ALISCIONI Sandra Silvina
The age of the grasses and cluster ed origins of C4
VICENTINI, A, J. BARBER, S. ALISCIONI, L. GIUSSANI & E. KELLOGG
GLOBAL CHANGE BIOLOGY
Lugar: Illinois; Año: 2008 vol. 14 p. 2963 - 2977
At high temperatures and relatively low CO2 concentrations, plants can most efficiently fix carbon to form carbohydrates through C4 photosynthesis rather than through the ancestral and more widespread C3 pathway. Because most C4 plants are grasses, studies of the origin of C4 are intimately tied to studies of the origin of the grasses. The earliest origins of C4 may have been in the Oligocene, coinciding with a reduction in global CO2 levels, although at least one possible fossil calibration suggests that C4 is much more ancient. Multiple transitions between C3 and C4 photosynthesis occurred subsequent to the first appearance of C4. A densely sampled phylogeny of the grass family with multiple calibration points shows that origins of the C4 pathway were significantly clustered in geological time, as were subsequent reversals to C3. This pattern of clustering was not influenced by the precise date assigned to the phylogeny. In the process of dating the origins of C4, we also show that the common ancestor of the grasses (the crown node) originated in the upper Cretaceous. Clustered changes of photosynthetic pathway occurred in the Mid- to Late Miocene, correlating with global climate and following a C4 origin in the Oligocene. The common ancestor of maize and rice, and hence the younger boundary for the genome duplication that characterizes most grasses, lived at 52 +/- 8 Ma. The genome duplication could, however, be as old as the stem node of the grasses, 89-94 Ma.