The evolutionary Tree of Life for flowering plants has been revealed using the largest collection of genomic data of these plants to date, report scientists from The University of Texas at Austin and University of Florida.
|Phylogenetic relationships among the major lineages of flowering plants based on plastid genome sequences. Pictured counter-clockwise from the root at the base of the circle tree are: Amborella trichopoda, Nymphaea odorata, Illicium floridanum, Chloranthus angustifolius, Piper longum, Liriodendron tulipifera, Ceratophyllum demersum, Ranunculus ficaria, Pelargonium exstipulatum, Helianthus annuus, Yucca filamentosa, Triticum aestivum and Acorus americanus. New Caledonia, home to Amborella trichopoda, is shown in the background.|
|Image: Gwen Gage|
The scientists, publishing two papers in Proceedings of the National Academy of Sciences this week online, found that the two largest groups of flowering plants, monocots (grasses and their relatives) and eudicots (including sunflowers and tomatoes), are more closely related to each other than to any of the other major lineages.
The analyses also confirmed that a unique species of plant called Amborella, found only on the Pacific island of New Caledonia, represents the earliest diverging lineage of flowering plants.
Robert Jansen, professor of integrative biology at The University of Texas at Austin, said the work sets the stage for all future comparative studies of flowering plants.
"If you are interested in understanding the evolution of flowering plants, you can't do that unless you understand their relationships," said Jansen.
The University of Florida team, led by Doug and Pam Soltis, also showed that the major diversification of flowering plants, so stunning that the researchers are calling it the "Big Bang," took place in the comparatively short period of less than five million years. This resulted in all five major lineages of flowering plants present today.
"Flowering plants today comprise around 400,000 species," said Pam Soltis, curator at the university's Florida Museum of Natural History. "To think that the burst that gave rise to almost all of these plants occurred in less than five million years is pretty amazing-especially when you consider that flowering plants as a group have been around for at least 130 million years."
The details of the flowering plants' rapid diversification have remained a mystery since Charles Darwin first suggested their evolutionary history is an "abominable mystery."
"One of the reasons why it has been hard to understand evolutionary relationships among the major groups of flowering plants is because they diversified over such a short time frame," said Jansen.
But by analyzing DNA sequences from completely sequenced chloroplast genomes, the scientists brought some clarity to the evolutionary picture.
Jansen and his colleagues at The University of Texas at Austin analyzed DNA sequences of 81 genes from the chloroplast genome of 64 species of plants, while the Florida researchers analyzed 61 genes from 45 species. The two groups also performed a combined analysis, which produced evolutionary trees that included all of the major groups of flowering plants.
As for the diversification's cause, it remains mysterious, Pam and Doug Soltis said.
It's possible it was spurred by some major climatic event. It's also possible that a new evolutionary trait—a more efficient water-conducting cell that transfers water up plant stems—proved so effective that it spurred massive plant growth. This cell type is not present in the first three flowering plant lineages, said Doug Soltis, professor of botany at Florida.
Michael Moore, a former postdoctoral associate in the Soltis lab and now a faculty member at Oberlin College, is lead author of the University of Florida study.
The scientists' work is funded by two grants from the Tree of Life program at the National Science Foundation.