Flowers in the wild

Biology

Flowers in the wild

Floral Treat: Flowering plants like the aquatic herb Trithuria, its seed shown in microscopic section, evolved with an extra set of genes. (Samuel Holloway via The New York Times)

Throughout his life, Charles Darwin surrounded himself with flowers. Despite his familiarity, Darwin once wrote that their evolution was “an abominable mystery.” He could see for himself how successful flowering plants had become. They make up the majority of living plant species, and they dominate many of the world’s ecosystems, from rain forests to grasslands.

They also dominate our farms. Out of flowers come most of the calories humans consume, in the form of foods like corn, rice and wheat.They are also impressive in their sheer diversity of forms and colours, from lush, full-bodied roses to spiderlike orchids to calla lilies shaped like urns.

The fossil record offered Darwin little enlightenment about the early evolution of flowers. At the time, the oldest fossils of flowering plants came from rocks that had formed from 100 million to 66 million years ago during the Cretaceous period. “There’s an energy that I haven’t seen in my lifetime,” said William Friedman, an evolutionary biologist at the University of Colorado, Boulder.

Wealth of clues in flowers, genes
The discovery of new fossils is one source of that new excitement. Scientists are also finding a wealth of clues in living flowers and their genes. They are teasing apart the recipes encoded in plant DNA for building different kinds of flowers. Their research indicates that flowers evolved into their marvellous diversity in much the same way as eyes and limbs have: through the recycling of old genes for new jobs.

Until recently, scientists were divided over how flowers were related to other plants. Thanks to studies on plant DNA, their kinship is clearer. “There was every kind of idea out there, and a lot of them have been refuted,” said James A Doyle, a paleobotanist at the University of California, Davis. It is now clear that the closest living relatives to flowers are flowerless species that produce seeds, a group that includes pine trees and gingkos.

The plants that might document the early stages in the emergence of the flower apparently became extinct millions of years ago. In the past few years, scientists have pushed back the fossil record of flowers to about 136 mn years ago. They have also found a number of fossils of mysterious extinct seed plants, some of which produce seeds in structures that look faintly like flowers. But the most intriguing fossils are also the most fragmentary, leaving paleobotanists deeply divided over which of them might be closely related to flowers. “There’s no consensus,” Doyle said.

But there is a consensus when it comes to the early evolution of flowers themselves. By studying the DNA of many flowering plants, scientists have found that a handful of species represent the oldest lineages alive today. The oldest branch of all is represented by just one species: a shrub called Amborella that is found only on the island of New Caledonia in the South Pacific.

Water lilies and star anise represent the two next-oldest lineages alive. If you could travel back to 130 million years ago, you might not be impressed with the earliest flowers. “They didn’t look like they were going anywhere,” Doyle said.

Flower fossils
Around 120 million years ago, a new branch of flowers evolved that came to dominate many forests. That lineage includes 99 per cent of all species of flowering plants on Earth, ranging from magnolias to dandelions to pumpkins. That also produced the burst of flower fossils that so puzzled Darwin. All flowers, from Amborella on, have the same basic anatomy. Just about all of them have petals or petal-like structures that surround male and female organs. The first flowers were probably small and simple, like modern Amborella flowers. Later, in six lineages, flowers became more complicated.

Mutations
In the late 1980s, scientists discovered the first genes that guide the development of flowers. They were studying a small plant called Arabidopsis, when they observed that mutations could set off grotesque changes. Some mutations caused petals to grow where there should have been stamens, the flower’s male organs. Other mutations transformed the inner circle of petals into sepals. And still other mutations turned sepals into leaves.

The discovery was a remarkable echo of ideas first put forward by the German poet Goethe, also a careful observer of plants. Two centuries later, scientists discovered that mutations to genes could cause radical transformations like those Goethe envisioned. In the past two decades, scientists have investigated how the genes revealed through such mutations work in normal flowers.

The genes encode proteins that can switch on other genes, which in turn can turn other genes on or off. Together, the genes can set off the development of a petal or any other part of an Arabidopsis flower. Scientists are studying those genes to figure out how new flowers evolved. They have found versions of the genes that build Arabidopsis flowers in other species, including Amborella.  Vivian Irish, an evolutionary biologist at Yale, and her colleagues are learning how to manipulate poppies because, Irish points out, “poppies evolved petals independently.” She and her colleagues have identified flower-building genes by shutting some of them down and producing monstrous flowers as a result.

The genes, it turns out, are related to the genes that build Arabidopsis flowers. In Arabidopsis, for example, a gene called AP3 is required to build petals and stamens. Poppies have two copies of a related version of the gene, called paleoAP3. But Irish and her colleagues found that the two genes produced different effects. Shutting down one gene transforms petals. The other transforms stamens.

New York Times News Service

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