The making of a blossom: a flower's evolutionary past may be read in the genes that influence its development

Natural History, May, 2002 by Enrico Coen

Bashford Dean had two passions in life. One was studying the development and evolution of fishes, which led to his becoming a professor at Columbia University in 1891 and a curator at the American Museum of Natural History in 1903. The other was a fascination with arms and armor that was first roused in early childhood, when Dean saw a beautiful European helmet in the house of a family friend. He was so taken with the helmet that he sat with it on the porch, where he studied it inside and out for a long time. Dean's interest in armor grew over the years, and in 1906 he became honorary curator of arms and armor at New York's Metropolitan Museum of Art. Eventually he retired from active duty as a scientist and a teacher and devoted himself to making the Met's collection of arms and armor one of the finest in the world.

Dean took his biological past with him, however. Diagrams he drew depicting the evolution of armaments such as helmets and shields have much the same branching pattern often used by scientists to illustrate the evolution of fishes or flowers. One diagram of helmets (at left) shows a simple, radially symmetrical ancestral helmet at the bottom. From this primitive form, various lineages emerge; some of them lead to highly elaborate, enclosed helmets with visors or chin guards, while others lead to dead ends or revert to simpler shapes.

[ILLUSTRATION OMITTED]

Such diagrams are a good way to organize objects and to show how they are related. But insofar as they give the impression that one object is transformed directly into another--that one helmet, say, is directly modified to give rise to the next in the series--they are misleading. What evolve, of course, are not the helmets themselves but the ways people make them. Bashford Dean's diagram tells a story about in how people fashion helmets in response to changing circumstances, materials, and traditions.

A similar principle applies to biological evolution. Although we commonly portray evolution as a branching tree or bush along which one type of organism seems to transform into another, it is not organisms themselves that change but the way they develop. During the evolution of flowers, for example, blossoms of one type are not directly modified to produce blossoms of another type. What changes is the way flowers develop from seed in each generation. More precisely, changes come from the genes that influence development and that underlie the evolution of flowers, fishes, and every other complex biological structure.

But how do evolutionary biologists unravel the history of developmental change when the ancestral organisms are no longer with us? Even when we are lucky enough to have a fossil record, we get only a few snapshots, not a dynamic view of how ancient plants and animals developed in each generation.

Recently, researchers have been approaching this problem from a new angle: studying how genes influence diverse organisms living today and then trying to infer what happened in the past. After all, genes, the units of heredity, are what connect us with our past. This approach, sometimes called evo-devo (short for evolution of development), became possible only in the last decade or so, when advances in our knowledge of genes allowed us to compare their roles in different types of organisms. Evo-devo has already yielded many surprises, prompting biologists to think afresh about some age-old problems, such as the evolution of the eye or the relationship between mammals and insects. In my own field--the evolution and genetics of flowering plants--I have been especially intrigued by how genes determine floral symmetry.

Flowers can be broadly divided into two types according to their symmetry. Radially symmetrical flowers, such as buttercups and tulips, have a single type of petal arranged the same way all around a center. There is more than one way to cut vertically through the center of these flowers to produce two halves that are mirror images. Bilaterally symmetrical flowers, such as snapdragons and sweet peas, have distinctive upper and lower petals and are therefore asymmetric from top to bottom. There is only one way you can cut one of these flowers to divide it into two mirror-image halves.

Like Bashford Dean's helmets, flowers are thought to have been radially symmetrical at first. Bilateral flowers evolved later in response to pollinators, the lower petals often providing a platform for insects to land on. Curiously, bilateral symmetry--and thus the developmental "trick" that makes it possible--seems to have evolved numerous times, independently. How was this possible?

One of the most familiar plants with bilateral symmetry is the snapdragon (Antirrhinum majus). Highly regarded as reliable and colorful members of the summer garden, snapdragons hold a different attraction for geneticists. Some of the key genes controlling flower symmetry have been identified in this plant, and one gene, called cycloidea, or cyc (from the Greek cyclo-, meaning circular), plays a particularly important role. With cyc, snapdragons produce the double-lipped blossom popular with small children, who like to squeeze the sides together to make the "dragon" open its mouth. Some snapdragons, however, produce radially symmetrical blossoms; in such mutant plants, the cyc gene is inactive.