Graduation Year

2025

Document Type

Open Access Senior Thesis

Degree Name

Bachelor of Arts

Department

Environmental Analysis

Reader 1

Tom E. X. Miller

Reader 2

Diane Thomson

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Terms of Use for work posted in Scholarship@Claremont.

Rights Information

© 2024 Audrey H Yung

Abstract

Epichloë is a genus of endophytes– symbiotic fungi residing within a plant’s tissue– that can provide mutualistic benefits to its host. The stress gradient hypothesis predicts that mutualism between species becomes stronger in conditions of high abiotic stress. We tested this idea by assessing how Epichloë prevalence in Elymus virginicus populations is affected by varying climatic conditions. We established plots of E. virginicus with known endophyte frequencies in the spring of 2023 at five sites across a precipitation gradient in Texas and Louisiana. We collected E. virginicus recruits in June and July 2024 at all field sites. To determine recruits’ endophyte status, we ran recruit samples through an immunoblot assay and did leaf peel microscopy to clarify any uncertainty from immunoblot endophyte status scores. Scans of immunoblot assay stains of endophyte-positive recruits were processed through ImageJ to quantify relative endophyte abundance in each recruit. Climate data was collected from HOBO data loggers that were placed directly in field sites. A binomial generalized linear mixed model was created for each of the five field sites to model the projected endophyte prevalence at equilibrium for given initial endophyte frequency. Projection values and observed changes in endophyte frequency after one year were plotted against average soil moisture and temperature data of the corresponding field site from November 2023 to May 2024. Weighted averages of each endophyte-positive recruit’s stain intensity was plotted by field site. We found that endophyte-plant interactions were favorable when soil moisture was high; conversely, Epichloë declined in drier sites, suggesting that hosting Epichloë is costly in stressful conditions. Similarly, endophyte abundance by individual recruits was greatest in wetter sites. These results are not consistent with the stress gradient hypothesis: mutualism weakened with increasing stress. This contradiction highlights unexpected complexity in how communities respond to stress and how plant-microbial communities may react to climatic shifts.

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