Photosynthesis research on yellowtops: Macroevolutionn in progress

The vast majority of angiosperms, including most of the agronomically important crop plants (wheat, etc.), assimilate CO₂ through the inefficient C₃ pathway of photosynthesis. Under ambient conditions these organisms loose about 1/3 of fixed carbon via photorespiration, an energetically wasteful process. Plants with C₄ photosynthesis (such as maize) eliminate photorespiration via a biochemical CO₂-pump and thus have a larger rate of carbon gain. The genus Flaveria (yellowtops, Asteraceae) contains not only C₃ and C₄ species, but also many C₃-C₄ intermediates, which have been interpreted as evolving from C₃ to fully expressed C₄ metabolism. However, the evolutionary significance of C₃-C₄Flaveria-intermediates has long been a matter of debate. A well-resolved phylogeny of nearly all Flaveria species has recently been published. Here, we review pertinent background information and combine this novel phylogeny with physiological data. We conclude that the Flaveria species complex provides a robust model system for the study of the transition from C₃ to C₄ photosynthesis, which is arguably a macroevolutionary event. We conclude with comments relevant to the current Intelligent Design debate.