(A little OT) All plants may have roots in single, slimy ancestor
Daniel B. Wheeler
dwheeler at teleport.com
Tue Jul 3 00:35:59 EST 2001
>From The Oregonian, May 23, 2001, p A21
All plants may have roots in single, slimy ancestor
Genetic analysis suggests one type of algae managed to spread to land
and evolve into myriad forms
By ROBERT COOKE, LA TIMES-WASHINGTON POST SERVICE
The first tentative moves that got life out of the water and onto the
land eons ago apparently were made by slimy green algae, and coming
ashore was far from easy, scientists say.
The first algae that managed to gain residence on land -
kick-starting the evolution of land plants - must have come out of
fresh water, not the sea, said paleobotanist Russell Chapman of
Louisiana State University.
And though four distinct types of algae managed to come ashore, only
one evolved enough complexity to eventually cover the land with
vegetation - trees, shrubs, flowers and grass, Chapman said.
Nonetheless, all four species of pioneering algae can still be found
Recent advances in techniques of genetic analysis are helping reveal
the ancient history of land plants. It's now possible to look at
individual genes in algal cells and higher plants and calculate their
similarity. Clues to the history of such organisms lie within the
chemical "spelling" - the sequence similarity - of the organisms'
genes. The closer they resemble each other, the closer they are
"The evolutionary history of various genes can be studied within the
lineage of green algae," Chapman said. That offers vital clues to how
the algal genes eventually evolved to produce plants.
Today's green plants are enormously varied, from the giant redwood
trees to the tiniest weeds - everything that blooms, including our
During a symposium on the genetics and evolution of green plants at
the annual meeting this year of the American Association for the
Advancement of Science in San Francisco, Chapman and several
colleagues laid out the evidence that today's multicellular plants,
such as corn, cabbages and all the other greenery, arose from a single
type of algae.
The genetic evidence now being uncovered shows that "the
multicellular land plants are all of one lineage," said plant
geneticist Brent Mishler of the University of California at Berkeley.
"The fossil evidence suggests that others (types of algae) tried" but
failed to evolve the needed complexity.
In other words, the three other algae that managed to wade ashore
didn't evolve beyond the single-cell stage, so they remain what they
Desiccation resistance the key
The discoveries and the ideas of how land plants arose "reminds
people of our humble origins," Chapman said. "This reminds people of
how important algae are in general, since without that one escape from
water and subsequent evolution, the half-million species of plants
that are so important to life on Earth might not exist.
"There would be no crops, no flowers, no fibers or foods. Also no us,
A key step that allowed slimy green algae to leave the water about
430 million years ago, the scientists said, was evolution of a trait
called desiccation resistance: They found a way not to dry up. Because
the algae originated in water, it wasn't easy for the green cells to
keep from dying once they ran out of moisture. The fact that many
plants have devised ways to withstand dryness, even droughts, goes all
the way back to a trait that emerged in algae.
The desiccation-resistance genes in algae may lead researchers to
similar genes in valuable crop plants. Desiccation resistance, a
plant's ability to recover despite losing as much as 90 percent of its
water, is a phenomenon seen in the reproductive structures of green
plants and ferns - such as seeds, pollen and spores.
Tolerance of dry conditions is widespread but still uncommon within
the plant kingdom generally, said Melvin Oliver of the USDA Plant
Stress and Water Conservation Laboratory in Lubbock, Texas. Most of
the plants with this ability are found among the algae, lichens and
mosses. There are also 60 or 70 species of ferns and another 60
species of flowering plants that exhibit some tolerance of dryness.
Researchers are trying to pull together data from this wide variety
of plants to "generate a working hypothesis as to the evolution of
desiccation tolerance in land plants," Oliver said. They suspect that
finding a way to resist dryness "was a crucial step in the
colonization of the land."
What also seems clear, the researchers said, is that as various plant
species evolved on land, many lost drought resistance as they began to
rely more on rapid growth, structural complexity and new mechanisms
that conserve water inside the plants.
One goal would be to seek ways to introduce drought resistance -
probably via genetic engineering - into crops to allow them to survive
in harsh conditions in which they cannot now endure. If that could be
done, marginal lands on which conditions are too harsh for existing
crops might come under cultivation using new drought-resistant plants.
COMMENT BY POSTER: Michael Amaranthus has said that the great leap of
evolution from aquatic plant life to the harsh dessication of dry land
was the development of fungal/plant symbiosis, known called
mycorrhizae. The earliest mycorrhizae currently known is Glomites.
Glomites was similar to today's Glomus species, but 300,000,000 years
Daniel B. Wheeler
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