New "Tree of Life" for green plants reveals evolutionary history of land plants and much more

By Robert Sanders, Public Affairs

St. Louis, Mo. -- 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 today 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.

Specifically, the team has overturned the traditional belief that the so-called "land-plant invasion" was led by seawater plants. Instead, the research team has found that primitive freshwater plants provided the ancestral stock from which all green plants now on land are descended, and that this ancestor spawned every green plant now alive on earth. (See below for more detail.)

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, Aug. 4, at a news briefing at the XVI International Botanical Congress. The worlds 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 will gather 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. For example, all land plants alive today share a single common ancestor at least 450 million years ago.

"Until now, it was commonly believed that within the green plant group, there were several lineages of land plants," said Mishler. "For example, people believed that mosses were derived from a different aquatic ancestor than were flowers or ferns. Now we've learned that at all levels of the green plant family tree, plants appear to be related to one another, indicating that they all share a common, freshwater ancestor. This is a real surprise."

"It seems that only one lineage actually made it," he added. "This indicates there's an Eve in the primordial soup."

The discovery that only one line of green plants seems to have left the sea for fresh water and thence to land has surprised scientists, in particular.

"The conquest of land by the plants didn't happen from the sea; it happened from freshwater streams and lakes," said Mishler. "This overturns the traditional thinking among scientists and what is taught in every text book in America.

"All life first arose in the ocean, then some moved to fresh water, and then a number of lineages went back to the sea. Green seaweeds, for example, originally came from fresh water."

According to Mishler, some plants, like the kingdom of red plants - mostly seaweeds - never left the ocean. In the brown plant kingdom, most remained as seaweeds and continued living in the ocean as well, but a few, such as "diatoms," moved into freshwater.

"Plants came out onto land probably many times, but only one lineage made it," Mishler said.

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."

Colonization 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 team has revealed that the group traditionally thought of as "plants" is really four separate lineages or "kingdoms," with one group -- fungi, including the mushrooms -- being more 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.

Darwin's Abominable Mystery: The Most Primitive Flower. The team's research also shed new light on which flowering plant living today is the most primitive -- the one that is closest to the earliest forms. Research will be presented to support two views on which plant groups are the closest living relatives of flowering plants, and the scientific discussion at the Congress will help determine which view is most likely to be accepted.

Much of the scientific world believes the Gnetophytes are the closest relative. These comprise a bizarre group of plants that includes Ephedra or "the Morman Tea," which is a shrub that grows in U.S. deserts, and Welwitschia, the strange "pile of rubber" plant that grows in the Namibian desert, one of the driest places on earth.

"The Gnetophytes don't technically flower, but have similar reproductive structures and so people have suspected them as relatives," said Mishler. "Some phylogenies have them as the sister (closest living relative) to the flowering plants. Others have them sister to the cone-bearing plants like pines and firs and then that assemblage, sister to the flowering plants. This does have big implications for where the traits of angiosperms came from. The primitive mode of moving pollen is thought to be wind, and then the standard story is that the flowering plants explosively radiated into many types once they began associating and co-evolving with insects. However, there are indications from fossils that some non-flowering plants were beginning to associate with insects, and the Gnetophytes have insects associated with their cones."

There will be further scientific discussion on this at the Congress, in addition to discussion on when the earliest plant branched off to form the rest of the flowering plants, and on how the flower fossil record fits in with the molecular data.

Using newly available DNA sequence data and more traditional morphological and anatomical data, the team has identified some fascinating relationships within the flowering plants as well. The team has discovered that flowering plants, recently discovered to be a single, but incredibly diverse lineage, are not divided neatly into monocots and dicots, as is commonly taught, but rather some of the dicots (including magnolias and water lilies) are on a branch with the monocots (including grasses and orchids).

A Microscopic Universe Distinct from "Plants" and "Animals." The team's research shows that all the plants and animals together form only a small branch on the tree of life, indicating that there is a universe of mostly single-celled and poorly known organisms that make up the most substantial parts of the tree.

Centuries ago, the scientific discovery that the sun is merely one star in a universe full of stars, planets and other astral bodies shattered mankind's preconceptions of space and led to the exploration of the cosmos. Similarly, the research team's findings could shatter some preconceptions about life here on Earth by proving that all the familiar biodiversity (including humans, along with the other animals and plants), are only one "tiny twig" on the tree of life on Earth.

The team has traced how the most basic of one-cell, bacteria-like organisms that had neither nuclei nor chromosomes "merged" with each other to become not only multi-cellular but also multi-functional. Although life on this planet has evolved to an astonishing level of richness and complexity in the form of plants and animals, most of the Earth's life forms are still found in these ancient bacteria-like organisms, according to Mishler. The majority of these organisms have not been identified. To date, scientists have identified about 1.4 million species of organisms on earth, and estimates of the numbers of undiscovered and undescribed species of organisms range from 10 million to more than 100 million.

"There are millions of microscopic organisms that live on and in plants and animals and in the soil, air and water," said Mishler. "They remain largely unexplored. Although some progress has been made, scientists are only just beginning to bring these creatures under the microscope much less to understand exactly how they interact with plant and animal ecosystems."

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.

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