I am interested in understanding the patterns and processes of plant diversification. My research, grounded in traditional systematics, uses techniques from phylogenetics, molecular genetics, and developmental biology to infer where, when, and how genes have evolved within grasses. Only by integrating data on model and non-model systems can we start to unravel the complexity of genetic mechanisms driving morphological change.

Post-doctoral research

The grass family contains over 10,000 species, including the cereal crops rice, barley, oats, wheat, corn, millet, and sorghum. Although a well-supported phylogeny for grasses exists, little is known about the developmental genetic changes that affected the morphological radiation of the family. My research uses the grass phylogeny as a baseline to interpret the evolution of developmental genes that affect floral morphology in rice and maize. These studies identify whether the same genes contributed to the diversification of the grass family.

A study of all 10,000 grass species is impractical. As a first sample before expanding our studies to other species, we are using representatives of the BEP (rice, Oryza sativa; barley, Hordeum vulgare; oats, Avena sativa) and PACCAD clades (common millet, Panicum miliaceum; foxtail millet, Setaria italica, Setaria viridis; pearl millet, Pennisetum glaucum; sorghum, Sorghum bicolor; maize, Zea mays).

SEPALLATA genes

In Arabidopsis and Petunia, SEPALLATA (SEP) genes have been shown to interact with and mediate the expression of floral organ identity genes that specify the sepal, petal, stamen and pistil whorls of flowers. In Arabidopsis, the ectopic expression of SEP and floral organ identity genes are sufficient to generate floral-like structures on the leaf. In grasses, SEP genes are very diverse: rice has at least five genes and maize has at least eight genes. This genetic diversity is matched with diverse patterns of expression, and presumably function. I am examining to what extent SEP genes are correlated with floral evolution. My expression studies suggest the genes have complex patterns of expression in grasses and may have several roles. For example, the SEP gene LEAFY HULL STERILE1 (LHS1) is restricted to the upper floret of the spikelet in rice and maize, but is expressed in the palea, lemma, and pistil of rice florets and all floral organs in maize. LHS1 has been proposed to act as a selector gene in grasses, specifying the uppermost floret in the spikelet. My expression data support this hypothesis in all species with spikelets that develop from top to bottom (rice, pearl millet, foxtail millet, sorghum and maize, fig. 1), but not in species with spikelets that develop from bottom to top (sea oats and oats, fig. 2). Studies such as this allow us to test hypotheses from model systems and formulate new hypotheses of gene function.

The expression of the gene TASSELSEED2 is involved in the production of unisexual florets in maize. Unisexuality has evolved at least six times in grasses; how many of the transitions are mediated by TS2 is not known. Molecular evolution studies suggest that TS2 is highly conserved across the grasses, with no evidence of positive selection based on several maximum likelihood tests. However, my RT-PCR expression studies indicate that TS2 is expressed in fertile and non-fertile tissues alike, suggesting the gene may be involved generally in cell-death rather than only in the evolution of unisexuality.

PhD research

My doctoral research concerned the systematics and evolution of Gaertnera (Rubiaceae or coffee family). Counter to expectations based on the wide-spread geographical distribution and large morphological variation of Gaertnera species, multiple molecular datasets suggested the genus radiated rapidly and recently. This study is the first example of a rapid radiation in a widespread, tropical taxon and raises interesting questions about the tempo and mode of evolution.