Phylogeny and Coevolution of Actinorhizal Rosaceae and their Nitrogen-Fixing Symbionts

八个开花植物科的物种与放线菌Frankia属的固氮细菌形成共生关系，其中原核共生体栖息在植物根部的结节上。系统发育分析表明，这种共生关系，被称为放线根协会，已经演变和/或失去了无数次的进化过程中，无论是在Frankia和植物家族之间的参与。这些植物科之一是蔷薇科，蔷薇科，一个具有相当大的经济重要性和进化兴趣的大组。在蔷薇科中，已知五个属（Cercocarpus，Chamaebatia，Cowania，Dryas和Purshia）的成员形成放线根协会。最近的分子证据表明，这五个属，虽然传统上被分类在不同的部落或亚科，而不是形成一个单系组内的家庭。从放线菌菌株感染这些物种的核苷酸序列数据表明，他们都属于三个已知的单系组感染性Frankia之一。无论是放线菌蔷薇科，也不是他们的Frankia共生体，然而，一直是彻底的系统发育研究的主题。相对较少的植物物种（约30）在这组中，使其成为一个理想的系统，在其中研究模式的两个参与者之间的共生放线菌。加州大学戴维斯分校的丹尼尔·波特博士和他在康涅狄格大学的同事大卫·本森博士及其同事将利用大分子数据来评估遗传多样性，并提出放线菌蔷薇科及其Frankia共生体的分类群内部和之间的系统发育关系的假设。在美国西部的实地考察和墨西哥的同事将致力于收集蔷薇科五个放线菌属所有物种的叶子和根瘤以及几个外类群的叶子材料。在可能的情况下，将从每个物种的整个地理范围内对多个种群和每个种群多个个体进行采样。还将收集同一地点出现的非蔷薇科放射根状茎植物物种的结节。核苷酸序列数据将从宿主植物（从叶材料提取的DNA）及其共生体（从根瘤提取的DNA）的基因组（在植物的情况下为叶绿体和核）的适当可变区获得。广泛的地理采样的多个人口将使调查的宿主特异性的Frankia菌株和混杂的植物物种就其共生体。将对数据进行遗传学分析，并将比较由此产生的系统发育树，以检查共同进化的程度和模式，并解决共同进化发生的分类学水平的问题。在确定感染特定地点特定宿主的共生体菌株时，地理和共生关系的相对贡献是什么？来自植物基因组的数据也将用于评估这一组中物种和属的分类学划界。这项研究的主要意义在于提高我们对共生进化的理解，特别是对生态学上重要的固氮共生体的理解。这项研究还将扩大我们的知识的分类和地理分布的放线菌协会，并将有助于澄清这组植物的分类，其中大部分发生在北美西部。

Species in eight families of flowering plants form symbiotic relationships with nitrogen-fixing bacteria of the actinomycete genus Frankia, in which the prokaryotic symbionts inhabit nodules on the plants' roots. Phylogenetic analyses have suggested that such symbiotic relationships, known as actinorhizal associations, have evolved and/or been lost numerous times over the course of evolution, both within Frankia and among the plant families involved. One of these plant families is the rose family, Rosaceae, a large group with considerable economic importance and evolutionary interest. Within Rosaceae, members of five genera (Cercocarpus, Chamaebatia, Cowania, Dryas, and Purshia) are known to form actinorhizal associations. Recent molecular evidence has shown that these five genera, although traditionally classified in different tribes or subfamilies, instead form a monophyletic group within the family. Nucleotide sequence data from the actinomcyete strains infecting several of these species have shown that they all belong to one of the three known monophyletic groups of infective Frankia. Neither the actinorhizal Rosaceae nor their Frankia symbionts, however, have been the subject of thorough phylogenetic study. The relatively small number of plant species (total of about 30) in this group makes it an ideal system in which to study patterns of coevolution between the two participants in the actinorhizal symbiosis. Dr. Daniel Potter of the University of California-Davis and his colleague at University of Connecticut, Dr. David Benson, and their associates will use macromolecular data to evaluate genetic diversity and generate hypotheses of phylogenetic relationship within and among taxa of actinorhizal Rosaceae and their Frankia symbionts. Fieldwork in the western U.S. and by colleagues in Mexico will be devoted to the collection of leaves and root nodules from all species of the five actinorhizal genera of Rosaceae and leaf material from several outgroups. Wherever possible, multiple populations, and multiple individuals per population, will be sampled from throughout the geographic range of each species. Nodules from non-rosaceous actinorhizal plant species occurring at the same localities will also be collected. Nucleotide sequence data will be obtained from appropriately variable regions of the genomes (both chloroplast and nuclear in the case of the plants) of the host plants (from DNA extracted from leaf material) and their symbionts (from DNA extracted from nodules). Broad geographical sampling of multiple populations will enable investigation of host specificity of Frankia strains and promiscuity of the plant species with respect to their symbionts. The data will be analyzed phylogenetically and the resulting phylogenetic trees will be compared in order to examine the extent and patterns of coevolution and to address the question of the taxonomic level at which coevolution has occurred. What are the relative contributions of geography and phylogeny in determining the symbiont strain(s) infecting a particular host at a particular locality? The data from the plant genomes will also be used to evaluate taxonomic delimitation of species and genera in this group. The primary significance of this study will be in improving our understanding of the evolution of symbiosis in general and of ecologically important nitrogen-fixing symbioses in particular. The study will also extend our knowledge of the taxonomic and geographic distributions of actinorhizal associations and will contribute to clarifying the classification of this group of plants, most of which occur in western North America.