Werner Gijsbert D A, Cornwell William K, Cornelissen Johannes H C, Kiers E Toby
Department of Ecological Science, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, The Netherlands;
Ecology and Evolution Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
Proc Natl Acad Sci U S A. 2015 Aug 18;112(33):10262-9. doi: 10.1073/pnas.1424030112. Epub 2015 Jun 3.
Understanding the origins and evolutionary trajectories of symbiotic partnerships remains a major challenge. Why are some symbioses lost over evolutionary time whereas others become crucial for survival? Here, we use a quantitative trait reconstruction method to characterize different evolutionary stages in the ancient symbiosis between legumes (Fabaceae) and nitrogen-fixing bacteria, asking how labile is symbiosis across different host clades. We find that more than half of the 1,195 extant nodulating legumes analyzed have a high likelihood (>95%) of being in a state of high symbiotic persistence, meaning that they show a continued capacity to form the symbiosis over evolutionary time, even though the partnership has remained facultative and is not obligate. To explore patterns associated with the likelihood of loss and retention of the N2-fixing symbiosis, we tested for correlations between symbiotic persistence and legume distribution, climate, soil and trait data. We found a strong latitudinal effect and demonstrated that low mean annual temperatures are associated with high symbiotic persistence in legumes. Although no significant correlations between soil variables and symbiotic persistence were found, nitrogen and phosphorus leaf contents were positively correlated with legumes in a state of high symbiotic persistence. This pattern suggests that highly demanding nutrient lifestyles are associated with more stable partnerships, potentially because they "lock" the hosts into symbiotic dependency. Quantitative reconstruction methods are emerging as a powerful comparative tool to study broad patterns of symbiont loss and retention across diverse partnerships.
理解共生伙伴关系的起源和进化轨迹仍然是一项重大挑战。为什么有些共生关系在进化过程中消失了，而另一些却对生存至关重要？在这里，我们使用一种数量性状重建方法来描述豆科植物与固氮细菌之间古老共生关系的不同进化阶段，探讨共生在不同宿主类群中的稳定性如何。我们发现，在分析的1195种现存结瘤豆科植物中，超过一半极有可能（>95%）处于高度共生持续性状态，这意味着它们在进化过程中表现出持续形成共生关系的能力，尽管这种伙伴关系仍然是兼性的而非 obligate（此处未明确该词含义，可根据专业知识补充完整）。为了探究与固氮共生关系丧失和保留可能性相关的模式，我们测试了共生持续性与豆科植物分布、气候、土壤和性状数据之间的相关性。我们发现了强烈的纬度效应，并证明年平均气温较低与豆科植物的高共生持续性相关。尽管未发现土壤变量与共生持续性之间存在显著相关性，但叶片中的氮和磷含量与处于高共生持续性状态的豆科植物呈正相关。这种模式表明，对养分需求高的生活方式与更稳定的伙伴关系相关，这可能是因为它们使宿主“锁定”在共生依赖关系中。数量重建方法正在成为一种强大的比较工具，用于研究不同伙伴关系中共生体丧失和保留的广泛模式。
