"By combining all these data," Edwards said, "we could get individual climate profiles for each grass species."
The pair then went a step further. They whittled the list to approximately 1,230 species for which the plants' genes had been sequenced and from there built a phylogenetic profile for the collection, the most comprehensive evolutionary tree to date for grasses. The reason for building the phylogeny, Edwards said, was to tease out the junctures at which C3 and C4 grasses diverged over time. The scientists zeroed in on 21 such "transition nodes" and examined the climatic conditions during those branching periods.
They found that in 18 of the 21 instances, precipitation, rather than temperature, had changed. "That was the clincher," Edwards said.
Looking more closely at the differences in rainfall, Edwards and Smith noticed the shifts in the amount of rainfall between C3 and C4 grasses in the tropics dictated in sharp relief how the different lineages evolved. Generally speaking, C3 grasses flourished in areas that received, on average, 1,800 millimeters (71 inches) of rain annually; C4 grasses took root in areas that received, on average, 1,200 millimeters (47 inches) of rain annually.
"Twelve-hundred millimeters isn't a desert," Edwards noted. "It's still a fairly mesic place. And so when you start looking at climate profiles, these closely related C3 and C4 lineages are straddling this transition zone between tropical forests and tropical woodlands and savanna."
So, did C4 grasses evolve in the tropical forest and then move out from the canopy or did they move out first and then adopt a different photosynthetic pathway? Edwards isn't sure, but she thinks the pathway may have begun to form with C3 grasses on the forest margins, where those plants would have been subje
|Contact: Richard Lewis|