Back from the Dead: New Hope for Resurrecting Extinct Plants
In January 1769, botanists Joseph Banks and Daniel Solander found a daisy in Tierra del Fuego, at the southern tip of South America. Later named Chiliotrichum amelloides, it is one of a thousand plant species unknown to European scientists that the two men collected during Captain Cook’s first voyage on the HMS Endeavor, braving treacherous seas and inhospitable landscapes to document every plant they encountered as they circumnavigated the globe. The plant was dried and pressed for future study. Today, the 254-year-old specimen is among the almost 8 million preserved plants in New York Botanical Garden’s William & Lynda Steere Herbarium.
For nearly five centuries, herbaria have helped botanists identify, name, and classify the world’s floral diversity. Now these vast botanical libraries are being tapped to try to create a new chapter in the 500-million-year history of Earth’s terrestrial plant life. In Nature Plants in December, an international group of biologists published the first-ever list of globally extinct plants they believe can be returned from the dead, using seeds available in herbarium specimens. Many of these plants are so-called “edge” species that represent a unique evolutionary lineage that has been lost.
“When a plant goes extinct,” says Giulia Albani Rocchetti, a postdoctoral researcher at Roma Tre University and the lead author of the paper, “we don’t just lose a species, we lose a member of a habitat community with a specific role and relations with other species; we lose millennia of evolution and adaptation; we lose genes which could have provided insight into the species and its community and yielded new pharmacological compounds and other products.” If the species can be brought back to life, there is a chance that all of that can be recovered.
Botanists are only now realizing the potential of specimen seeds to resurrect life forms believed lost forever.
In recent decades, the seeds of rare and imperiled species have been preserved in seed banks at low humidity and temperatures that ease the embryos inside into a kind of state of suspended animation to maximize their longevity. However, species already lost remain only as specimens in the collections of dried and pressed plants known as herbaria, and only in some (lucky) cases. Herbaria were never designed to prolong life. Only a few of these plants happened to be in fruit and in seed when they were collected. And even when herbarium seeds are discovered, there is no easy way to tell if the embryos inside are dead or lying dormant, waiting to sprout when conditions are right.
While herbarium specimens with seeds have been available for centuries, botanists only now are realizing their potential to resurrect life forms believed lost forever. Abby Meyer, executive director of Botanic Gardens Conservation International in the United States, points to the rise in recent decades of the field of bioinformatics, which has transformed the trove of biodiversity information once locked up in natural history collections — such as herbarium specimens of extinct plants that contain seeds — into browsable digital databases. New York Botanical Garden (NYBG), for example, began digitizing its herbarium specimens in the mid-1990s, and today some 4 million, or about half of its preserved plants, have been scanned and can now be called up on a computer screen by anyone around the globe. Data aggregators such as the Global Biodiversity Information Facility provide researchers looking for seeds with instant access to millions of scanned specimens, along with associated “metadata” such as the GPS coordinates where the plants were collected. At the same time, scientists have been refining in vitro embryo rescue techniques, increasing the odds that old or weak seed embryos can grow into viable plants.
In the words of McKenna Santiago Coyle, who showcased 16 extinct plants in an online gallery on the NYBG herbarium website, these specimens “are remarkable glimpses into the past, capturing a moment before something tragic happened.” For example, proposed de-extinction candidate Degener’s peperomia, Peperomia degeneri, is an evolutionarily unique member of the pepper family known from a single collection. In 1928, botanist Otto Degener plucked the plant from moist, shaded slopes on the spectacular eastern end of the Hawaiian island Moloka’i, home of the world’s highest sea cliffs. The species once perched on other plants and derived its moisture and nutrients from the air, rain, or debris that accumulated around it. During the past century, however, the forests of Moloka’i have been overrun by introduced invasive plants and animals, and the species has never been seen again.
While attempts to de-extinct the dodo, the woolly mammoth, and other charismatic megafauna continue to grab headlines, they would result at best in a hybrid, genetically engineered animal — a proxy of an extinct species. By contrast, recovering plants by germinating or tissue-culturing any surviving seeds or spores preserved in herbaria would result in the resurrection of the actual species.
