Puzzling Out The Tree Of Life Of Green
Plants
A
five-year effort to reconstruct the evolutionary relationships among all
green plants has resulted in the most complete "tree of life" of any
group of living things on the planet, including animals, scientists
announced Wednesday in St. Louis.
The up-to-date family
tree has revealed several surprises about the evolution of green plants,
such as how they emerged from the sea onto land and how they are related
to the four other major kingdoms: the brown plants, the red plants, the
fungi and the animals.
This knowledge will help scientists
in a broad range of disciplines, from those trying to develop new and
better crops to those prospecting for new medicines.
"Knowing the tree of life is important because of its predictive
value," said Brent Mishler, a spokesman for the so-called "Deep Green"
project who also is a professor of integrative biology and director of
the Jepson and University Herbaria at the University of California,
Berkeley. "When you find a new plant, the first thing you want to know is
how it is related to other plants. That is essential in predicting its
traits."
Mishler spoke Wednesday at a news briefing at the
XVI International Botanical Congress. The world's largest gathering of
plant scientists, it is held every six years and, for the first time
since 1969, is taking place in the United States.
Some
5,000 scientists from around the world have gathered for a week, Aug.
1-7, to discuss the importance of plants for human survival and improved
quality of life and to announce breakthrough research in the field of
plant science.
The Green Plant Phylogeny Research
Coordination Group, or Deep Green, has sponsored eight separate symposia
on green plants during the meeting, plus a keynote symposium on Friday,
Aug. 6, 9:00-11:30 a.m., that will place the green plants in the context
of the rest of the tree of life. Mishler will speak at the Friday
symposium, summarizing five days of presentations on green plant
phylogenetics -- that is, how green plants developed and evolved over
time.
The five-year project, funded by the U.S. Department
of Energy, the National Science Foundation and the Department of
Agriculture, was initiated by plant biologists as a way to make sense of
the reams of data that exist on plant relationships.
"We
got together and decided to make a concerted effort to understand one
branch of the tree of life, the green plants, whose impact, dollar for
dollar, you can't beat," Mishler said. "Their economic importance as
sources of medicine, structural materials, food and chemicals is
immense."
Despite the wealth of information -- much of it
molecular data comparing DNA from different organisms -- the data
represented so many different plants as to make synthesis difficult if
not impossible. Mishler likens the evolutionary tree to a real tree
buried in the sand, with only the tips of the twigs -- the million or so
plants living today -- visible. We can reconstruct the buried branches
and trunk, he said, but only by concentrating on a small number of twigs,
perhaps as few as 100.
"Certain groups were over-studied,
other groups nearly unknown," he said. "Data sets derived from different
molecules and different morphological character systems rarely included
the same basic taxa, thus they couldn't be compared."
To
remedy the situation, more than 200 scientists from a dozen countries
joined forces to select standard organisms that scientists are being
encouraged to use for comparison. The first results of their efforts are
being reported this week at the botanical congress.
In the
process of producing a detailed tree of life for green plants, Deep Green
has clarified many suspected relationships among green plants and has
pointed out many new ones. Among these are:* We're All Related: At all
levels of the green plant family tree, plants appear to be related to one
another. This is a real surprise, Mishler said. In theory, throughout
history various families would have evolved and left descendants, so that
today we would see distinct subgroups related only in the distant
past.
Instead, at each stage of evolution only one family
or lineage seems to have survived. Even at the very root of the green
plant family tree, just one line leads to today's living green
plants.
"It seems that only one lineage actually made it,"
Mishler said. "This indicates there's an Eve in the primordial soup."
As an example, only one line of green plants seems to have left the
water and survived on land. Thus all green land plants are descended from
one group. Some of those land plants subsequently returned to the sea to
join the green plants that failed to make the transition to land.
"Plants came out onto land probably many times, but only one lineage
made it," Mishler said. In fact, the trail leads through fresh water.
"All life first arose in the ocean, then some moved to fresh water,
and then a number of lineages went back to the sea," he said. "Green
seaweeds, for example, originally came from fresh water."
