Date of Award

Summer 8-15-2016

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type



The grass family, Poaceae, is one of the most ecologically and economically important plant groups on Earth. However, the large size (over 11,000 species) and geographic range of the family makes complete resolution of the evolutionary relationships in Poaceae challenging, with some significant groups remaining neglected. Two such understudied clades, subfamilies Arundinoideae and Micrairoideae, possess an incredible amount of morphological and ecological diversity for their sizes, making them a potentially rewarding system in which to study evolution of a wide range of grass features. In this dissertation, I resolved many of the long-standing systematic issues in Arundinoideae and Micrairoideae and used this improved phylogenetic framework to investigate evolutionary issues of broad importance to the grasses.

First, I conducted a molecular phylogentic analysis of the grass family using high-throughput sequencing of chloroplast genomes, focusing sampling on the taxonomically problematic Arundinoideae. I then used this phylogeny along with observations of herbarium specimens to explore patterns in the evolution of lemma traits across the diverse PACMAD clade, a group containing roughly half of all grass species. I found that the Arundinoideae are polyphyletic, with several genera belonging in other subfamilies of PACMAD. Possession of a straight awn near the apex of the lemma is found to be the ancestral state in PACMAD, with the evolution of a geniculate awn and loss of lemma awns each evolvoing multiple times across the clade. Possession of a hairy callus and hairs on the body of the lemma are strongly associated with presence of a lemma awn, supporting the existence of a dichotomous burial syndrome of either smooth and round diaspores or elongate, awned and hairy ones. However, burial syndromes were not associated with changes in diversification rate at this phylogenetic scale.

I explored the origin of the polyploid genomes in Arundinoideae using a phylogenetic analysis of transcriptomic sequence data for four species: Arundo donax, Hakonechloa macra, Molinia caerulea, and Phragmites australis. I found strong support for a shared whole genome duplication in the ancestor of the latter three species, with possible support for another such duplication shared by all four. However, limited sampling in Arundinoideae and closely-related subfamilies makes the placement of this second genome duplication equivocal.

Lastly, I tested whether the unique origin of C4 photosynthesis in tribe Eriachneae of Micrairoideae meets some of the expectations of an adaptive radiation. I used carbon isotopes and plastome phylogenetics of 24 species of Eriachneae that I collected in northern and northwestern Australia to test the phylogenetic limits of the C4 pathway. Eriachneae were found to all be C4, with the rest of Micrairoideae using the C3 pathway. An analysis of bioclimate data in Micrairoideae showed that the shift to C4 is associated with a transition to hotter and drier climates. However, counter to expectations based on other instances of C4 evolution in grasses, Eriachneae did not undergo rapid lineage accumulation or habitat diversification following this transition, suggesting that in this case C4 photosynthesis did not facilitate an adaptive radiation.


English (en)

Chair and Committee

Barbara A. Schaal, Elizabeth A. Kellogg

Committee Members

Garland Allen, Gerrit Davidse, Allan Larson, Peter Raven


Permanent URL: https://doi.org/doi:10.7936/K7N87853