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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]






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