Research
Phylogenetic systematics of milkweeds (Asclepias)

My primary areas of research are focused on discovering the phylogenetic relationships among plant species and using these hypotheses as frameworks for studying evolutionary processes. I have worked extensively on the phylogeny of Asclepias, which is a large genus in North America. There are also a few species in South America and a very large number of species in Africa that, although closely related, are often excluded from the genus. I am interested in how these geographically remote groups of species are related to one another and how this biogeographically puzzling distribution of species originated. Analyses of non-coding chloroplast DNA sequences suggest that South American species are a single group derived from North American ancestors and that American and African species largely form distinct clades. These data also strongly refute the monophyly of taxa comprising the most recent infrageneric classificatin of the genus. My collaborators on this research include Steve Lynch (ex Louisiana State University-Shreveport), David Goyder and Mark Chase (Royal Botanic Gardens, Kew), and Roberta Mason-Gamer (University of Illinois-Chicago). Ongoing research seeks to further resolve the phylogeny of the genus using additional plastid and nuclear sequences. This research is supported by the U.S. National Science Foundation.

Insights into the phylogeny of Asclepias have provided a framework for investigating the patterns and modes of evolution of suites of traits related to defense against herbivores. Plants employ diverse chemical, morphological, and developmental strategies to deter or mitigate the effects of herbivory. Some theories predict that tradeoffs will occur among alternative defenses, whereas others predict the evolution of multiple traits into suites that are more effective than the component traits. My colleague Anurag Agrawal (Cornell University) and I have analyzed the pattern of evolution of defense traits in Asclepias and determined that there is no evidence of tradeoffs among defense traits in this genus. There is some evidence that morphologically independent traits, such as latex and trichomes (hairs), evolve in concert ^ref. We have also investigated evolutionary trends in defense traits, finding that traits involved in resistance to herbivory have declined in investment during the diversification of Asclepias, whereas traits involved in tolerance to herbivory have increased ^{ref ref}.

Microevolutionary studies of plant reproduction

My interests also include the ecology and evolution of plant reproduction in populations, especially the evolution of flowers. Using Asclepiadaceae, particularly Asclepias, as a model study system, I use a variety of approaches to study the evolution of flower structure and function.

Pollinators play an important role in the evolution of flowers by being the agents mediating successful sexual reproduction. Thus, they strongly influence which flower morphologies have high fitness. I have studied the interaction between the milkweed, Asclepias tuberosa, and its pollinators ^ref.

Milkweeds are excellent examples of a common phenomenon in plants: very low fruit set. Evolutionary biologists have long wondered why plants often mature very few fruits after producing a large number of flowers. This would appear to be a wasteful trait that should be eliminated by natural selection. It's been theorized that low fruit set only appears wasteful when we forget that many plants (including milkweeds) produce flowers that act as males (by producing pollen) as well as females (by producing ovules). Thus, massive flower production can be explained by sexual selection through male function--plants with many flowers are superior at getting genes into the next generation of seeds by contributing gametes in their pollen. I have helped to develop a theory for how natural selection operates on the number of flowers and have tested this theory on Asclepias tuberosa^ref.

Evolution of floral traits in Apocynaceae/Asclepiadoideae

My study of milkweed evolution extends to the entire "family" Asclepiadaceae (now considered a subfamily of Apocynaceae). This level of milkweed phylogeny is of great interest in the study of flower evolution because of structural complexity and a specialized pollination mechanism ^ref. Stamens and carpels are so intimately fused and modified that they appear unrecognizable without close scrutiny. Also, pollen grains are contained in hard sacs that are attached by clips to the legs and mouthparts of insects--a unique pollination mechanism that is similar only to that found in orchids. Milkweeds also have unusual structures located between the petals and the stamens termed coronas, which have various functions in the pollination process. I have begun phylogenetic study of this clade using chloroplast DNA sequences and will use a phylogeny of the family as a basis for studying how milkweed flowers have diversified, particularly the coronas. My collaborators on this research include Laure Civeyrel (Université Paul Sabatier, Toulouse). The study will also incorporate the use of developmental data for the purposes of homology assessment and investigation of mechanisms of morphological diversification.

Mechanisms of gene flow between species

The increase use of transgenic crops has heightened concerns over the impacts of unintended transfer of transgenes into non-target crops or wild species. For genes to move between species, hybrid offspring must successfully mate (backcross) with an individual from one of the parental species. However, little is known about how the traits of hybrid offspring influence the probability of successful backcrossing and the introgression of traits from one species into another. With collaborators Steve Broyles (State University of New York-Cortland), Rob Raguso (University of South Carolina), and Robin O'Quinn (Eastern Washington University), I am investigating how traits of hybrids determine the probability of introgression using milkweeds as a model system.

Hybridization among milkweed species is not common, but has been documented between the common milkweed of the eastern U.S. (Asclepias syriaca) and the relatively uncommon poke milkweed (A. exaltata). These species are ecologically segregated over a broadly sympatric range—A. syriaca in prairies and open, disturbed habitats, A. exaltata in forest understories. At Shenandoah National Park, Virginia, these species have large populations and hybridize extensively. Steve Broyles has shown that allozymes introgress much more extensively than would be predicted by the few first generation hybrids that can be found at the site, and that introgression is asymmetric—alleles of A. exaltata are found more often in A. syriaca than vice versa. With undergraduate researchers sponsored by a NSF REU site, I have conducted an intensive study of pollinators and floral scents of both parental species and hybrids. These studies have documented pronounced differences in scents of A. syriaca and A. exaltata and shown that hybrids are chemically similar to A. syriaca, but show high levels of expression of some compounds found only at trace levels in A. syriaca. This is the first documentation of transgressive expression of floral scent in hybrids. Similar scents between hybrids and A. syriaca, and relatively low levels of pollination in A. exaltata help to explain why introgression is asymmetric in the direction of A. syriaca. Using microsatellite markers ^ref, Robin O'Quinn and I are gaining a more detailed picture of gene flow between these species.

Systematics, Taxonomy, and Floristics

As fun as it is to sit down in front of the computer and compile, analyze, and interpret data, sometimes I need to undertake the onerous task of conducting field work. I am especially fascinated by plant biogeography. I have collected extensively in the southwestern U.S. and northwestern México. I have focused on the "sky island" mountains of the U.S.-México border region and the Río Mayo watershed on the west slope of the Sierra Madre Occidental in southern Sonora and Chihuahua. I plan to continue floristic work in this area, with the future goal of figuring out something about the origin of diversity and endemism, especially in such diverse communities as the pine-oak forests and evergreen oak woodlands these mountains. Such intensive local studies have allowed me to compile revisions of my favorite plants, the Asclepiadaceae, of this region.

Phylogenetic studies of genera like Asclepias and floristic studies inevitably lead to the discovery of new species. This is a new species from Querétaro, México that was discovered by Victor Steinmann ^ref.