Milkweeds (the genus Asclepias) are beautiful plants that are well known and commonly planted for their ability to attract butterflies. There are 135 species of milkweed in North America that have all originated fairly rapidly. This project will use genomic sequencing technology to investigate how the species are related to each other. DNA sequences will be obtained from complete chloroplast genomes and 10 nuclear genes from two or more samples of each species. The goal is to achieve a species level phylogeny of a large, rapidly radiating genus.
The resolution of systematic relationships among milkweed species will help to improve the classification of the genus. Furthermore, these results will provide a robust foundation for studies of milkweed floral diversity, pollination biology and biogeography. Milkweeds have long served as a model system for the evolution of plant defenses against herbivory, and results from this research will contribute to a better understanding of how plants co-evolve with their pests. The methods developed here achieve a savings in cost, time, and effort beyond any previously attained in comparative genomics, and will be directly applicable to phylogenetic, population genomic and ecological genomic studies of animals, fungi and plants. A post-doctoral associate and three undergraduate students will be trained, and two workshops designed to introduce plant systematists to genomic sequencing approaches will be conducted at the annual Botany meetings in 2010 and 2011.
This project developed new laboratory and computational tools for investigating the evolutionary history of plant species. These tools permit extremely low cost sequencing of hundreds of genes from many related organisms at the same time. The new techniques permit tremendous cost savings to projects that require sequencing large quantities of the same genetic informartion from numerous related species. The methods were employed to investigate the evolutionary history of milkweeds. Milkweeds are an important subject of study because they are the host plants of monarch butterflies, a species of significant conservation concern. The coevolutionary interaction between milkweeds and monarchs is but one example of milkweed interactions with insects that have the ability to counter the toxic chemicals and other defenses that milkweeds deploy against the organisms that eat them. These ecologically important interactions have been the subject of numerous studies that make use of esimates of the evolutionary relationships among milkweed species. However, evolutionary relationships among the 130 species of American milkweeds have remained difficult to discern because milkweeds seem to have evolved very rapidly. A clear picture of the relastionships among milkweed species is essential for discovering general processes underlying the adapation between plants and the animals that eat them. Such knowledge is crucial not only for improving long term conservation efforts of monarch butterflies, but more generally for devloping long term, sustainable methods for defending agricultural crops against pests that quickly evolve resistance to pesticides. In this project, the feasibility of sequencing large components of the genomes of milkweed species was demonstrated by obtaining sequences from 150 samples representing 100 species. Analyses of these data showed that this approach does provide sufficient data to clearly resolve the relationships among milkweed species. They also permitted the identification of an ancient transfer of DNA in milkweed ancestors from the cellular organelle that conducts respiration (mitochondrion) to the organelle that conducts photosynthesis (plastid). Such transfers have only recently and rarely been demonstrated. The project also provided an initial list of candidate genes that control the devlopment of one of the defenses that milkweeds use against insects--plant hairs (trichomes). This project contributed to the training of a postdoctoral researcher at Oregon State University, graduate students at Oklahoma State University and Oregon State University, and many undergraduate students at both universities. The techniques developed through this project were presented at scientific meetings where workshops providing instruction in using the techniques were held. Resources such as datasets and computer software have been made available to the scientific community through the project website. Thus far, five scientific publications have been produced reporting the results of the project. At Okahoma State University, a display garden of milkweed species was installed that highlights the interaction of milkweeds with monarchs and other insects.
