Eardly Bertrand, Elia Patrick, Brockwell John, Golemboski Daniel, van Berkum Peter
Biology Department, Penn State University, Berks College, Reading, Pennsylvania, USA
Soybean Genomics and Improvement Laboratory, Agriculture Research Service, U.S. Department of Agriculture, Beltsville, Maryland, USA.
Appl Environ Microbiol. 2017 May 1;83(10). doi: 10.1128/AEM.03446-16. Print 2017 May 15.
Here, we describe a novel clade within and consider how geographic and ecological isolation contributed to the limited distribution of this group. Members of the genus are best known for their ability to form nitrogen-fixing symbioses with forage legumes of three related genera, L., Mill., and L., which are members of the tribe Trifolieae. These legumes have a natural distribution extending from the Mediterranean Basin through western Asia, where there is an unsurpassed number of species belonging to these genera. L. is unusual in that it is the only species in the tribe Trifolieae that is native to Australia. We compared the genetic diversity and taxonomic placement of rhizobia nodulating with those of members of an reference collection. Our goal was to determine if the rhizobial strains, like their plant host, are naturally limited to the Australian continent. We used multilocus sequence analysis to estimate the genetic relatedness of 56 symbionts to 28 reference strains. Sequence data were partitioned according to the replicons in which the loci are located. The results were used to construct replicon-specific phylogenetic trees. In both the chromosomal and chromid trees, the Australian strains formed a distinct clade within The strains also shared few alleles with reference strains from other continents. Carbon source utilization assays revealed that the strains are also unusual in their ability to utilize 2-oxoglutarate as a sole carbon source. A strategy was outlined for locating similar strains elsewhere. In this study, we employed a biogeographical approach to investigate the origins of a symbiotic relationship between an Australian legume and its nitrogen-fixing rhizobia. The question of the ancestral origins of these symbionts is based on the observation that the legume host is not closely related to other native Australian legumes. Previous research has shown that the legume host is instead closely related to legumes native to the Mediterranean Basin and western Asia, suggesting that it may have been introduced in Australia from those regions. This led to the question of whether its rhizobia may have been introduced as well. In this study, we were unable to find persuasive evidence supporting this hypothesis. Instead, our results suggest either that the rhizobia are native to Australia or that our methods for locating their close relatives elsewhere are inadequate. A strategy to investigate the latter alternative is proposed.
在此，我们描述了一个新的进化枝，并探讨地理和生态隔离如何导致该类群分布有限。该属成员以其与三个相关属（L.、Mill. 和 L.）的饲用豆科植物形成固氮共生体的能力而闻名，这三个属是三叶草族的成员。这些豆科植物的自然分布范围从地中海盆地延伸至西亚，在那里这些属的物种数量无与伦比。L. 不同寻常之处在于它是三叶草族中唯一原产于澳大利亚的物种。我们将与 L. 形成根瘤的根瘤菌的遗传多样性和分类地位与一个参考菌库的成员进行了比较。我们的目标是确定 L. 的根瘤菌菌株是否像其植物宿主一样自然地局限于澳大利亚大陆。我们使用多位点序列分析来估计 56 个 L. 共生体与 28 个参考菌株的遗传相关性。序列数据根据基因座所在的复制子进行划分。结果用于构建特定于复制子的系统发育树。在染色体树和质粒树中，澳大利亚菌株在该属内形成了一个独特的进化枝。这些菌株与来自其他大陆的参考菌株也很少共享等位基因。碳源利用分析表明，这些菌株在利用 2 - 氧代戊二酸作为唯一碳源的能力方面也很不寻常。概述了一种在其他地方寻找类似菌株的策略。在本研究中，我们采用生物地理学方法来研究一种澳大利亚豆科植物与其固氮根瘤菌之间共生关系的起源。这些共生体的祖先起源问题基于这样的观察：豆科宿主与其他澳大利亚本土豆科植物关系不密切。先前的研究表明，豆科宿主 L. 反而与地中海盆地和西亚的本土豆科植物关系密切，这表明它可能是从那些地区引入澳大利亚的。这就引出了其根瘤菌是否也可能被引入的问题。在本研究中，我们未能找到支持这一假设的有说服力的证据。相反，我们的结果表明要么 L. 的根瘤菌原产于澳大利亚，要么我们在其他地方寻找其近亲的方法不够充分。提出了一种研究后一种可能性的策略。
