Finding Genes That Fit
Desperate to break up the genetic monotony that cripples endangered species, researchers are outfitting populations with borrowed genes. The payoff is survival. The price is uniqueness.
By Joe Roman
The Florida panther was in a desperate state. Its habitat was shrinking, and highway deaths were commonplace. After a recovery plan was drawn up in 1981, the Florida Fish and Wildlife Conservation Commission began to capture cats for radio telemetry. They found fewer than 25. Of these, only about a dozen lived in protected areas; the rest were on private lands, mostly cattle ranches on the northern edge of what was left of the big cat’s range. The remaining panthers looked closely related, sporting the kinked tails and cowlicks considered signs of inbreeding. More than 80 percent of the males were cryptorchid, with only one descended testicle or none at all. They had poor sperm quality and low testosterone levels. Many panthers had heart defects and high loads of parasites and infectious pathogens. When author Charles Bergman visited South Florida in 1986, he wrote that the panther was a “deeply endangered animal, perhaps unsavable.”
For years, biologists claimed that the Florida panther was a unique subspecies of puma and should be preserved in its pure form, kinked tails and all. The panther appeared on license plates; school kids elected it the state animal. But with only about two dozen left in the world, state and federal managers seemed faced with a choice: preserve the Florida panther as a unique subspecies and probably watch it disappear, or introduce some new cats to increase genetic diversity and reduce inbreeding. Such a genetic rescue could protect the felid’s place in the ecosystem, if not the original cat itself.
Historically, there had been gene flow between Texas cougars and Florida panthers, so how much would it hurt to re-establish that corridor, even if it involved the transport of a few cats from Big Bend? Genetic rescues had worked for prairie chickens in Illinois and for adders in northern Europe. Perhaps the import of new genes from Texas could save the panther as well. As with earlier endangered-species interventions, such as bringing all the black-footed ferrets and California condors into captivity to prevent their extinction, the decision was controversial. Some argued that the kinked tails and isolation were part of being a panther in the twentieth century; new genes would only contaminate it. Others claimed that only assertive habitat protection would allow the cat to increase its numbers and expand its range. But time was running out. If nothing were done, models predicted a 95-percent likelihood of extinction within 20 years.
John Lukas, director of White Oak Plantation in northern Florida, was eager to break the impasse. In October 1992, he invited scientists and decision-makers to the plantation—a 7,400-acre resort and posh wildlife-conservation center that held a couple of captive panthers—to consider the options and perhaps come to an agreement. Stephen O’Brien, who now heads the Laboratory of Genomic Diversity at the National Cancer Institute, and Melody Roelke, a field veterinarian who had worked with the panthers for seven years, voiced strong concerns about the genetic and reproductive health of the cats, arguing for a radical change in federal and state strategy. O’Brien told the group that it would take only a few new individuals to improve population numbers, increase depleted genetic diversity, and reverse the death spiral of inbreeding. At the workshop, a resolution was passed in support of a genetic rescue—but nothing happened for three years.
Then, in spring 1995—after much bureaucratic foot-dragging and political wrangling—the rescue finally began to take shape on the ground. Roy McBride, a big-cat tracker from Alpine, Texas, captured eight female cougars in the Big Bend region along the Rio Grande. The cats were held in quarantine, checked for disease, and then flown to Florida on commercial airlines in crates built by McBride. Six of the animals were released among the core population just east of Naples, and two were let out in the Everglades. The results have been convincing: in 1990, 88 percent of the Florida population had kinked tails. By 2000, not a single first- or second-generation kitten born to the Texas females had the characteristic 90-degree crook in the last five vertebrae. The hybrid kittens had a higher survival rate than the purebred panthers, the females lived longer, and most bred. Most of the young males had healthy reproductive systems. The spread of the new lineages wasn’t always peaceful: Florida panthers usually lost their battles with the new hybrid males, fights that sometimes resulted in death. Panthers started expanding their range into habitats once thought unsuitable, and the population increased from about 30 cats to at least 95. Every one of them has some Texan DNA.
