Regional differences in leaf evolution facilitate photosynthesis following severe drought

AbstractCharacterizing physiological and anatomical changes that underlie rapid evolution following climatic perturbation can broaden our understanding of how climate change is affecting biodiversity. It can also provide evidence of cryptic adaptation despite stasis at higher levels of biological organization. Here we compared evolutionary changes in populations of Mimulus cardinalis from historically different climates in the north and south of the species’ range following an exceptional drought. We grew seeds produced from pre-drought ancestral plants alongside peak-drought descendants in a common greenhouse and exposed them to wet and dry conditions. Prior to the drought, northern ancestral populations expressed traits contributing to drought escape, while southern ancestral populations expressed drought avoidance. Following the drought, both regions evolved to reduce water loss and maintain photosynthesis in dry treatments (drought avoidance), but via different anatomical alterations in stomata, trichomes, and palisade mesophyll. Additionally, southern populations lost the ability to take advantage of wet conditions. These results reveal rapid evolution towards drought avoidance at an anatomical level following an exceptional drought, but suggest that differences in the mechanisms between regions incur different trade-offs. This sheds light on the importance of characterizing underlying mechanisms for downstream life-history and macromorphological traits.
MethodsClimate data was collected from ClimateWNA. Seeds of Mimulus cardinalis (scarlet monkeyflower) were originally field collected prior to a severe drought (2010 and 2011) and during the peak drought (2014 and 2015) that occurred throughout the range . Following a refresher generation, the seeds were grown in a common garden experiment and exposed to either wet or dry treatments. Plants were monitored to maintain a similar drought across all plants. Leaves were sampled with a licor 6800 photosynthetic instrument for physiological traits. The leaves were collected and sectioned to examin leaf cell layers and epidermal peels were made using nail polish to examine leaf surface architecture. Data was directly input into a spreadsheet for anatomical traits. Data from Licor instrument was imported into Excel and cleaned for just the variables of interest using R software.