Character displacement is an evolutionary process whereby two or more species exhibit increased divergence when occurring together. Because the composition of communities varies geographically, a single species may co-occur with a variety of different species across its geographic range; as a consequence, variation in interactions between species can cause selection promoting within-species divergence among populations. The proposed research examines this process in the flowering plant Pedicularis cranolopha, which exhibits substantial within-species floral variation. Genomic markers will be used to trace population genetic histories to test how floral variation among populations relates to community similarity and whether it affects gene flow among populations. By examining the evolution of reproductive isolation in a community context, this research will improve our understanding of how species interactions affect rates of evolution and speciation.
This study will be conducted in the Hengduan Mountains in China, a biodiversity hotspot that is poorly studied with little known about processes generating or maintaining diversity. By working with Chinese scientists this work will promote future international collaborations. In addition, it will provide educational outreach through training of young scientists both in field and laboratory skills. Results will be disseminated to the general public (including K-12 students) through outreach portals available at the Field Museum of Natural History.
Understanding the process by which two populations of a single species differentiate to become new species -- termed speciation -- is important for explaining variation the diversity of life around us, as well as for determining policy decisions about how we should invest in protecting biodiversity. The simplest and most functional way of delimiting species is to identify groups of organisms that are no longer able to intermate with one another; a definition which highlights the importance that reproduction plays in maintaining species cohesion. Because differences in reproductive traits, such as flowers, can affect the probability that individuals mate with one another, reproductive divergence is often considered to arise early in the process of speciation. This research project focuses on a widespread plant species that shows considerable variation in floral morphology across its geographic range in the eastern edge of Tibetan plateau, and examines the effect of this morphological divergence on the genetic similarity of neighboring populations to test the hypothesis that floral divergence promotes species divergence. The NSF DDIG grant was used to travel to the study area to measure plants, collect tissues for DNA extractions, to perform cross-pollination experiments between divergent populations, and to sequence genomic DNA from the samples. The morphological data show a strong geographic pattern of similarity. A northern morphotype and southern morphotype converge in the eastern part of the range where an intermediate phenotype is observed. This matches with the genetic data which show a hybrid population in the eastern part of the range, but also several isolated demes separated by major geographic barriers. In light of these results two highly divergent populations, in terms of both genotype and phenotype, which occur in close proximity were selected to be used in a cross-pollination experiment. These two populations vary greatly in the length of their flowers (see photo), particularly with respect to the style of the flower, which can be thought of as being analagous to the reproductive tract of a female animal. Pollen tubes, like sperm, have to travel the distance of this tract to reach the ovules. As such, it is expected that the large different in style lengths and pollen size between these populations may lead to reproductive incompatibility. Pollen tube growth inside the crossed flowers are currently being measured in the lab to test this. Statistical analyses are also being performed to detect whether selection has played a role in driving the divergence of floral phenotypes among the studied populations, and whether this floral divegence can explain differences in gene flow among populations after accounting for their relatedness. We plan to publish these findings within the next six months. A major component of this research has been in the development of new software for the analysis of next-generation sequence data. Restriction-site associated DNA is gaining rapid use for the study of population level divergences, and now increasingly also for phylogenetic questions, but little software was previously availble for such analyses. I developed a new software pipeline called PyRAD which is freely available, and which outperforms existing software at recovering homologous sequences across samples. It is now being used by many researchers from a number of institutions to answer a wide variety of evolutionary questions. Broader impacts of this research include a significant amount of graduate and undergraduate training in molecular lab techniques related to the preparation and analysis of restriction-site associated genetic data. In the Pritzker laboratory at the Field Museum in Chicago we have now established an affordable and effective protocol that is being used by researchers throughout the Chicago area, as well as by visiting researchers to the Field Museum, and which provides important and relevant training to the many undergraduate and high school interns in the lab to learn about the newest technologies in molecular genetics.
