Understanding the roles of polyploidy and the environment on nordihydroguaiaretic acid variation in Larrea tridentata

Abstract

Nordihydroguaiaretic acid (NDGA) is the principal compound in the resinous leaf coating of Larrea tridentata (Creosote bush), the dominant shrub of North American deserts. L. tridentata exists as three polyploid races: diploid (2X = 26), tetraploid (4X = 52), and hexaploid (6X = 78). The distributions of these ploidy levels are strongly associated with the three major deserts of the region where diploids primarily reside in the cooler, wetter Chihuahuan desert, tetraploids in the Sonoran desert, and hexaploids in the hot, dry Mojave desert. NDGA is a secondary metabolite of creosote bush that functions to protect plants from biotic and abiotic stressors such as extreme drought, harmful UV radiation, and herbivory. Here, I investigated the role of polyploidy and environmental variables on the production of NDGA by quantifying concentrations from field and greenhouse-grown polyploids. Citizen scientists were utilized to facilitate simultaneous sampling across the entire distributional range of this species, for one full year. Under natural conditions, shrubs produced significantly higher NDGA concentrations than when removed from the harsh desert environment. In field and greenhouse treatments, hexaploids exhibited higher NDGA concentrations than diploids or tetraploids.Within the diploid cytotype, I documented environmental influences on NDGA concentration based on comparisons between a field site experiencing Severe drought, a watered field site, and greenhouse-grown diploids. Principal components analysis revealed that NDGA response to environmental variables successfully predicts the current ploidy distribution of this species. These observations highlight the complexity of plant-environment-genotype interactions and suggest that evolution in production of secondary metabolites may be driven by long-term changes in environmental conditions, and potentially influence species distribution regimes.