Ancient shrub unlocks a clue to Darwin's 'abominable mystery'

Scientists have found a possible 'missing link' between flowering plants and their nonflowering ancestors.

To millions of moms, the Mother's Day bouquet still gracing the dining room table symbolizes gratitude and love. To Charles Darwin, however, they also would stand as colorful characters in what he called an "abominable mystery" - the origin of flowering plants.

"It's no different now," sighs biologist William Friedman. The mystery remains abominable. But Dr. Friedman may have helped put plant biologists on a promising scent. He says he's discovered a unique trait in an ancient line of plants that could represent a "missing link" between flowering plants and more-ancient seed plants, such as pines, ginkos, and cycads, some of which can be mistaken for palms.

It takes a microscope to see the difference between the remarkable Amborella and other flowering shrubs. Passersby on its home island of New Caledonia in the South Pacific wouldn't give it a second thought. But to evolutionary biologists, "it's akin to finding a fossil amphibian with an extra leg," notes Henry Gee, a zoologist by training and a senior editor at the journal Nature, which will publish the results in Thursday's issue.

Essentially, female plants among the unisexual Amborella plant build embryo sacs hosting cells common to all flowering plants. But these sacs also carry a sterile "extra" cell that appears to tie Amborella back to its nonflowering ancestors. No other flowering plant known contains this unique combination.

The question behind the quest: How is it that flowering plants, which include grasses, exploded some 130 million years ago to around 250 million species?

Other seed-bearing plants didn't do as well. Of at least 20 seed-plant lineages found in fossil records, only five remain today.

Conifers are abundant in certain ecosystems, but they don't display a wide range of species. Ginkos are the last of a line. Cycads, such as the misnamed sago palm, are still found in the tropics and subtropics, and have become garden favorites in areas with hospitable climates. The fourth lineage is represented by the "Mormon tea," a shrub that grows in the deserts of the southwestern United States.

Scientists are befuddled about how the fifth branch, flowering plants, blossomed into so many distinct species.

"After the origin of seed plants some 300 million years ago, things were fairly conservative," Friedman explains. "Plants grew slowly and you evolved long-lived trees. Then 130 million years ago, something got going that broke the mold."

For decades, he continues, evolutionary biologists interested in Darwin's mystery studied members of the magnolia family - long thought to be the oldest line of flowering plants.

Then in 1999, researchers published the results of DNA studies that suggested that Amborella is older. Indeed, Dr. Gee notes, it is considered "the last remnant of one of the most ancient lineages" of flowering plants.

But DNA evidence alone only "tells you where to look," Friedman explains. "Then you have to roll up your sleeves and do the biology."

That meant trips to tropical rain forests on the New Jersey-size Pacific island - the plant's only known home. There, Friedman's colleagues collected specimens, later carefully cultivated in a greenhouse at the University of Colorado, where Friedman teaches evolutionary biology and conducted the studies.

Although Amborella's productive features give it a missing-link look, Friedman cautions that its unique cell arrangement also could represent a case of parallel evolution, where similar traits emerge among unrelated species. Even if that turns out to be the case, in a field where subtle details of reproductive systems often provide significant clues about the evolutionary relationships among organisms, his results appear to represent the first new embryo-sac design to emerge from studies of flowering plants in half a century.

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