Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Biology

First Advisor

Scott Poethig

Abstract

As plants age, the morphology and physiology of the shoot changes—a

phenomenon known as vegetative phase change. Often these changes are

important in the life history of the plant and contribute to shifting strategies in

growth, defense, and competition. The temporally regulated microRNAs miR156

and miR157 are known to control many aspects of vegetative phase change

making them ideal for examining how this pathway contributes to the evolution of

life history strategies. In this work, I examine this question using two genera of

woody legumes.

First, I use the New World Ant-Acacias (genus Vachellia) to examine if

age-dependent changes in plant defenses evolve as a consequence of neutral

constraints on development. I find that the miR156/157 pathway likely controls

the age-dependent appearance of the swollen-thorn syndrome—a suite of traits

important for interaction with mutualistic ants. This finding is in disagreement with

the idea that the temporal regulation of the swollen-thorn syndrome has evolved

as a consequence of selectively neutral processes such as ontogenetic drift and

supports the idea that the timing of syndrome emergence is somehow adaptive. I

discuss the possible agents of this selection and their relation to the miR156/157

pathway.

I also investigate the frequency of life-history heterochrony and its

mechanism using plants of the genus Acacia, which undergo a conspicuous shift

in leaf morphology during their life cycle. This transition from a compound leaf to

a simple-horizontally expanded leaf, known as a phyllode has previously been

linked to the miR156/157 pathway. Using this morphological marker of vegetative

iiiphase change I examine the timing of this transition in 147 species within a

phylogenetic context. I find that the evolution of a prolonged or persistent juvenile

phase has evolved independently at least 7 times in Acacia. For two of these

events the rate and magnitude of miR156 decline is reduced supporting the idea

that these species are neotenous. I use genome sequencing to characterize the

MIR156/MIR157 gene families, and identify a gene with a mutation in a putative

cis-regulatory element that may contribute to phenotypic differences between

species.

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