Genome reshuffling drove CAM evolution in tropical Clusia

New genomic research on Clusia trees shows their water-saving photosynthesis evolved through repeated genome reorganization over millions of years. The study analyzed three species to uncover how Crassulacean Acid Metabolism (CAM) arose in the only known trees that use this strategy.

CAM photosynthesis allows plants to absorb carbon dioxide at night and keep their stomata closed during the day, conserving substantial amounts of water. The research group compared the genomes of Clusia rosea, Clusia minor, and Clusia major. All three are ancient polyploids, meaning they share an ancestral genome duplication event. The genomes then underwent a long process called diploidization. Wolfram Weckwerth, who led the study, stated: "The genomes have not simply multiplied; over millions of years, they have been reorganized, reduced and functionally rewired." Gene copies were lost, deactivated, or took on new functions, especially those involved in nighttime carbon dioxide storage. This flexible, repeated reshuffling produced fine-tuned versions of CAM across the genus, not a single evolutionary jump.

Species-specific CAM use reflects gene function shifts

Each Clusia species uses CAM to a different degree, matching shifts in gene expression. Clusia rosea relies heavily on strong CAM, storing large amounts of CO2 as malic acid at night. Clusia major employs a hybrid strategy, mixing conventional daytime C3 photosynthesis with CAM. Clusia minor mostly uses C3 photosynthesis and only switches to CAM under stressful conditions like drought. The research paired genomic analysis with physiological measurements in realistic environmental conditions. These behavioral differences aligned directly with patterns of gene activity, protein production, and metabolism across the three species.

Historical observation links to modern CAM understanding

The unique photosynthesis in these trees was hinted at centuries ago. Around 1800, naturalist Alexander von Humboldt observed a tropical tree leaf that did not produce oxygen bubbles in sunlight when submerged in water. This foreshadowed the modern understanding of CAM, where the plant absorbs CO2 at night and stores it as acid until sunrise. The mechanics of this water-saving strategy have been known for some time, but its evolutionary origins in trees remained unclear until now.

Study details and publication

The research was led by Wolfram Weckwerth at the University of Vienna and co-led by Hannes Kramml and Johannes Herpell. It combined detailed genomic analysis with physiological measurements. The genus Clusia is uniquely valuable for this research because its species cover the full photosynthetic spectrum from standard C3 to intense CAM. The genomic insights into how these trees evolved their drought resilience may help inform the development of crops better adapted to arid conditions.