Oldest known flowering plants

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.