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Author ORCID Identifier


Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program

Plant Biology

Year Degree Awarded


Month Degree Awarded


First Advisor

Madelaine Bartlett

Second Advisor

Ana Caicedo

Third Advisor

Sam Hazen

Fourth Advisor

Jill Preston

Subject Categories

Plant Biology


This research focuses on a specific example of replicated evolution: the grass awn. Awns are typically extensions of the lemma, but may also appear on glumes or paleas. The lemma is a leaf-like organ on the exterior of the grass flower, the glumes are a pair of bracts subtending the basic unit of grass inflorescences, the spikelet, and the palea is the floral organ opposite the lemma. Awns are often described as "hair-" or "bristle-" like, but appear in many different shapes. Many awns are “twisted & geniculate", in which the awn has two sections, a lower twisted column, and an upper straight section (the "subule"). These sections are separated by a bend, or "knee", which gives rise to the term "geniculate". Awns may also be attached, or inserted, to the lemma at different points: either at the tip, apically, or at a point on the back of the lemma, abaxially. Awns are critical for several functions in grasses, and likely contribute to grass dispersal, seedling establishment, and reproductive success. Some awns, like those in barley and wheat, photosynthesize and increase grain weight. Awns are part of a suite of characters that have likely played a key role in the spread of grasses to thrive in almost all terrestrial biomes. I performed a series of experiments aimed at understanding the replicated evolution of awns. I hypothesized that the awned lemma shares homology with the grass leaf, and that conserved leaf genes have been recruited for awn development. To determine the evolutionary history of awns in the Pooideae (the largest grass sub-family), and to select focal species for further study, I performed several ancestral state reconstructions for awn traits in the Pooideae. I discovered that awns have been independently derived at least 30 times in the Pooideae. To test the hypothesis that lemmas and leaves are homologous, I examined the anatomy and micromorphology of several awns from Pooideae species, and found that leaf traits are shared between awns and leaves. Similarly, I examined awn development in three species with independently derived awns and found that they all shared underlying ontogeny with grass leaves. To determine whether the function of straight awns in photosynthesis and contributing to grain weight is conserved in Brachypodium distachyon, I performed an awn removal experiment, and found that awns in B. distachyon do not have a significant role in contributing to grain weight. Finally, I characterized and cloned an awnless mutant in B. distachyon, and showed that a gene with conserved functions in leaf development is responsible for the awnless trait. Further, I find that the ortholog of this gene is also necessary for awn development in Alopecurus myosuroides, a species with independently derived awns. These experiments all support the main hypothesis that lemmas are homologous to leaves, and share the same genetic pathways for development. I find that these instances of replicated evolution are likely supported by developmental constraints, and that heterochrony has a key role in shaping awn development in different species.


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Creative Commons Attribution-Noncommercial 4.0 License
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