Ancient genome duplications and millions of years of rewiring built CAM in Clusia trees

Around 1800, Alexander von Humboldt put a leaf from a tropical tree in water and found it made none of the oxygen bubbles he had seen before, even in sunlight ^[11]. The reason is a schedule. The pores stay shut while the sun is up, CO2 is taken in at night, bound chemically and held as malic acid, and closed pores in the heat mean less water lost to evaporation ^[8].

All three genomes the Vienna group compared carry the signature of an ancient multiplication, followed by reorganization and reduction ^[4]. The plants did not simply keep extra copies of the same genes; over time the duplicated genomes were transformed into something different ^[14]. Lead author Hannes Kramml said: "In the process, gene copies are lost, deactivated or take on new functions" ^[5]. The copies involved were not a random draw from the genome. "Genes crucial for nocturnal CO2 storage in CAM metabolism are particularly affected," second lead author Johannes Herpell said ^[6].

Weckwerth attributes the differences between species to that reorganization. "The genomes have not simply multiplied; over millions of years, they have been reorganized, reduced and functionally rewired. This enormous plasticity explains the physiological diversity of CAM in the genus Clusia," he said ^[7].

The comparison covers three genomes, from Clusia rosea, Clusia minor and Clusia major, one lineage for each of three different CAM phenotypes ^[2][13]. That design links genome structure to physiology across three points. It cannot separate the rearrangements that produced a phenotype from the ones that came along with it, and with one genome per phenotype there is no replication within a strategy. The physiology itself came from trees monitored through the day in near-natural greenhouse conditions with water availability varied ^[10]. Those were controlled dry-downs under glass, a step short of a field season.

The University of Vienna summary says the work could inspire drought-resistant crops ^[12]. What it describes is a genome comparison plus greenhouse measurement ^[15]. If night-time CO2 uptake in Clusia rests on duplicate copies whose functions diverged over millions of years ^[5][6], then the target for engineering is a set of regulatory relationships across many copies. Clusia is a good place to keep asking: it holds the only known CAM trees, and its species span conventional C3 photosynthesis to very strong CAM ^[9].