Adaptation to Environmental Stressors Conferred by Gene Copy Number Variation in Herbicide Resistant Amaranthus palmeri

Many rampant agricultural weeds have been found to contain gene copy number variation, a genetic adaptation that can facilitate the evolution of stress resistance. Amaranthus palmeri is a highly competitive and problematic agricultural weed in the United States which continues to naturalize, spreading northward to Canada. A.palmeri has gained attention due to its rapid evolution of herbicide resistance— a mechanism conferred by gene copy number variation of EPSPS. Researchers hypothesize that this gene is contained within an amplicon alongside other stress tolerance related genes. This raises the question of whether increased EPSPS gene copy number is associated with higher tolerance to environmental stressors, beyond herbicide stress. To prepare for addressing this hypothesis, I conducted a pilot project testing the ideal conditions for germination of A.palmeri. I planted 200 seeds divided between greenhouse and growth chamber environments. Many seeds benefit from a period of cold as a signal to break dormancy, thus, each category was further divided into two treatments: control and cold stratified (seeds were incubated in a –23 cold room for a week). I was interested in whether the increased consistency of temperature, light intensity, and photoperiod provided by a growth chamber, relative to a greenhouse, would increase germination rates. Results indicate that greenhouse grown seedlings had a 71% germination rate and within each treatment, 86% of cold stratified plants germinated while only 56% of the control group germinated. Whereas in the growth chamber, surprisingly only 26% of plants germinated with a breakdown of 22% cold stratified and 30% control. Thus, to germinate enough plants for a large-scale experiment, I chose to cold stratify seeds and grow them in the greenhouse for my undergraduate thesis. In this common garden experiment, I am currently growing 500 plants which will be divided into four stress treatment categories: control, salinity, drought, and salinity x drought. Using phenotypic measurements and digital droplet PCR (ddPCR) to quantify EPSPS gene copy number, I will test for a possible association between plants with a higher tolerance to these simulated environmental stressors and the presence of increased EPSPS gene copy number.