae）为实验对象，开展了人工选择实验，分别筛选出感染倾向增强与减弱的种群。本研究中，感染性以螨类附着于宿主海德果蝇（Drosophila hydei）的程度进行表征，并被建模为阈值性状（threshold trait）。实验结果显示，螨类对增强感染性的选择呈现正向响应；感染行为的实现遗传力（realized heritability）显著不为零，估算值为16.6%（标准误±4.4%）。此外，我们在选择结束后对感染率进行了连续20代的监测，发现经选择的种群仍维持较高水平的感染率，表明感染性具备一定的遗传稳定性。本研究首次对兼性寄生物的感染行为的遗传力及加性遗传变异进行了定量估算，研究结果表明自然选择可作用于兼性策略，这对寄生现象的演化具有重要意义。