Flora in Excelsis
By Jay Mayfield
As the world celebrates the 200th anniversary of the birth of famed biologist Charles Darwin, scientists continue to search for answers to the questions he raised generations ago.
For UT Knoxville researcher Joe Williams, the quest for answers led him to dangle from ropes 80 feet above the Australian soil, placing pollen in the flowers of the Austrobaileya vine, which grows high within the forest canopy.
What he found there and in other locations from Knoxville to New Caledonia helped shed light on a question that has flummoxed researchers since the time of Darwin, and it earned Williams international recognition in the process. An assistant professor of ecology and evolutionary biology, Williams used his findings to help explain the amazing diversity of flowering plants—angiosperms—compared to nonflowering plants around the world.
Diversity Driven by Survival
The heart of the evolutionary process that drives all living things to adapt to their surroundings is survival. When a plant or an animal is able to survive and reproduce, it passes its genes to the next generation, and within those genes is presumably the code for the trait that helped it to survive. The survival of some individuals but not others leads to diversity. In trying to understand that diversity, Williams looked at how plants are fertilized. His curiosity focused on the time it takes from pollination to fertilization. Williams noticed a recurring theme in the research papers he read: “They would usually describe how fertilization was occurring,” he says, “but they never tell you much about timing.”
For a flowering plant to be fertilized, pollen that lands on the flower must grow a tube to carry sperm to the egg. In non-flowering plants, such as conifers and ferns and mosses, the pathway is usually short because the pollen tube must destroy cells in its path, which is a time-consuming process. In flowering plants, though, pollen tubes are able to cover longer distances to the egg by essentially “squeezing” between cells. It is a trait that Williams says is vital to angiosperm diversification.
“The longer a plant takes to be fertilized, for the pollen to reach the egg,” says Williams, “the more chance there is for it to die.”
When he studied the data he had collected through the years, Williams found that older lineages of flowering plants—those on lower branches of the angiosperms’ evolutionary family tree—grew shorter pollen tubes than those that went on to evolve into the diverse array of flowering plants that exists today.
Confirming the Data
That’s what brought Williams to dangle from that treetop in the Australian forest. To confirm what he found in the data analysis, he pollinated—by hand—an ancient vine known as Austrobaileya scandens that grows high in the canopy. He chose that plant, along with another plant found only on the Pacific island of New Caledonia, and a water lily that grows high in the Colorado Rockies, to test because they developed as species early in flowering plants’ evolution.
He found that the older species grow shorter pollen tubes and take longer to do so than more diverse modern species. According to Williams, this indicates that these pollen tubes likely played a previously unknown role in spurring the evolution of the roughly 250,000 species of flowering plants we see today.
“As these plants gained the ability to grow pollen tubes faster and over longer distances,” says Williams, “It gave them the ability to develop the much larger and more complex flowers as well as deeper ovaries with more seeds—that is to say, larger fruits—that we see around us today.”
Williams’ findings, which were published in the Proceedings of the National Academy of Sciences in July 2008, drew attention from around the world, both within and outside the scientific community. In fact, the article was named one of the top 100 science stories of 2008 by Discover Magazine. It’s an honor that reflects the feedback and interest he’s received in his work since publishing the original research.
Williams has presented his findings at a number of conferences and has heard from colleagues across the spectrum of biology, many of whom would not normally pay attention to articles examining questions of evolution.
“Talking with people who don’t normally look at evolution—they’re eating it up,” says Williams. “It’s not that I did anything new, though I looked at some previously unexamined species. I just put together a lot of existing information in a way that made sense.”
Of course, the progress of research never really ends, and that’s especially true in evolutionary biology, where each new finding leads to another chapter in the story of how life on Earth came to exist as it does today. Williams is continuing to explore the topic of how plants are pollinated and fertilized. He is pursuing the question of how ancient flowering plants evolved a completely new reproductive biology, including studies that will seek the specific genes that govern how pollen gets sperm to the egg.
Two hundred years after Darwin’s birth and 150 years after the publication of The Origin of Species, the quest to understand evolution continues.Tags: Biology • Evolution • Joe Williams • Plants