The age of the grasses and clusters of origins of C4 photosynthesis

VICENTINI A., J. C. BARBER, S. A. ALISCIONI, L. M. GIUSSANI, & E. A. KELLOGG

GLOBAL CHANGE BIOLOGY

Blackwell Publishing

Año: 2008 p. 2963 - 2978

1354-1013

At high temperatures and relatively low CO2 concentrations, plants can most efficientlyfix carbon to form carbohydrates through C4 photosynthesis rather than through theancestral and more widespread C3 pathway. Because most C4 plants are grasses, studiesof the origin of C4 are intimately tied to studies of the origin of the grasses. We presenthere a phylogeny of the grass family, based on nuclear and chloroplast genes, andcalibrated with six fossils. We find that the earliest origins of C4 likely occurred about 32million years ago (Ma) in the Oligocene, coinciding with a reduction in global CO2levels. After the initial appearance of C4 species, photosynthetic pathway changed atleast 15 more times; we estimate nine total origins of C4 from C3 ancestors, at least twochanges of C4 subtype, and five reversals to C3. We find a cluster of C4 to C3 reversals inthe Early Miocene correlating with a drop in global temperatures, and a subsequentcluster of C4 origins in the Mid-Miocene, correlating with the rise in temperature at theMid-Miocene climatic optimum. In the process of dating the origins of C4, we were alsoable to provide estimated times for other major events in grass evolution. We find thatthe common ancestor of the grasses (the crown node) originated in the upper Cretaceous.The common ancestor of maize and rice lived at 52 8 Ma.