Clusia genomes shed light on the evolution and diversity of CAM physiotypes

225 years ago, Alexander von Humboldt observed the first crassulacean acid metabolism (CAM) tree, Clusia rosea (Clusiaceae). Since then, the photosynthetic and ecophysiological plasticity of Clusia
species have captivated the minds of plant scientists worldwide. CAM is a physiological adaptation to low water availability. While stomata are closed during the day, RuBisCO is supplied with CO2 via
decarboxylation of organic acids that have been stored and synthesized during the night by phosphoenolpyruvate carboxylases (PEPC). How the physiological reprogramming necessary for CAM
evolved remains enigmatic. Photosynthetic physiotypes of CAM, including weak CAM, inducible CAM, and CAM-cycling have additionally fueled a debate on the evolutionary constraints of CAM and the prospects of engineering CAM into C3 crops. Here, we de novo sequenced the genomes of three Clusia species to capture genetic snapshots along an evo-ecophysiological continuum from weak over inducible to strong CAM. Through a combination of phased chromosome level assembly and annotation, comparative multiomics, and physiological experiments, we demonstrate that diploidization of polyploids explains the physiotype diversity of CAM. We illustrate that Clusia major, a plant that exhibits a C3-type mode of photosynthesis, retained almost all hallmarks of CAM. Transposon-mediated genic diploidization, however, acted upon homoeologs in CAM-related gene families, preferentially those involved in phosphoenolpyruvate (PEP) recycling via phosphorolytic leaf starch metabolization. In effect this rendered a plant capable of constitutive C3+CAM with open stomata during the day by shifting carbohydrate supply (PEP) to viable soluble sugars. Our findings indicate that polyploidization during genus evolution and subsequent diploidization shaped the emergence of extant C3+CAM physiotypes in Clusia. This study of evolutionary intermediates provides crucial insights into the convergent evolution of physiotype diversity and plasticity of CAM.