As different as they may seem, corn and daylilies have a lot in common. So do towering palm trees and diminutive lady’s slipper orchids. Thanks to a common ancestor 137 million years ago, the roots, seeds, and sometimes leaves of these flowering plants—known as monocots—look alike. Now, a new genetic study reveals why: Even though all of these plants are landlubbers today, their ancestor lived in water.
The work is convincing, says Peter Stevens, a systematist at the University of Missouri in St. Louis who was not involved with the study. “It allows you think about the origin of monocot features.”
Scientists have long had trouble placing monocots, whose seeds contain just one embryonic leaf, on the plant family tree. (Most flowering plants are eudicots, which have two such leaves in their seeds.) That tree is key to understanding the evolutionary relationships of the world’s 85,000 monocots, which include staple crops like corn and rice, the grasses eaten by cows, palm trees, and some of the world’s prettiest flowers, such as orchids and lilies.
“In virtually every one of the [monocot] families, you can point to beautiful and economically and ecologically important members,” says Elizabeth Kellogg, a plant biologist at the Donald Danforth Plant Science Center in St. Louis who was not involved with the work.
Knowing how important an accurate family tree was—especially for crop breeding and basic research—Thomas Givnish, an evolutionary biologist at the University of Wisconsin in Madison, pulled together about 19 fellow biologists to draw up the most definitive version to date. They sequenced the DNA in the chloroplasts of 545 monocots and of 22 other plants. Based on similarities in the plants’ DNA, the team worked out family connections and estimated the age of each branch. “We have very strong support for most of the relationships,” Givnish says. Among their discoveries: Bananas branch off closer to gingers and heliconia (flowering plants known as “lobster claws”) than previously thought.
“What is really new is the amount of data that they have thrown at the whole problem,” Stevens says. Many of the relationships—including the banana-ginger one—had been suggested before.
Most striking is what’s at the base of the tree, Givnish says. The nonmonocots most closely related to that base indicate the first monocots were aquatic plants, Givnish’s team reported last month in the American Journal of Botany. Botanists in the 1800s were the first to suggest this idea, and several researchers also explored this origin in the 1990s, but none had the genetic data that now back it up, he says. Not just seeds, but monocot leaves and roots are different from those of other flowering plants, and the aquatic origin may explain why.
For example, monocot leaves tend to have parallel veins running the long way up the leaves, whereas other flowering plant leaves have branching veins. The branching veins keep the paper-thin leaves stiff; otherwise gravity would make them flop over. But leaves in monocots’ aquatic ancestors presumably floated and thus could do with a less extensive—and expensive—support system. Also, leaves in most flowering plants attach to the stem through a base called a petiole. But leaf bases in monocots tend to clasp the stem with an array of “fingers,” which makes sense if swirling water tossed the leaves every which way, Givnish says. Monocot roots also show little branching, like aquatic plant roots. And most monocots are herbaceous, not woody; if their watery ancestors put on wood layers every year like most trees, the new growth would have interfered with air tubes reaching from leaves to the plants’ underwater parts.
As comprehensive as this new family tree is, it needs refining, Kellogg says, so that more than just monocots’ larger groups are in their proper places. To do that, Stevens says the team would need to compare DNA, not from the chloroplasts, but from the much larger amount of DNA stored in cells’ nuclei. This work is already under way, says Givnish, whose team has analyzed 500 genes from nuclear DNA from a wide array of species. The team’s new findings “largely support the same patterns of relationships,” and should be published in a few months.