The evolution of C photosynthesis.

C photosynthesis is a series of anatomical and biochemical modifications that concentrate CO around the carboxylating enzyme Rubisco, thereby increasing photosynthetic efficiency in conditions promoting high rates of photorespiration. The C pathway independently evolved over 45 times in 19 families of angiosperms, and thus represents one of the most convergent of evolutionary phenomena. Most origins of C photosynthesis occurred in the dicots, with at least 30 lineages. C photosynthesis first arose in grasses, probably during the Oligocene epoch (24-35 million yr ago). The earliest C dicots are likely members of the Chenopodiaceae dating back 15-21 million yr; however, most C dicot lineages are estimated to have appeared relatively recently, perhaps less than 5 million yr ago. C photosynthesis in the dicots originated in arid regions of low latitude, implicating combined effects of heat, drought and/or salinity as important conditions promoting C evolution. Low atmospheric CO is a significant contributing factor, because it is required for high rates of photorespiration. Consistently, the appearance of C plants in the evolutionary record coincides with periods of increasing global aridification and declining atmospheric CO . Gene duplication followed by neo- and nonfunctionalization are the leading mechanisms for creating C genomes, with selection for carbon conservation traits under conditions promoting high photorespiration being the ultimate factor behind the origin of C photosynthesis. Contents Summary 341 I. Introduction 342 II. What is C photosynthesis? 343 III. Why did C photosynthesis evolve? 347 IV. Evolutionary lineages of C photosynthesis 348 V. Where did C photosynthesis evolve? 350 VI. How did C photosynthesis evolve? 352 VII. Molecular evolution of C photosynthesis 361 VIII. When did C photosynthesis evolve 362 IX. The rise of C photosynthesis in relation to climate and CO 363 X. Final thoughts: the future evolution of C photosynthesis 365 Acknowledgements 365 References 365.