Data from: Genetic conflict with a parasitic nematode disrupts the legume-rhizobia mutualism

Genetic variation for partner quality in mutualisms is an evolutionary paradox. One possible resolution to this puzzle is that there is a tradeoff between partner quality and other fitness-related traits. Here, we tested whether a susceptibility to parasitism is one such tradeoff in the mutualism between legumes and nitrogen-fixing bacteria (rhizobia). We performed two greenhouse experiments with the legume Medicago truncatula. In the first, we inoculated each plant with the rhizobia Ensifer meliloti and with one of 40 genotypes of the parasitic root-knot nematode Meloidogyne hapla. In the second experiment, we inoculated all plants with rhizobia and half of the plants with a genetically variable population of nematodes. Using the number of nematode galls as a proxy for infection severity, we found that plant genotypes differed in susceptibility to nematode infection, and nematode genotypes differed in infectivity. Second, we showed that there was a genetic correlation between the number of mutualistic structures formed by rhizobia (nodules) and the number of parasitic structures formed by nematodes (galls). Finally, we found that nematodes disrupt the rhizobia mutualism: nematode-infected plants formed fewer nodules and had less nodule biomass than uninfected plants. Our results demonstrate that there is genetic conflict between attracting rhizobia and repelling nematodes in Medicago. If genetic conflict with parasitism is a general feature of mutualism, it could account for the maintenance of genetic variation in partner quality and influence the evolutionary dynamics of positive species interactions.共生关系中伙伴品质的遗传变异是一个进化悖论。针对这一难题的一种可能解决路径是，伙伴品质与其他适合度相关性状之间存在进化权衡。本研究在豆科植物与固氮细菌（根瘤菌，rhizobia）的共生体系中，检验了寄生易感性是否属于此类权衡之一。我们以蒺藜苜蓿（Medicago truncatula）为实验材料开展了两项温室实验：第一项实验中，我们为每株植物接种苜蓿中华根瘤菌（Ensifer meliloti），并接种寄生性根结线虫北方根结线虫（Meloidogyne hapla）的40个基因型中的一个；第二项实验中，我们为所有植株接种根瘤菌，其中半数植株接种遗传变异的线虫种群。以线虫虫瘿数量作为感染严重程度的替代指标，我们发现植物基因型间的线虫感染易感性存在差异，且线虫基因型间的侵染能力亦存在分化。其次，我们证实根瘤菌形成的共生结构——根瘤（nodules）数量与线虫形成的寄生结构——虫瘿（galls）数量之间存在遗传相关性。最后，我们发现线虫会破坏根瘤菌共生体系：被线虫侵染的植株形成的根瘤数量更少，且根瘤生物量低于未侵染植株。本研究结果表明，在蒺藜苜蓿中，招募根瘤菌与抵御线虫之间存在遗传冲突。若与寄生生物的遗传冲突是共生关系的普遍特征，则这一现象或可解释伙伴品质遗传变异的维持机制，并对物种间正相互作用的进化动态产生影响。