Evolution of heavy metal hyperaccumulation in wild sunflowers (Helianthus)
Goolsby, Eric Wayne
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This dissertation proposes novel methods phylogenetic comparative methods for studying complex biological traits and investigates the evolutionary history of heavy metal hyperaccumulation in a comparative framework. Current phylogenetic comparative methods, which limit trait observations to univariate means with normally distributed errors, are unable to incorporate several biologically important trait types, including phenotypic plasticity and dose-response curves. The ability to study such phenotypes, known as function-valued traits, in a comparative framework is critical to unifying the historically disparate fields of evolutionary biology and toxicology. Two methods-based chapters lay out a framework for explicitly accounting for function-valued traits in in the context of macroevolution, representing a substantial improvement in comparative methods which are currently limited regarding the capacity to incorporate complex multivariate traits. The first of these chapters extends phylogenetic generalized least squares methods to allow for function-valued traits assuming a Brownian motion model of evolution. The next chapter expands function-valued and other high-dimensional comparative analyses to allow for the incorporation of alternative evolutionary models, fixed effects, within-species variation, and missing data, and also addresses issues of statistical power, model flexibility, and computational tractability. The next two chapters explore the evolution of metal hyperaccumulation in the Helianthus genus (sunflowers). First, the elemental defense hypothesis, an adaptive hypothesis for metal hyperaccumulation which predicts a deterrent effect of leaf metals on herbivory, is evaluated in Helianthus and the generalist herbivore Vanessa cardui (the Painted Lady butterfly) using a comparative approach. Mixed support for the elemental defense hypothesis is found, with herbivores exhibiting a preference for leaves from non-metal-treated plants over metal-treated plants in certain species. However, in the absence of a choice, V. cardui were not deterred by leaf metals. Next, the evolutionary history of As, Cd, Cr, Cu, Ni, Pb, Se, and Zn accumulation was investigated across Helianthus. Hyperaccumulation of Cd, Ni, and Zn is widespread throughout Helianthus. Cd and Zn hyperaccumulation, as well as elevated Ni accumulation, likely evolved in wild Helianthus prior to sunflower domestication. These results, in conjunction with the comparative methods developed in this dissertation, provide a framework for further investigation of metal hyperaccumulation as a function-valued trait.