The Bizarre, Putrid Beauty of the Corpse Flower

Scientists struggle to understand the parasitic behavior of one of the world's largest and smelliest flowers before it disappears.

By Hillary Rosner|Wednesday, December 11, 2013
Blooming corpse flower
Blooming corpse flower
A Rafflesia, one of the world’s largest and smelliest flowers, blooms on Borneo, in Southeast Asia.
Frans Lanting, Mint Images/Science Source
On a table in Harvard University’s herbarium, six slimy-looking brownish lobes, each roughly the size of your palm, float in a tub of ethanol solution. They’ve been sliced in half and their flat sides face the ceiling, revealing strange patterns of grooves, bumps and tentacle-like projections. If you had to guess, you might say
they were sections of something that grew in the deep ocean.

In fact, they’re pickled flower buds. And though their drab hues and soggy miens suggest otherwise, they’d have bloomed, if left in their rainforest homes, into some of nature’s largest, weirdest and worst-smelling flowers. The plants, which graduate student Lachezar Nikolov collected in the jungles of Malaysia and Thailand, belong to the genus Rafflesia. Their bright red and yellow flowers can grow up to 3 feet in diameter and weigh more than 15 pounds. But unlike most plants, Rafflesia possess no leaves, roots or stems. They don’t even practice photosynthesis, the basic method of energy-making common to nearly all plants. Instead they bloom from within another species, stealing nutrients from massive vines that are members of the grapevine family. Rafflesia are enormous parasites.

Add in the fact that they emanate the smell of rotting flesh, and it’s no wonder Rafflesia, known as corpse flowers, have captivated naturalists for more than two centuries. Yet these rare organisms are only just beginning to reveal their secrets. Because Rafflesia are so incredibly unusual, scientists have only recently figured out where they came from — which ancestral plants ultimately morphed into these biological curiosities. “There are so many fundamental questions,” says Nikolov.

But in the lab of Nikolov’s adviser, Charles Davis, provocative facts about Rafflesia evolution are starting to pile up, and they could prove useful in a range of fields, from the biology of scent to parasitology.

Corpse flower bud
Corpse flower bud
A Rafflesiaceae bud is cracked open in the Harvard lab of graduate student Lachezar Nikolov.
Lachezar Nikolov

Evolutionary Mystery

The first time Davis saw a Rafflesia flower blooming in a Malaysian rainforest, he couldn’t believe it was real. It was a sweltering day in northern Borneo in 1995 and Davis, then an undergraduate, was collecting plants at the foot of Mount Kinabalu. He’d been studying plant diversity, and this flower, curiously emerging from a twisted thicket of vines, seemed to defy all rules. Davis was transfixed. “It was warty like a toad, smelled like a dead animal, felt wet like a sponge,” he recalls. “It was just so totally strange and otherworldly to me.”

Davis was hardly the first to succumb to the lure of the corpse flower (not to be confused with Amorphophallus titanum, which has a giant phallic protrusion and also happens to smell like rancid meat and bear the nickname corpse flower). In 1818, naval surgeon and naturalist Joseph Arnold, one of the first Europeans to collect a Rafflesia, called it “the greatest prodigy of the vegetable world.”

The mystery of this bizarre flower only deepened over the decades that followed, as a succession of biologists was drawn to it but unable to classify it, or the dozen or so other species of the family Rafflesiaceae they discovered in the forests of Southeast Asia, all lurid and stinking of decay.

Davis returned to his undergraduate studies, but he never forgot the flower he’d seen. A decade later, as a postdoctoral researcher, he was working on a multi-university collaboration called the Tree of Life, to piece together the phylogeny — that branching tree of evolutionary history — for all flowering plants.

The puzzle, known as “Darwin’s abominable mystery,” was to figure out how a quarter-million-plus species of flowering plants came, in a very short span of evolutionary time, to dominate the Earth.

Fly on corpse flower
Fly on corpse flower
A fly, and pollinator, is attracted to the mottled flesh tones and rancid smell of a Rafflesia flower.
Lachezar Nikolov
Davis’ section of the puzzle was a large order of around 20,000 plants called Malpighiales, which first appeared more than a hundred million years ago and quickly diversified to include a large percentage of the planet’s tropical forest diversity.

In 2004, a group of researchers made an important discovery as they compared the genetic makeup and mitochondrial DNA (one of three types of DNA in plants, but the one that researchers had tended to ignore because it evolves relatively slowly) of Rafflesiaceae species to known Malpighiales: It turns out that the Rafflesiaceae family is part of Malpighiales. Up until then, it had been anyone’s guess where these bizarre parasitic plants came from. Suddenly, the flower that had captivated Davis that steamy day on the island of Borneo was officially part of his funded research agenda. He pounced.

