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Date of Award

Summer 8-15-2019

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Geographically widespread species experience varied selection across their ranges, and adaptation to local environments plays a critical role in their ability to persist. Understanding the genetic basis of local adaptation is a longstanding goal in evolutionary biology and provides practical information for agriculture and conservation. However, the genetic architecture of local adaptation has been characterized in relatively few plant species, primarily those with short lifespans and high rates of self-fertilization. Moreover, for plants, chemical defenses are known to play an important role in adaptation, but the extent to which they contribute to local adaptation is less understood. This dissertation provides a genome-wide, multi-environment assessessment of the importance of a well-studied chemical defense polymorphism for local adaptation, relative to other genetic factors, and addresses fundamental questions in evolutionary biology about the genetic architecture of local adaptation in an outcrossing plant. White clover (Trifolium repens L.) is a perennial, obligately outcrossing legume and an important forage crop. Naturalized populations occur across a wide range of climates, from subtropical to near arctic. White clover populations display adaptation related to a chemical defense polymorphism, cyanogenesis—the production of hydrogen cyanide upon tissue damage. Adaptive cyanogenesis clines have repeatedly evolved across the species range, such that higher proportions of cyanogenic plants are found in warmer climates. However, the relative adaptive importance of the cyanogenesis polymorphism for local adaptation, compared to other genetic factors, is unknown. Chapter 1 in this dissertation provides evidence of local climatic adaptation in white clover by documenting correlations between fitness traits and home-site climate variation for 15 widespread populations grown in a central North American common garden experiment. Chapter 2 demonstrates that divergent life history strategies associated with early flowering versus multi-year persistence contribute to local adaptation across three common garden experiments in locations spanning the U.S. latitudinal range of white clover. It also suggests that allelic trade-offs at major-effect loci are common for local adaptation in this outcrossing species. We did not find significant fitness differences that were attributable to cyanogenesis in the experiments presented in Chapters 1 and 2, which focused on mature adult plants; however, Chapter 3 documents significant shifts in cyanogenesis frequencies from the seedling to adult life stages and also from benign greenhouse to field germination environments, specifically at the seedling stage, across three environments. These results suggest that cyanogenesis may be more important for local adaptation at the earliest life stages, thereby promoting clinal evolution in this chemical defense trait.

Language

English (en)

Chair and Committee

Kenneth M. Olsen

Committee Members

Christine Edwards, Justin Fay, Allan Larson, Jonathan Myers,

Available for download on Wednesday, November 04, 2020

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