Some seeds have the astonishing ability to survive adverse conditions and sprout after decades, even centuries.
But bringing plants like Degener’s peperomia back from the dead is a formidable challenge. One of the biggest hurdles is figuring out how to germinate the precious few seeds of often genetically unique plants found only on dried specimens. There is little margin for error, and before attempting to germinate the extinct species itself, scientists must perfect methods for germinating seeds of any closely related species that survive. Should researchers succeed, they will revolutionize plant conservation and demonstrate that, at least for some species, extinction is not forever.
In December 2019, Giulia Albani Rocchetti sat in Florence’s Central Herbarium, marveling over the remains of Ranunculus mutinensis, an endemic buttercup that once grew in moist floodplain forests of the Po River, as it threads through northeastern Italy. “I already had a vague idea of how difficult it was to find herbarium specimens with seeds, and how rare and valuable specimens of extinct species were,” she recalls. So it was a thrill for her to find not just one but two Ranunculus specimens with numerous mature fruits called achenes. She then spent months at her desk in Rome, blowing up digitized images of extinct plants from herbaria across the globe on her computer screen in the improbable search for seeds.
She was also spurred on by the knowledge that some seeds have the astonishing ability to survive adverse conditions and sprout after decades, even centuries — such as the Judean date palm, which a team of scientists successfully germinated in 2005 from a 2,000-year-old seed. And even though the chance of finding living seeds in herbarium specimens is slim, it is not impossible, given the nearly 400 million plant specimens in 3,000 herbaria worldwide.
Albani Rocchetti and colleagues at Roma Tre University began the project by analyzing online databases created by botanists all over the world, generating a diverse list of 361 globally extinct species, spanning 92 families of flowering plants. Scrutinizing digital herbarium specimens by using data aggregators such as the Global Biodiversity Information Facility, they identified 556 specimens that contained seeds, representing 161 of the extinct plant species.
She and her coauthors proceeded to devise a pioneering roadmap for prioritizing species for de-extinction. Assuming that species whose close kin produce long-lived seeds and newer specimens are the most likely to contain seeds that survive, they combined data on the seed behavior and longevity of closely related plants, as well as the age of each specimen, to create a DEXSCO, or best de-extinction candidate score for each species. Finally, they complemented these scores with a determination of each plant’s evolutionary distinctiveness. The plants they recommend as candidates for de-extinction are the 50 species with the highest scores.
The seed-bearing specimen of a small, grass-like plant called short-fruited spikerush, Eleocharis brachycarpa, was collected almost 190 years ago and is comparatively old for a de-extinction candidate. But the species, which was reported only from marshes in Texas and the neighboring Mexican state of Tamaulipas, scores high on evolutionary distinctiveness. Unlike fellow members of the sedge family, it had tiny leaves and photosynthesized mostly in its green stems.
“These are such rare commodities that you can’t just throw them in the soil,” a botanist says of the seeds of extinct plants.
Streblorrhiza speciosa, a spectacular member of the pea family, was another marvel of evolution, so unique that it is considered the only member of its genus, or closely related group of plants. The species’ striking cascades of pink blossoms clambered exuberantly over trees on Phillip Island in the Pacific Ocean east of Brisbane, Australia. Collected in 1804 by Austrian botanist Ferdinand Bauer, the Phillip Island glory pea was an instant hit in Europe, coveted by every wealthy family with a conservatory. Meanwhile, however, Phillip Island was being overrun by pigs, goats, and rabbits introduced by British officers overseeing a nearby penal settlement, leaving barely a scrap of the remote island’s unique vegetation, and the glory pea was never seen in the wild again. But the plant, which proved difficult to grow in pots without its native rich volcanic soil, appears to have died out in cultivation before 1850 in Europe. The glory pea is now presumed extinct, but at number three is near the top of the list of recommended de-extinction candidates.
