December 16, 1999
Harvard
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HARVARD GAZETTE ARCHIVES

Oldest Known Flowering Plants Identified By Genes

By William J. Cromie
Gazette Staff


The water lily's origins go back more than 140 million years. Photo by Michael Donoghue.

It’s a nondescript shrub with small, unimpressive flowers, and it’s found in only one place in the world – New Caledonia, a minor tropical island in a remote corner of the southwest Pacific. But it has suddenly catapulted to botanical eminence as a key piece in the puzzle of the origins of flowering plants.

Called Amborella, the plant is the one remaining species of a lineage that first appeared on Earth more than 140 million years ago, while dinosaurs still ruled the planet. The other flowering plants, from which it branched, evolved and diversified until they came to dominate Earth at about the same time as the mammalian ancestors of humans were replacing dinosaurs. Flowering plants now number 250,000 different species, including virtually all the vegetables and grains we eat, as well as most of the food of the animals that we consume.

"It’s difficult to imagine a world without flowering plants," says Michael Donoghue, professor of biology at Harvard University. As a result of analyzing the genes from all flowering plants suspected of being among the world’s oldest, Donoghue and research associate Sarah Mathews concluded that Amborella and water lilies are the first two branches on the family tree of flowering plants.


Michael Donoghue (right) and Sarah Mathews examine a Herbaria sample of Amborella, which their research has identified as the oldest known flowering plant. Photo by Justin Ide.
Mathews and Donoghue’s analysis revealed that another early branch includes Austrobaileya, a group represented by one species found only in Australia, and by the more common star anise, which boasts bright red, saucer-shaped flowers.

Tracing flowering plants back so close to their roots ranks as one of the major botanical discoveries of this century.

"We knew Amborella was a possible candidate, but to actually pin it down after decades of speculation was a cause for great excitement," Donoghue says. "One of the wonderful things is that three other teams of scientists have come up with the same result."

The other scientists did different types of genetic analyses at universities in the United States, Canada, and Switzerland. Mathews and Donoghue published their results first, in the Oct. 29 issue of the U.S. journal Science. Two other teams published their findings in the Nov. 25 issue of the British journal Nature.

Flowers on the Tree of Life

Despite these new findings, plenty of mysteries remain. None of the analyses reveals when the first flowering plant appeared on Earth. Amborella is not the first one but, rather, a representative of the first branch from that unknown ancestor.

Sometime before 140 million years ago, flowering plants, known as angiosperms, diverged from nonflowering seed plants known as gymnosperms. Biologists imagine a tree of life with different groups of animals or plants as branches. Flowering plants branched off from within the branch of seed plants. The first branch within flowering plants separated Amborella from all the rest.

Fossils of gymnosperms, such as pine trees and other evergreens, go back at least 350 million years, when cockroaches as big as house cats and dragonflies with the wingspans of modern hawks prowled the planet. Therefore, the birth of flowering plants could have occurred anytime between 140 million and 350 million years ago. Botanists would like to narrow that gap.

Another mystery involves how flowering plants were able to diversify so much and spread so rapidly. Donoghue and Mathews see similarities in this expansion with that of mammals after dinosaurs became extinct about 60 million years ago.

The key, they believe, lies in newly evolved structures not available to nonflowering plants. One of these is the carpel, a womb-like vessel that encloses the seeds of flowering plants. The word "angiosperm" means "seed born in a vessel." "Gymnosperm" means "naked seed," a reference to the lack of a protective structure enveloping the developing seed.

In modern plants, a leaflike carpel folds over one or more seeds and its edges fuse together. The young plant develops inside the seed with the help of a placenta-like nutritive tissue known as the endosperm. Amborella, water lilies, and the star anise have folded carpels, but they do not fuse shut. Instead, they secrete a fluid that sticks the carpel edges shut.

That doesn’t sound like much difference, but it gives botanists a rare glimpse of a modern structure as it appeared more than 140 million years ago. "This is probably how the carpel looked in the distant ancestors of flowering plants," Donoghue says. "It’s a neat observation that increases our understanding of how flowering plants originated and what the first ones looked like."

The carpel probably helped flowering plants disperse their seeds around the Earth. Undoubtedly, other features evolved and gave certain lineages advantages that allowed them to adapt to many environmental niches closed to gymnosperms. Today, the latter are limited to about 750 species compared with 250,000 angiosperms.

Mathews and Donoghue speculate that this glory road to diversification and expansion contained many ruts of extinction. Lineages of larger, woody, flowering plants are thought to have given rise to smaller, herbaceous plants. Continued evolution of these less woody species eventually made possible farming and the rise of modern civilizations.

It’s something to think about the next time you look at a water lily.

 


Copyright 1999 President and Fellows of Harvard College