I am broadly interested in the ecology and genetics of adaptation and speciation, and I explore these topics using a variety of methods and systems. In particular, I use field and greenhouse experiments, genetic mapping, population genomics, and functional genetics. I use these tools to work mainly with flowering plants in the genera Helianthus, Penstemon, and Ipomoea.
My work falls into three categories:
(1) The evolution of reproductive barriers
I am currently working to understand the evolution of a strong crossing barrier between a mixed mater, Ipomoea cordatotriloba, and a selfer, I. lacunosa. This barrier may have arisen because of direct selection (i.e. reinforcement), as a byproduct of selection on other traits (e.g. the selfing syndrome), or through genetic drift. To distinguish between these possibilities, I am surveying the strength of the reproductive barrier in allopatric and sympatric populations of the species. I am also working with Joanna Rifkin to QTL map genes involved in the incompatibility. With QTL in hand, we will look for evidence of pleiotropy and signatures of natural selection.
During my PhD, I measured reproductive barriers between dune and non-dune ecotypes of the sunflower Helianthus petiolaris. I found that many barriers, including selection against immigrants, different flowering times, different pollinators, post-pollination assortative mating, and selection against hybrids, separate the ecotypes despite less than 10,000 years of divergence. This demonstrates that several types of reproductive barriers (e.g., prezygotic, postzygotic, intrinsic and extrinsic) can arise early in the speciation process.
(2) The genetic basis of parallel evolution
The bulk of my thesis work focused on dune adaptation in the sunflowers Helianthus petiolaris and H. neglectus. In both cases, dune plants have larger seeds than their non-dune relatives and this difference is likely maintained by divergent natural selection. I used QTL mapping to determine that some of the same genomic regions underlie these differences in seed size (2 of 3 regions). To complement this analysis, I am now using whole genome sequence data to further explore whether the dune genomes are diverging in parallel (preliminary analyses suggests that they are).
I am involved in a collaborative project about parallel evolution of pollination syndromes with Carolyn Wessinger, Lena Hileman and Mark Rausher. There are many independent transitions from bee to hummingbird pollination in the genus Penstemon, and these pollinator shifts are coupled with extensive parallel phenotypic change. For example, hummingbird flowers tend to have longer, narrower and redder corollas with larger volumes of more dilute nectar than related bee flowers. Together, we are working to determine how often phenotypic changes are explained by the same genetic changes, and thus the extent of genetic constraint. My part of this project focuses on the sister species Penstemon davidsonii and P. newberryi, which are found along elevational gradients in the Sierra Nevada Mountains.
(3) Mate choice and sexual selection in flowering plants
I’m collaborating with Daniel Ortiz-Barrientos et al. to explore relationships between the strength of sexual selection and diversification in flowering plants. This project takes a broad view and looks for correlations between ovule number (a proxy for the strength of sexual selection) and multiple stages of diversification. My primary contribution to this project was identifying stronger signatures of positive selection on reproductive proteins in lineages with few ovules. This pattern, along with the other correlations, suggest that sexual selection could be an important force in plant evolution.
As part of my postdoc, I am building transcriptome libraries from several tissues (e.g. pollen, pistils, leaves, roots) in Ipomoea and Penstemon species. I will use these data to determine whether genes expressed in sexual tissues tend to evolve more quickly than genes expressed in somatic tissues. This pattern has been observed in several animal systems where sexual selection is thought to be an important part of evolution but is rarely observed in plants.
Finally, I am involved in a wide variety of other projects. Some of my past and present research excursions are listed below:
- Understanding the genetic history of a highly invasive thistle with Kathryn Turner
- Assessing gene flow between an economically important Ethiopian oil seed crop and its wild relative
- Determining the potential pollination ability of male bumble bees
- Exploring how leaf shape polymorphisms are maintained in morning glory