Mechanisms of Recurrent Cyanogenesis Cline Evolution in Trifolium repens (White Clover)
Date of Award
Doctor of Philosophy (PhD)
Clines reflect adaptive differentiation in response to environmental heterogeneity. Recurrently forming adaptive clines provide the opportunity to compare multiple replicates of the process of adaptation. This dissertation investigates the molecular, microevolutionary and ecological mechanisms underlying the evolution of parallel clines in a classic chemical defense polymorphism. Cyanogenesis, the ability to produce hydrogen cyanide after tissue damage, is a presence/absence trait in white clover that, when present, functions as an energetically costly herbivore defense. The presence of cyanogenesis depends on the presence of two biochemical components, cyanogenic glucosides and their hydrolyzing enzyme, linamarase. Each of these components can also be present or absent and may have other physiological roles in nutrient storage and transport. The cyanogenesis polymorphism is manifest spatially as recurrent clines where there is a higher frequency of cyanogenic plants in warmer climates. I establish that parallel adaptive clines in frequency of cyanogenic plants have formed after introduction of white clover into the central U.S., New Zealand and the U.S. Pacific Northwest. Cyanogenesis clines form from standing genetic variation from the native range rather than by de novo mutation occurring in each introduced region. Although parallel cyanogenesis clines have evolved, there is not parallel clinal variation in the frequency of cyanogenic glucosides and linamarase in each region, indicating that the selection pressures and the genes underlying selection differ among clines. A series of ecological experiments suggests that, in contrast to previous hypotheses, differential herbivore pressure is not responsible for evolution of cyanogenesis clines. Instead, manipulative experiments in the greenhouse and field suggest that clines result from a fitness tradeoff where plants producing cyanogenic glucosides have higher fitness under moderate, frequent periods of drought, but have a fitness cost when water is abundant. Additional observational data support this conclusion, including an adaptive cline in the frequency of cyanogenic glucosides that corresponds to a precipitation gradient. This dissertation establishes that adaptive differentiation between populations occurs in this system on short temporal and spatial scales. Results indicate that parallel spatial patterns do not necessarily form via similar mechanisms, but can occur in response to multiple spatially-correlated selection pressures acting on correlated phenotypes.
Chair and Committee
Kenneth M. Olsen
Barbara A Schaal, Alan R Templeton, Justin C Fay, Eleanor A Pardini, Jonathan A Myers
Kooyers, Nicholas John, "Mechanisms of Recurrent Cyanogenesis Cline Evolution in Trifolium repens (White Clover)" (2013). Arts & Sciences Electronic Theses and Dissertations. 52.