Plants produce a broad array of defenses that reduce damage from insect, vertebrate, and microbe pests. These defenses are diverse, ranging from spines and thorns to toxic chemicals like nicotine, and type and amount of defense varies substantially within and among populations. This study will advance understanding of the basis for maintenance of diverse defenses by documenting the specific factors that maintain variation in defensive chemistry of the common cocklebur (Xanthium strumarium) in replicated experimental gardens.
Variation within and among populations in ecologically significant traits supplies the basis for biodiversity. Better understanding of the basis of plant defense has significant applications to agriculture. This project also includes interdisciplinary field and laboratory training for undergraduates and an active public outreach program.
A common goal of evolutionary ecologists is to understand and explain how ecological processes both generate and maintain the forms of organisms (i.e. plants, animals, microbes, etc.) observed in nature on an evolutionary timescale. Plant defensive chemical traits are particularly interesting because 1) they exhibit a high amount of variation at the level of populations, species, genera, and families, and 2) these traits are often assumed to be subject to strong selective pressures. Polymorphisms in plant defensive chemical traits (where different forms are found within the same population or species) are intriguing, because the commonly observed form of natural selection, directional selection, is insufficient to explain patterns observed in nature. In this research, we examined a polymorphic defensive chemical trait in the common cocklebur (Xanthium strumarium). Plants in this species produce a suite of defensive chemicals called sesquiterpene lactones, which occur in either a cis-fused or trans-fused form (referring to relative bond geometry, see figure). Individuals and populations often produce only one type of these chemicals; all sesquiterpene lactones produced have either a cis-fused or a trans-fused lactone ring. Our previous research indicated that plants with trans-fused sesquiterpene lactones experienced more damage from leaf chewing insects such as grasshoppers, and this damage reduces plant. Theory would suggest that producing the cis-fused form would be outcompeted by plants producing the trans-fused form, but both forms are observed in nature. We hypothesized that the impact that insect herbivores have on these plants varies spatially, such that each form (cis-fused or trans-fused) could outperform the other in different places – a phenomena termed variable selection. Using a series of common gardens spread across central and Eastern Texas (where both forms naturally occur), we found that both the intensity and specificity of herbivore damage varied between sites. Additionally, in some places, plants producing trans-fused sesquiterpene lactones outperformed plants producing cis-fused sesquiterpene lactones (had higher mean seed set), yet at other sites they had equal performance, adhering to our hypothesis of variable selection. The patterns of fitness (performance) were correlated with patterns of chewing insect damage, indicating that the intensity and specificity of the herbivorous insects in different locations could provide sufficient variable selection to maintain this polymorphism in nature. This work is one of the first to document the selective basis maintianing a defensive chemical polymorphism in nature, and will help in the goal of explaining the vast diversity of variation observed in plant defensive chemistry.
