Cacti sprouted during cool down

BROWN (US) — Cactus burst onto the global scene between 5 and 10 million years ago, at a time when the earth cooled, aridity increased, and carbon dioxide levels lowered.

“The cacti, as a group, have been around for a while, but most of the species diversity that we see today was generated really recently,” says Monica Arakaki, a postdoctoral researcher at Brown University and the paper’s lead author.

Scientists interested in dating the origins of the cacti (Cactaceae) sequenced the chloroplast genomes (the organelles inside plant leaves that engineer photosynthesis) for a dozen cacti and their relatives and combined their new genomic data with existing genomes to build a phylogeny, or evolutionary tree, for angiosperms, the genealogical line of flowering plants. Angiosperms represent roughly 90 percent of all plants worldwide.

From there, they deduced that Cactaceae first diverged from its angiosperm relatives roughly 35 million years ago but didn’t engage in rapid speciation for at least another 25 million years.

Details are published in the journal Proceedings of the National Academy of Sciences.

“Cacti were actually present on the landscape for millions of years—looking like cacti and acting like cacti—before they began their major diversification,” says Erika Edwards, assistant professor of biology at Brown and corresponding author on the paper.

Cactus diversification was compared to that of other succulents from around the world—including aloes, agaves of North America, and ice plants of South Africa—all plants with shallow roots and water-storing tissue that exist where water is limited.

All the succulents were found to have undergone major speciation around the same time, as did C4 grasses, the tropical grasses that are now up to 20 percent of Earth’s vegetative covering.

“It isn’t overly surprising that most of the standing cactus diversity is relatively young. But when you put these species radiations in the context of all the other changes in plant communities that were happening at that very moment, all over the world, it begs some sort of global environmental driver,” Edwards says.

The most plausible causes, the scientists thought, were a drying out of the planet and lowering of atmospheric carbon-dioxide levels. Earth underwent a drop in temperature at the time, which probably led to reduced rainfall and increased aridity worldwide.

The carbon-dioxide link is more nuanced and controversial. An earlier study inferred atmospheric CO2 levels spiraled downward beginning roughly 15 million years ago. Combined with global cooling, a drop in CO2 concentration would therefore immediately expand the ecological space in which drought-adapted succulent plants, with their high photosynthetic water use efficiency, would be competitive.

“We suggest that a rapid expansion of available habitat (rather than any particular new key innovation) during the late Miocene was a primary driver of the global diversification of plant lineages already possessing a preadapted succulent syndrome,” the researchers write.

“Against a backdrop of increasing global aridity, a sharp CO2 decline is a plausible driver of the simultaneous expansion of C4 grasslands, the clustering of new C4 origins, and the diversification of succulent lineages.”

Researchers from Oberlin College and the University of Zurich contributed to the study, that was funded by the National Science Foundation.

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