Data from: Evolution of Drosophila resistance against different pathogens and infection routes entails no detectable maintenance costs

Pathogens exert a strong selective pressure on hosts, entailing host adaptation to infection. This adaptation often affects negatively other fitness-related traits. Such trade-offs may underlie the maintenance of genetic diversity for pathogen resistance. Trade-offs can be tested with experimental evolution of host populations adapting to parasites, using two approaches: (1) measuring changes in immunocompetence in relaxed-selection lines and (2) comparing life-history traits of evolved and control lines in pathogen-free environments. Here, we used both approaches to examine trade-offs in Drosophila melanogaster populations evolving for over 30 generations under infection with Drosophila C Virus or the bacterium Pseudomonas entomophila, the latter through different routes. We find that resistance is maintained after up to 30 generations of relaxed selection. Moreover, no differences in several classical life-history traits between control and evolved populations were found in pathogen-free environments, even under stresses such as desiccation, nutrient limitation, and high densities. Hence, we did not detect any maintenance costs associated with resistance to pathogens. We hypothesize that extremely high selection pressures commonly used lead to the disproportionate expression of costs relative to their actual occurrence in natural systems. Still, the maintenance of genetic variation for pathogen resistance calls for an explanation.病原体（pathogens）对宿主施加了强烈的选择压力，进而驱动宿主演化出针对感染的适应性。这类适应性往往会对其他与适合度相关的性状产生负面影响。此类演化权衡或许是病原体抗性相关遗传多样性得以维持的核心机制。可通过宿主种群适应寄生虫（parasites）的实验演化来验证这类权衡，主要包含两种研究策略：其一，检测松弛选择系（relaxed-selection lines）的免疫能力（immunocompetence）变化；其二，在无病原体环境中对比进化组与对照组种群的生活史性状。本研究同时采用上述两种方法，探究黑腹果蝇（Drosophila melanogaster）种群在感染果蝇C病毒（Drosophila C Virus）或昆虫假单胞菌（Pseudomonas entomophila，后者采用不同感染途径）的条件下连续演化超过30代后的演化权衡情况。结果显示，即便经过多达30代的松弛选择，宿主的病原体抗性仍得以维持。此外，在无病原体环境中，即便处于干燥胁迫、营养限制与高密度等应激条件下，对照组与进化组种群的多项经典生活史性状均未出现显著差异。据此，本研究未发现与病原体抗性维持相关的适应性成本。我们推测，当前实验中普遍采用的极高选择压力，会使得抗性成本的表现程度远超其在自然系统中的实际发生水平。尽管如此，病原体抗性相关遗传变异的维持仍需进一步的合理解释。