Testing the Proximate Mechanisms for the Process of Character Displacement on the Evolution of Root Traits.

Abstract

Character displacement, is a process wherein closely resembling species diverge in their resource-linked traits as a response to intense competition. Research evaluating whether character displacement can influence the evolution of a plant’s belowground root system remains unreported in the literature, despite the importance of root systems in capturing resources from the soil environment and mediating belowground competition. Thus, my dissertation addresses the overarching question, Can character displacement influence the evolution of root traits between two closely related species?

In the first two data chapters of this dissertation I tested for the potential that root traits can evolve via character displacement using Ipomoea purpurea and I. hederacea. In my first data chapter (Chapter 2) I performed a greenhouse common garden experiment to test if root traits were genetically variable and a competition field experiment to test if belowground competition can impose selection on root traits. In my second data chapter (Chapter 3) I expanded on my findings from Chapter 2 and performed a second competition field experiment to test for the main prediction of character displacement.

In addition to the root system, the root-associated microbiome can play a major role in a plant’s realized niche and affect how plants access and compete for belowground resources. Moreover, the root-associated microbiome can potentially influence root phenotypes and vice versa. Consequently, plant-microbe interactions can potentially feedback into plant ecology and evolution and alter the outcome of processes such as belowground plant-plant competition. To this end in my third data chapter (Chapter 4). I asked the broad question, Does the rhizosphere microbial community composition and structure vary with root phenotypes and what are their relative effects on plant fitness according to competitive environment? I subsampled and analyzed the bacterial microbiome from rhizosphere soil taken from individuals of I. purpurea and I. hederacea grown in the presence and absence of belowground competition. I tested if root phenotypes and measures of the rhizosphere microbial community were linked with each other and determined the relative impact of the rhizosphere microbial community on plant fitness in context of belowground competition

In brief, my thesis demonstrates that belowgound competition and root traits represent a viable and overlooked agent and target of selection. Most importantly, it demonstrates that belowground competition may potentially result in character convergence, not displacement of root traits. It provides initial evidence for the possibility that the rhizosphere microbiome and root traits can influence each other and effect how plants compete belowground. My work demonstrates the potential for belowground competition to shape plant evolution and diversity and suggests that plant-microbe interactions itself may play an important role in how plants respond and adapt to belowground competition. Collectively, this work represents a novel first step in linking plant ecology and evolution to the ‘hidden’ half.