Data from: Addicted? Reduced host resistance in populations with defensive symbionts

Heritable symbionts that protect their hosts from pathogens have been described in a wide range of insect species. By reducing the incidence or severity of infection, these symbionts have the potential to reduce the strength of selection on genes in the insect genome that increase resistance. Therefore, the presence of such symbionts may slow down the evolution of resistance. Here we investigated this idea by exposing Drosophila melanogaster populations to infection with the pathogenic Drosophila C virus (DCV) in the presence or absence of Wolbachia, a heritable symbiont of arthropods that confers protection against viruses. After nine generations of selection, we found that resistance to DCV had increased in all populations. However, in the presence of Wolbachia the resistant allele of pastrel—a gene that has a major effect on resistance to DCV—was at a lower frequency than in the symbiont-free populations. This finding suggests that defensive symbionts have the potential to hamper the evolution of insect resistance genes, potentially leading to a state of evolutionary addiction where the genetically susceptible insect host mostly relies on its symbiont to fight pathogens.

能够保护宿主免受病原体侵害的可遗传共生菌（heritable symbionts），已在多种昆虫物种中被报道。这类共生菌可通过降低宿主的感染发生率或感染严重程度，削弱昆虫基因组中增强抗病性的基因所承受的选择压力。因此，这类共生菌的存在可能会延缓宿主抗病性的演化进程。
本研究针对这一假说展开探究：我们将黑腹果蝇(Drosophila melanogaster)种群分别置于携带与不携带沃尔巴克氏体(Wolbachia)的环境中，用致病性果蝇C病毒(Drosophila C virus, DCV)进行感染处理——沃尔巴克氏体是一种可侵染节肢动物的可遗传共生菌，能够为宿主提供抗病毒保护。
经过九代选择培育后，我们发现所有种群对DCV的抗性均有所提升。但在携带沃尔巴克氏体的种群中，pastrel基因（pastrel）的抗性等位基因——该基因对DCV抗性具有显著调控作用——的频率显著低于无共生菌的种群。
这一结果表明，具有防御功能的共生菌有可能阻碍昆虫抗病基因的演化，甚至可能引发一种演化依赖状态：原本遗传上易感的昆虫宿主，将主要依赖共生菌来抵御病原体侵染。