Mysterious and Important: The Role of Standing Variation in the Rapid Evolution of Rhizobium Leguminosarum

Schwarz, Chase P

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https://hdl.handle.net/2142/128298 Copy

Description

Title

Mysterious and Important: The Role of Standing Variation in the Rapid Evolution of Rhizobium Leguminosarum

Author(s)

Schwarz, Chase P

Issue Date

2025-05-09

Director of Research (if dissertation) or Advisor (if thesis)

Heath, Katy D

Department of Study

Honors Integrative Biology

Discipline

Biology

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Level

Bachelors

Date of Ingest

2025-05-09T11:34:05-05:00

Keyword(s)

• Evolution
• Genomics
• Drought
• Rhizobia

Language

eng

Abstract

A plant’s ability to grow and respond to environmental stressors is partially due to the microbial community in the rhizosphere. Plants may be particularly dependent on their microbial partners during periods of rapid environmental shifts because the short generation times and extensive genetic variation of microorganisms allows for relatively fast adaptation compared to plants. However, the mechanisms of symbiont evolution and the stability of symbiosis under intense environmental stress remain unknown. To examine how the environment shapes microbial evolution in relation to their plant hosts, we subjected a natural population of 28 strains of Rhizobium leguminosarum to drought conditions over four plant generations. We found evidence of adaptation: strains evolved in drought display greater benefits to plants in contemporary drought than strains evolved in wet conditions. Here, we sequence 80 descendant strains to investigate the source of this rapid evolution. We find that selection acts on standing variation in the ancestral R. leguminosaurum population, but not de novo mutations, resulting in improved legume health during drought. Population genetic and pan genomic analyses indicate selection acts on genes associated with drought resistance and nitrogen fixation. We find that these genes are located on a single plasmid, and the phylogeny of this plasmid is correlated with a strain’s relative representation in drought conditions. These results suggest that R. leguminosarum populations are sufficiently diverse to rapidly adapt to novel environmental conditions, and that their plasmids may act as reservoirs of variation that modulate plant-microbe interactions under abiotic stress.

Sponsor(s)/Grant Number(s)

NSF: 2022049

Type of Resource

text

Copyright and License Information

Chase P. Schwarz

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