Sex among the Flowers

Article excerpt

A bouquet of botanical breakthroughs is shedding light on the exuberant evolution of the earliest flowering plants and their mysterious sexual history.

In an oft-quoted letter written in 1879, Charles Darwin confessed, with his usual candor, that the "rapid development as tar as we can judge ot [flowering plants] within recent geological times is an abominable mystery." In fact, through much of Darwin's later life, he was keenly interested in and vexed by the evolutionary origin of flowering plants, or angiosperms. In 1875, for instance, Darwin confided to a colleague that the "sudden appearance of so many [angiosperms] in the [Cretaceous period] appears to me a most perplexing phenomenon to all who believe in any form of evolution, especially to those who believe in extremely gradual evolution." Even just months before his death in 1882, Darwin continued to maintain that "nothing is more extraordinary in the history of the Vegetable Kingdom .. . than the apparently very sudden or abrupt development of the [flowering plants]."

Darwin's frustration stemmed, in large part, from the remarkably rapid origin and diversification (as shown by the fossil record) of flowering plants within a brief period of earth's history-and bis own strong views that evolutionary change was typically a slow and gradual process. Darwin was convinced that the origin of angiosperms was one of the great challenges in the effort to decipher the evolutionary history of life.

What so impressed him remains impressive today: how some 250,000 species of flowering plants have, come to dominate the earth's vegetation.They flourish in the tropics and in the arctic, in alpine terrain, as well as in deserts and lakes. They range from mighty oak trees to woody and herbaceous vines, from underground parasites to carnivores that prey on insects, from floating aquatics to epiphytes (plants that live on other plants), such as orchids and bromeliads. Darwin was also astute enough to recognize the importance of a question that intrigues evolutionary biologists today: how to square such diversity with the fact that flowering plants are tar and away the youngest major lineage ot plants.Their evolutionary origin can be traced in the fossil record to the Early Cretaceous, some 130 million years ago. In contrast, conifers had a head start of more than 170 million years. By all measures, angiosperms have diversified to a greater extent and in a shorter time than any other group of plants.

What were the progenitors of angiosperms? What did the first ones look like? How did their many unique biological features evolve? Why did the origin and early diversification of flowering plants proceed so rapidly? To shed light on such questions, I have studied extant plant species belonging to the same ancient flowering plant lineages that flourished in the days of dinosaurs. My findings, together with the recent work of other botanists, indicate that many of the century-old assumptions about the biological features of the first flowering plants and the subsequent diversification of early angiosperms are fundamentally wrong.

Like gymnosperms, angiosperms propagate via seeds, which house and nourish the developing embryo. Hut unlike gymnosperms, whose seeds are exposed to the environment (in cones, for instance) angiosperms envelop their future seeds in one or more protective structures called carpels. The word "angiosperm," the botanical name for flowering plant, comes from the Greek words tor "vessel" and "seed."

The recent breakthroughs in reconstructing early angiosperm evolution and diversification have come from work in three interconnected fields of evolutionary biology: paleontology, phylogenetics (the analysis of the genealogical relationships of life), and morphology (the study of the development and structure of organisms).

The fossil record remains silent about the ancestors of flowering plants, just as it was in Darwin's day. …