Nicole Tarnowsky removes the folder containing Mangarevan chaff flower, Achyranthes mangarevica, from a steel cabinet in NYBG’s temperature- and humidity-controlled herbarium. As assistant director of the largest herbarium in the Western Hemisphere, she may handle dozens of specimens a day. But this one is different. “It’s sad. So sad,” Tarnowsky says. When it was found in 1934, the small tree with pointed oblong leaves and small, straw-colored blooms crowded on its multi-branched inflorescences was confined to a small fragment of moist forest on the precipitous southern slope of Mount Mokoto on the French Polynesian island of Mangareva. By then, the island’s native vegetation already had been decimated by continuous burning and grazing by goats.
Chaff flower seeds on the herbarium specimens, which may or may not be alive, are the only hope for the plant’s future. The prognosis is iffy. Old seeds from herbaria have been successfully germinated, but there are as yet no documented examples of plant de-extinction using seeds from herbarium specimens. One major complication is that little is known about the habits and preferences of such rare and often genetically unique creatures. In most cases, the only clues about the conditions where they grew are found on each specimen’s label. Occasionally, a few notes were scrawled on an attached piece of paper by the botanist who collected the plant. This information will be critical for the long, painstaking process of devising protocols for coaxing them to sprout.
“These are such rare commodities that you can’t just throw them in the soil,” says Wesley Knapp, chief botanist of NatureServe. For the past couple of years, he has been working with Valerie Pence, director of plant research at the Cincinnati Zoo and Botanical Garden, to attempt to germinate herbarium seeds of a few extinct species, including the blunt-flower rush, Juncus pervetus, which has not been seen in its sole recorded site, a Massachusetts saltmarsh, since 1928. So far, they have not had much luck.
Another challenge of plant de-extinction is the lack of financial support for pursuing it. But on the bright side, plant de-extinction has not kicked up the controversy surrounding attempts to resurrect, say, the wooly mammoth or passenger pigeon. “For whatever reason, the human brain doesn’t seem to be as concerned about plants as about animals,” Knapp says. “But in this case, we’re literally just germinating seed. We’re not reconstructing a genome. And that’s way less intimidating. Everyone can understand that.”
The increasing number of scanned herbarium specimens have allowed scientists to better document plant losses.
Yet some scientists have reservations about potential damage that could be caused by extracting seeds from precious herbarium specimens. In a 2021 paper in the journal Taxon, Albani Rochetti and colleagues report on a survey of the botanical community in which most survey respondents said they would permit the collection of a small number of seeds, preferably from duplicate specimens of the candidate species, but would favor stricter controls of others, such as so-called “type specimens” on which the description and name of a plant species is based.
A short walk from the herbarium in the New York Botanical Garden is Brugmansia suaveolens, a shrub or small tree native to Brazil commonly known as angel’s trumpet or angel’s tears. It is named after its dazzling foot-long, yellow or white, pendulous, and sweetly fragrant trumpet-shaped flowers. The species is extinct in the wild and is one of at least 39 plants worldwide that endure only in cultivation, one short step from extinction. “Genetic bottleneck is one of the largest obstacles for long-term survival of these plants,” says Meyer of Botanic Gardens Conservation International. De-extinction has the potential to not only resurrect species long gone but offer much-needed genetic variation for extinct-in-the-wild species, most of which cling precariously to life in extremely small numbers.
According to the current best estimate, almost 600 plants have gone extinct globally in the past 250 years — a rate about 500 times greater than the “baseline” rate at which plants would disappear without human influence. An analysis of plants in North America north of Mexico since European settlement found nearly twice as many had gone extinct, over a larger area, than previously estimated. “The actual extinction rate is undoubtedly much higher,” the researchers write, since some plants probably disappeared when European settlers moved into new areas, particularly the American West, before botanists could document the species that lived there.
The increasing number of scanned and easily accessible herbarium specimens have allowed scientists to better document plant losses. But according to a 2020 study, only about 38 percent of the vascular or higher plant specimens in herbaria are available in digital form; much of the rest remains “dark data” confined to smaller, regional herbaria.
As global efforts to digitize specimens continue, and more herbaria enter the world of big data, the number de-extinction candidates will inevitably increase. If researchers prove able to unlock the secrets of long-dormant embryos in herbarium collections, it may truly be possible not only to halt but reverse the rising tide of plant extinction.