DISSERTATION RESEARCH: Evolution of hyperaccumulation in wild Helianthus: a phylogenetically explicit assessment of evolutionary history and adaptive hypotheses

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Over 500 species of flowering plants are metal hyperaccumulators, meaning they possess the ability to take up extremely high concentrations of normally toxic metals into their shoot tissues. This natural ability may be harnessed for useful technological applications, including phytoremediation (the use of plants for environmental cleanup of polluted areas) as well as biofortification (the enrichment of crops with key nutrients like iron). The broad objective of the proposed research is to understand how the development of metal hyperaccumulation ability depends on metal tolerance, and how hyperaccumulation ability may benefit wild plants. Proposed benefits of hyperaccumulation to plants have included reduced herbivory and disease, improved drought tolerance, and reduced competition. This research will test these proposed benefits using the genus Helianthus (the sunflowers), a group of diverse species found in habitats across North America. Sunflowers are known to many for the agriculturally and horticulturally important crop species Helianthus annuus, which is itself a known hyperaccumulator of at least five different heavy metals. In addition to improving our understanding of plant ecology, this research will provide direct insights into hyperaccumulator physiology for the improved breeding and use of sunflower in technological applications.The evolutionary interactions of metal tolerance and metal hyperaccumulation will be assessed using 35 wild Helianthus species, through a manipulative greenhouse study generating a soil nickel gradient consisting of ten separate treatment levels, in which the evolution of tolerance and hyperaccumulation are parsed apart as separate and distinct traits. The putative costs and benefits of hyperaccumulation will be examined in greenhouse experiments which will quantify fitness-related costs while simultaneously evaluating five leading adaptive hypotheses. A phylogenetically explicit approach to the study of metal hyperaccumulation evolution can dramatically improve the understanding of the ecological and evolutionary context of metal hyperaccumulation. More broadly, this research will promote further development of the newly emerging field of evolutionary toxicology and will implement novel phylogenetic comparative methods for assessing environmentally responsive traits, allowing for an improved range of tools to address existing and emerging environmental concerns.