Phenotypic and genetic variation within a species set the foundation for evolution and adaptation. This project will study local adaptation in natural populations of a wild chili, Capsicum chacoense, which has a polymorphism in fruit pungency (heat) that varies geographically in association with moisture availability. Field experiments will be used to test the environmental and biotic causes of variation in pungency and to evaluate the costs/benefits of pungency, the extent of local adaptation in pungency, and the primary ecological drivers maintaining this polymorphism.
This project examines the evolutionary processes that have shaped a stable polymorphism in wild chili peppers and explores the role of drought stress, an important agronomic factor, in limiting adaptation. Chilies are a major crop that plays important roles in human diet, and the chemicals that cause its pungency, the capsaicinoids, have medical and security applications. This project includes public outreach and education, as well as training of undergraduates and international collaborations.
Natural Selection in Wild Chili Peppers A central challenge in evolutionary biology is to describe the historical processes leading to current patterns of phenotypic differentiation. This is particularly true for secondary compounds where selection is often presumed to maintain variation. Few studies have documented the role of reduced gene flow between populations in shaping variation in chemical production. Capsaicinoid production (compounds responsible for the "heat" in chilies) was presumed to be under selection. Our work demonstrates the role of natural selection, specifically diversifying selection, in maintaining variation in the production of capsaicinoids among natural populations of the wild chili pepper Capsicum chacoense. We evaluated the amount of genetic exchange between 12 populations of chilies that showed marked differentiation for capsaicinoid content in ripe fruit (fig. 1, pie charts). We also determined the relative neutral genetic structure (proportion of shared ancestry) among the populations (fig. 1, colors in the bargrpahs). The population structure analysis suggests that there is some differentiation in neutral genes, however, a test of the neutral genetic variation among the populations fails to explain the geographic pattern in phenotypic differentiation. Thus, we have demonstrated that natural selection maintains spatially explicit variation in capsaicinoid production, which is tightly correlated with average annual rainfall (fig. 1 heat map) and the presence of a fungal seed pathogen. The broad appeal and economic importance of chili peppers generates much public interest in chilies which we have leveraged to convey our work and evolutionary perspective to broad audiences through diverse media. We are firmly committed to educating the public about the fundamental role of science and as part of this project have implemented a program aimed at teaching very early learners the principles of evolution. We also embrace the importance of broadening participation of groups underrepresented in science, and we have included two Bolivian nationals and one First Nations individual in the training associated with this project. Finally, our work is explicitly international which fosters the independence of a training scientist at the global scale. Equally important, we are building professional capacity in Bolivia (the poorest country in South America) by mentoring Bolivians.
