The factors that maintain defensive chemical variation within and between plant species has intrigued ecologists for decades. However, there have been few experimental studies examining the selective mechanisms responsible for the maintenance of polymorphisms in nature. Here, we report on a chemical polymorphism in a widespread annual weed, Xanthium strumarium, in which plants produce sesquiterpene lactones with either a cis or trans fused lactone ring. Our previous work has shown that this chemical trait is strongly correlated with damage from chewing insect herbivores, and that damage is negatively correlated with fitness. With a series of common garden experiments conducted throughout Eastern Texas we asked the question: What factors maintain the chemical polymorphism in Xanthium strumarium? Specifically, is there evidence that particular chemotypes have superior performance in particular places or times? Over the summer of 2010 we established six common gardens spanning a range of habitats where both cis and trans forms of Xanthium naturally occur (prairie, wetland, forest clearing, river bank). Plant growth and survival were surveyed throughout the growing season, and herbivore damage was estimated visually twice. Seeds from plants were collected after senescence and counted to estimate reproductive fitness.
Results/Conclusions
Insect herbivore damage rates varied between sites with mean damage per plant ranging from 7.4% to 69.9%. There was a significant stereochemistry by site interaction with stereochemistry influencing damage at some sites but not others (p=0.0024). At sites where stereochemistry was significant, cis plants received more damage than trans plants (p<.02). Plant survival to reproduction differed dramatically across sites (19% to 97%), with differential mortality between cis and trans plants found at three of six sites. Interestingly, sites near wild cis populations did not experience differential mortality, while at sites near wild trans populations more cis plants died than trans plants. Lifetime reproductive fitness differed between sites and stereoforms, with trans plants outperforming cis plants at three of six sites (site by stereochemistry interaction p=0.0102). In conclusion, cis plants experienced higher herbivore damage, lower survival to reproduction, and lower mean fitness than trans plants at numerous sites. However, these same variables did not differ at other sites, suggesting that the chemical polymorphism in Xanthium strumarium may be maintained by variable selection.