Reconciling continuous and discrete models of C4 and CAM evolution

Area of Science:

• Plant biology
• Evolutionary biology
• Photosynthesis research

Background:

• Current CAM biology literature debates whether CAM evolution is a smooth continuum or involves discrete steps.
• This debate has implications for the evolvability of CAM, with a continuum potentially increasing it and discrete steps making the transition from C3 photosynthesis more difficult.

Purpose of the Study:

• To reconcile the viewpoints of a continuum versus discrete steps in CAM evolution.
• To propose that the phenotypic space of CAM evolution is both a continuum and contains discrete phenotypes.
• To argue that the evolutionary mechanics of CAM are similar to C4 photosynthesis and other complex traits.

Main Methods:

• Conceptual analysis and synthesis of existing literature on CAM and C4 photosynthesis evolution.
• Identification of phenotypic gaps in current CAM evolutionary models.
• Exploration of potential connections to C3 plant physiology under stress.

Main Results:

• The dichotomy between continuum and discrete steps in CAM evolution is considered a false one.
• The phenotypic space connecting C3 and strong CAM/C4 species is characterized by both continuous quantitative traits and discrete phenotypes.
• The evolutionary process of CAM is comparable to that of C4 photosynthesis and other complex traits.

Conclusions:

• Embracing discrete phenotypic phases of CAM evolution is crucial for progress.
• Key gaps in understanding CAM evolution include the establishment of the rudimentary CAM cycle and the recruitment of the accessory CAM cycle.
• Investigating C3 plant behavior under physiological stress may reveal connections to CAM evolution.