From the DNA, Davis teased out more details on the plant’s origins. He discovered that Rafflesia evolved from plants with some of the world’s smallest flowers, whose blossom size ballooned nearly eightyfold over 46 million years. For scale, if a common white Shasta daisy grew 80 times bigger, its petals would stretch the length of a shipping container. “It makes the plants that much more mysterious,” says Davis. “What drove the evolution of these ginormous flowers?”

Corpse flower cells
Corpse flower cells
Rafflesiaceae cells (colored yellow) invade and start to grow in the tissue of a Tetrastigma vine.
Lachezar Nikolov

The Smell of Success

Davis and his colleagues suspect the answer lies in Rafflesia’s pollination strategy. The corpse flower employs what scientists call carrion mimicry. “They go in for this phenomenon of reproductive deceit,” Davis says, referring to the plants’ method of attracting flies that can aid in pollination. “They look like big rotting pieces of flesh — darkish reddish-burgundy colored, mottled with white, really faithful to this model they’re mimicking.” And then there’s the smell.

Unpleasant as it may be to humans, that sickly odor is clearly not intended for us. It is meant to entice pregnant flies, which, in the process of looking for a place to lay eggs, can spread pollen from male to female plants. Eager to uncover more clues about how this olfactory trick might help shape evolution, Davis is now working with Robert Raguso, a chemical ecologist at Cornell University who studies communication between flowers and pollinators. The odor of death may be what initially attracts the flies, but once they enter the flowers another scent may cue particular behaviors the plants want. Such a highly choreographed pollination dance could easily drive natural selection, allowing Rafflesia to attract pollinators and, over time, grow larger. But exactly how or why remains an enigma.

Part of the corpse flowers’ evolutionary success could also be due to their unusual parasitic behavior. Under a microscope, before they bloom, Rafflesia appear as distinct little necklaces of cells that wind their way through their host vine’s tissue. To the naked eye, though, Rafflesia are invisible until they finally bud, emerging like a spongy tumor from the skin of their tangled jungle hosts. While trying to explain how the flowers grew so peculiar and huge, Davis has also made other discoveries. A few years ago, he showed that in addition to stealing carbohydrates, minerals and other resources from its host vines, Rafflesia are also stealing genes. While this phenomenon, known as horizontal gene transfer, is fairly common in bacteria and other microorganisms, this was the first compelling evidence that multicellular organisms were swapping genes.

Corpse flower copperplate engraving
Corpse flower copperplate engraving
A hand-tinted copperplate engraving circa 1820 captures Rafflesia arnoldii.
Paul Stewart/Science Source

More recently, Zhenxiang Xi, a postdoc in Davis’ lab, used new DNA sequencing methods to show that far more of the hosts’ genes are pirated than anyone suspected — more than 2 percent of expressed nuclear, or primary, genes and up to 40 percent of mitochondrial DNA are stolen from the vines. One theory, Davis says, is that costuming themselves in pieces of their hosts’ DNA may help the flowers carry out their heists undetected. If true, the discovery could help explain how all sorts of parasites — including those that prey on humans — take up residence without attracting attention. Xi’s work also shows that even the genes that Rafflesia plants inherited from their own ancestors have appropriated the characteristics of the vines’ genes; up to 30 percent of the corpse flowers’ native genes code for proteins (the process that helps to determine physical traits) in a way that’s more like the vines’ than like its fellow Malpighiales’. This could prove to be yet another stealth strategy.

A Rare Find

Hard science is finally chipping away at a centuries-old riddle, but the quest now involves another obstacle: a race against the clock. Evolutionary prodigy though it is, Rafflesia aren’t doing too well these days. Those enormous buds that bobbed in Nikolov’s ethanol solution take nine months to form, and when they finally bloom, the flowers last only a week before turning into what Nikolov calls “a slimy mush” on the forest floor. Male and female plants appear to live far from one another. As rainforest is slashed and burned in Southeast Asia, corpse flower romance is increasingly a matter of luck — “highly improbable,” as Nikolov puts it. “I’ve never seen two open at the same time,” he says. And yet, somehow, the plants have survived.

Back when Arnold, the intrepid 19th-century naval surgeon, journeyed into the Sumatran interior, it was an arduous journey involving ships and trains, dinghy voyages through unknown swamps and nights spent in primitive huts under the specter of malaria. Today, getting to the rainforests where Rafflesia still cling to existence is as simple as suffering through some long-haul flights and checking into a bed-and-breakfast in Kuching or Chiang Mai. Yet the lure of the plant is no less seductive. Generations of naturalists have traveled deep into its jungle home, but for modern DNA explorers like Davis, the odyssey is just beginning.

[This article originally appeared in print as "Exquisite Corpse Flowers."] 

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