<p> Legume crops are significant agriculturally and environmentally for their ability to form symbiosis with specific soil bacteria capable of nitrogen fixation. Nitrogen fixation for a given legume in a given soil is limited by the availability of the plant’s bacterial partners, and by variation in the effectiveness of those symbionts. We used a global-level hierarchical sampling scheme to comprehensively characterize the evolutionary relationships and distributional limitations of nitrogen-fixing bacterial symbionts of the legume crop chickpea. This has been accomplished using culture-dependent and independent approaches to generate over 1,200 draft whole-genome assemblies at the level of bacterial populations, as well as 14 finished-quality genomes using the Pacific Biosciences platform. These strategies reveal that chickpea’s symbionts across the globe are confined to the genus <i>Mesorhizobium </i>, but a diversity of taxa within the genus (chapter 1 and 3). Comparative phylogenomic analysis reveals that despite chickpea’s symbionts within and across regions coming from different taxa, all share almost identical genes for symbiosis. PacBio genome-assemblies reveal that this is due to the horizontal transfer of a 500 kb chromosomal island known as a symbiosis island, between unrelated strains of the genus <i>Mesorhizobium </i>. Analyzing the symbiosis island at the population level reveals that the symbiosis island spreads repeatedly once introduced to a region, suggesting that strains well-adapted to a particular soil climate continue to dominate once the new host (chickpea) has been introduced, through repeated acquisition of the symbiosis island. This dataset provides additional insights into the functional and taxonomic diversity of other bacteria associated with chickpea nodules (chapter 2).</p><p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:10837756 |
| Date | 15 November 2018 |
| Creators | Greenlon, Alex |
| Publisher | University of California, Davis |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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