Sawada Hiroyucki, Kuykendall L David, Young John M
National Institute for Agro-Environmental Sciences, Tsukuba 305-8604, Japan.
J Gen Appl Microbiol. 2003 Jun;49(3):155-79. doi: 10.2323/jgam.49.155.
As of February 2003, bacteria that form nitrogen-fixing symbiotic associations with legumes have been confirmed in 44 species of 12 genera. Phylogenies of these taxa containing legume symbionts based on the comparative analysis of 16S rDNA sequences show that they are not clustered in one lineage but are distributed in the classes Alphaproteobacteria and Betaproteobacteria, and dispersed over the following nine monophyletic groups, being intermingled with other taxa that do not contain legume symbionts (shown in parentheses below): Group 1, which comprises Rhizobium and Allorhizobium species containing legume symbionts (intermingled with Agrobacterium and Blastobacter species, which are nonsymbionts); Group 2, Sinorhizobium and Ensifer species (with unclassified nonsymbionts); Group 3, Mesorhizobium species (with nonsymbiotic Aminobacter and Pseudaminobacter species); Group 4, Bradyrhizobium species and Blastobacter denitrificans (with nonsymbiotic Agromonas, Nitrobacter, Afipia, and Rhodopseudomonas species); Group 5, 'Methylobacterium nodulans" (with nonsymbiotic Methylobacterium species); Group 6, Azorhizobium species (with nonsymbiotic Xanthobacter and Aquabacter species); Group 7, 'Devosia neptuniae" (with nonsymbiotic Devosia species and unclassified nonsymbionts); Group 8, symbiotic Burkholderia strains (with nonsymbiotic Burkholderia species); and Group 9, Ralstonia taiwanensis (with nonsymbiotic Ralstonia species). For Groups 5, 8, and 9, the present classification, in which 'each monophyletic group comprises one genus wherein legume symbionts and nonsymbionts are intermingled with each other, " is considered to be retained as is because they are clearly separated from other genera at high bootstrap values and have already been sufficiently characterized based on polyphasic taxonomy. As for the remaining six monophyletic groups, on the other hand, there are currently three options for emending their current classification (definitions and circumscriptions) at the generic level: A) the current classification shall be retained as is; B) all the genera within each monophyletic group shall be amalgamated into one single genus in conformity with the results of phylogenetic analysis; or C) each subordinate lineage in each monophyletic group shall be proposed as a genus. It is considered that research and discussions will be continuously conducted for emending the classification of these monophyletic groups based chiefly on Options B and C as preferable candidates.
截至2003年2月，已在12个属的44个物种中确认了与豆科植物形成固氮共生关系的细菌。基于16S rDNA序列的比较分析，这些包含豆科植物共生菌的分类群的系统发育表明，它们并非聚集在一个谱系中，而是分布在α-变形菌纲和β-变形菌纲中，并分散在以下九个单系群中，与其他不包含豆科植物共生菌的分类群混合在一起（以下括号中显示）：第1组，包括含有豆科植物共生菌的根瘤菌属和异根瘤菌属物种（与非共生菌土壤杆菌属和芽生杆菌属物种混合）；第2组，中华根瘤菌属和剑菌属物种（与未分类的非共生菌）；第3组，中慢生根瘤菌属物种（与非共生的氨基杆菌属和假氨基杆菌属物种）；第4组，慢生根瘤菌属物种和反硝化芽生杆菌（与非共生的土壤单胞菌属、硝化杆菌属、阿菲彼亚菌属和红假单胞菌属物种）；第5组，“结节甲基杆菌”（与非共生的甲基杆菌属物种）；第6组，固氮根瘤菌属物种（与非共生的黄色杆菌属和水生杆菌属物种）；第7组，“海王星德沃斯氏菌”（与非共生的德沃斯氏菌属物种和未分类的非共生菌）；第8组，共生伯克霍尔德氏菌菌株（与非共生的伯克霍尔德氏菌属物种）；第9组，台湾嗜麦芽窄食单胞菌（与非共生的嗜麦芽窄食单胞菌属物种）。对于第5、8和9组，目前的分类“每个单系群包含一个属，其中豆科植物共生菌和非共生菌相互混合”被认为按原样保留，因为它们在高自展值下与其他属明显分开，并且已经基于多相分类法得到了充分的表征。另一方面，对于其余六个单系群，目前在属水平上有三种修改其当前分类（定义和范围）的选择：A）当前分类应按原样保留；B）每个单系群内的所有属应根据系统发育分析结果合并为一个单一属；或C）每个单系群中的每个下属谱系应被提议为一个属。人们认为将主要基于选项B和C作为更合适的候选方案，持续进行研究和讨论以修改这些单系群的分类。