Some plants, like the kingdom of red plants -- mostly seaweeds --
never left the ocean. In the brown plant kingdom, most remained as
seaweeds but a few, such as diatoms, also moved into fresh water.
Multicellular life forms also no doubt arose more than once, but
today all five major kingdoms of multicellular organisms are descendants
of only one unicellular lineage.
"Multicellular life was
another great idea that almost certainly arose more than once. A
one-celled organism has to do everything -- it's a jack of all trades and
master of none -- but in a group of cells, individuals can specialize and
be very good at one specific thing," Mishler said. "But again, only a
very few major lineages made it through to the present, and those all
from one basic stock." * The Conquest of Land: To invade the land,
green plants had to learn some new tricks: not only new ways to
reproduce, but also ways to keep from drying out.
"In the
ocean you just dump your gametes - reproductive cells, such as egg and
sperm - into the water and they find one another," Mishler said.
"Everything swims."
On land that doesn't work. The first
land adaptation was to a wet environment where plants can rely on a film
of water through which gametes can swim. Mosses and ferns still reproduce
this way, but only on a rainy day.
This limitation, which
tends to make sexual reproduction less successful, was overcome by the
angiosperms or flowering plants, which essentially rely on a whole male
plant, the pollen, traveling to the female plant.
"This is
the most successful technique in terms of number of sperm, diversity, et
cetera," Mishler said.
Recent DNA analysis by Mishler and
his colleagues shows that the simplest, most primitive land plants, the
bryophytes -- which include the mosses -- could tolerate dry conditions.
However, the researchers found that this ability was lost in the lineage
of land plants and re-evolved later in both ferns and flowering
plants.
As a result of this realization, researchers are
beginning to study mosses for tips on how to make economically important
crops like corn tolerant to desiccation.
"The study of
desiccation tolerance has really benefited from knowing the phylogeny,"
he said. "We can tell really interesting stories about land plants with
what we know now." * Invasion by Chloroplasts: Plant cells are
characterized by pigment-containing organs called chloroplasts, which
convert sunlight into energy for the cell. Scientists have known for some
time that these chloroplasts were once free-living organisms that
colonized cells and eventually became a symbiont -- essential to the host
cell and unable to live without it.
Surprisingly, the red,
green and brown chloroplasts that distinguish the red, green and brown
plant kingdoms all appear to have arisen from the same group of
organisms.
"Many people thought that the red, green and
brown plants were all colonized at different times by different groups of
pigmented organisms, but it appears that one group of blue-green algae is
responsible for all the chloroplasts in plants," Mishler said. "In fact,
the chloroplasts are more closely related than the plants
themselves." * Are Fungi Plants?: The fungi, including the mushrooms,
actually are more closely related to animals than to plants.
Nevertheless, all five kingdoms of higher organisms - as distinguished
from the bacteria - are closely related to one another. Aside from
animals and fungi, these kingdoms are the green plants, which include
most land and all the flowering plants; the red plants, mostly seaweeds;
and the brown plants, also mostly seaweeds, including the large brown
kelp.
Many other questions remain to be answered, Mishler
said, including one of life's most fundamental questions: From which
original stock have all plants evolved?
To obtain
definitive answers, however, plant scientists must work together. Mishler
emphasizes the need for many fields of biology to coordinate their
efforts in order to create family trees such as the genealogy of the
green plants. Biologists working on fungi are embarking on a similar
effort, led by John Taylor and Thomas Bruns, professors of plant and
microbial biology at UC Berkeley, while those studying the red and brown
plants will soon follow.
In addition, Mishler insists on
the importance of maintaining plant collections, such as the efforts by
UC Berkeley's herbaria to preserve plant specimens for study.
"Only a very few universities, the ones that have retained natural
history collections in addition to cutting-edge laboratories, are able to
contribute in a major way to synthetic projects such as this," he
said.
Deep Green is headed by Mark A. Buchheim as
principal investigator, and Brent D. Mishler and Russell L. Chapman as
co-principal investigators. - By Robert L. Sanders Related
websites: Deep
Green. The Botanical Congress
[Contact: Brent
Mishler, Robert
Sanders]
05-Aug-1999 |
