Dissertation Research: Coevolution in the Legume-Rhizobiaceae Symbiosis: A Phylogenetic Test of Codivergence

根瘤菌科细菌家族的成员以固氮和与多种豆科寄主植物的共生关系而闻名。土壤细菌在豆科植物的根上形成根瘤，将大气中的氮固定成植物可以利用的形式。本研究的目的是测试这两个共生伙伴之间的共同进化。系统发育中分支模式的相似性（或系统发育一致性）是共生体之间共享进化历史的有力证据，并将用于测试根前科及其豆科宿主之间的共同进化。研究将确定在寄主植物多样化过程中，使细菌成为植物共生体的遗传信息，即系统区域，是否与寄主植物的基因共同进化。研究结果还将说明地理分离对根瘤菌遗传分化的贡献，并表明根瘤菌进化历史中结瘤基因是否在不同根瘤菌染色体谱系之间转移。了解根瘤菌科和豆科植物之间关联的进化具有重要的农业意义。豆类包括重要的农作物，大豆，豌豆和鹰嘴豆。与根瘤菌的固氮共生也被开发为不使用化肥向土壤补充固定氮的一种方式。如果要通过控制共生关系来提高作物产量和土壤肥力，了解根瘤菌/宿主关系是如何产生的，在豆科植物多样化过程中这种联系的稳定性，以及维持自然界共生关系的进化压力的类型是至关重要的。

9623537 Riley Members of a bacterial family Rhizobiaceae are known for their nitrogen-fixing, symbiotic relationship with a variety of legume host plants. The soil bacteria forms nodules on legume roots, where it fixes atmospheric nitrogen into a form which the plant can use. The goal of this study is to test for coevolution between these two symbiotic partners. Similarity of branching patterns in phylogenies (or, phylogenetic congruence) is strong evidence of a shared evolutionary history between symbionts, and will be used to test for coevolution between the Rhizobeaceae and their legume hosts. Research will determine whether genetic information enabling the bacteria to become a symbiont of the plant, the sym region, has coevolved with genes of the host plant during the process of host plant diversification. Findings will also illustrate the contribution of geographic separation among spatially isolated strains to genetic divergence in rhizobia and show whether nodulation genes have been transferred between different rhizobial chromosomal lineages over the evolutionary history of rhizobia. Understanding the evolution of associations between Rhizobiaceae and legumes is important agriculturally. Legumes include the important crop plants, soybeans, peas, and chickpeas. The nitrogen-fixing symbiosis with rhizobia has also been exploited as a way to replenish soil with fixed nitrogen without using chemical fertilizers. Understanding how rhizobia/host relationships arose, the stability of the association throughout the diversification of legumes, and the types of evolutionary pressures which maintain the symbiosis in nature is critical if crop production and soil fertility are to be improved by manipulating the symbiosis.