A few years ago, I got a first-hand look at the “new” Florida panther. I accompanied Darrell Land, a panther biologist with the Florida Fish and Wildlife Conservation Commission, on a routine telemetry flight over southwestern Florida. Land and his colleagues flew three times a week, 52 weeks a year, to pinpoint the location of about 20 panthers east of Naples. As we waited for a thick fog to lift, I asked Land, who is stout with closely cropped hair and who speaks directly, about the restoration project. “In my mind,” he said, “it’s less about the uniqueness of the subspecies and more about healthy cats.” Land acknowledged that this was one of the first attempts to restore an inbred population in the U.S., though captive-raised animals were becoming increasingly common.
Peregrines, he said, were successfully reintroduced to the eastern U.S. starting in 1974. The local bird, known as the “rock peregrine,” had already been extirpated. After DDT was banned, a captive stock made up of seven subspecies from four continents was used to re-falcon the Midwest and East. With widely different genetic heritages, the birds have thrived. Some birders protested the artificial introductions and the lack of genetic counseling, but Land remarked, “A peregrine is a peregrine is a peregrine.”
Lifting off from Naples airport, we left the terra-cotta roofs, turquoise pools, and slate-gray roads behind and flew over a gray-green landscape of swamp forest, pinelands, and hardwood hammocks: core habitat of the Florida panther. O’Brien claimed that it was this land—with its mosquitoes and poisonous snakes—that saved the swamp cat. “A hundred and fifty years ago, the Florida panther had a range that spanned the American Confederacy. But by the time we got to the seventies, everybody thought it was extinct. The only reason it wasn’t is that it was hiding in the swamp, where nobody went because it was unpleasant.”
Aerial surveys have made the search for panthers much easier. The H-shaped antennas strapped to the airplane’s struts allowed Land to pinpoint the location of a radio collar across miles of open land. That morning, from the bumpy skies above Fakahatchee Strand and the Florida Panther National Wildlife Refuge and adjacent private ranches, he located 22 panthers.
With a tilt of his hand, Land instructed the pilot to circle over each radio signal, and he marked the location on well-worn U.S. Geological Survey topological maps. Once green, the quadrants with favored panther rest areas were now cratered with the black ink of Land’s minute scrawl. We circled Panther 83 on the edge of the strand, just north of the Tamiami Trail. Panther 59 was resting off the Janes Scenic Drive, east of Picayune Strand. One cat was in a hardwood hammock not far from Wild Cow Island, another within earshot of State Road 832 between Collins Slough and Tom Still Hammock. Almost able to make out the individual palmettos, I hoped to catch a glimpse of a tawny coat among the fronds. But by the eighth or ninth panther, I started to dread the 45-degree tip of Land’s left hand, the abrupt tilt of the horizon, and the stomach-churning spiral above yet another radio pulse—and there were still many panthers to go before we landed.
Farther north, we circled above forested scraps in the midst of a cattle ranch, the cows and calves lazing in the sun just a hundred yards from a sleeping panther. Ranchland, citrus groves, sugar cane, and truck crops—the tomatoes, cucumbers, red peppers, and other produce that filled salad bowls in winter—did not necessarily exclude big cats.
When McBride started tracking the Florida panther in the 1970s, he and his hounds often spent weeks looking for a new cat. “Now,” he said, “I can’t find a place to exercise my dogs without running into one.” At first McBride didn’t see much behavioral difference between the new hybrid offspring and their pure Floridian relatives. But now, he says, they’re more vigorous. When the offspring of the Texas cats are treed, they escape more often or put up more of a fight.
Looking back, one wonders: what really was the difference between the Florida panther and the Texas cougar? Not much, it turns out. Both cats are pumas—Felis concolor, one of the most widespread carnivore species in the world, found from Tierra del Fuego to the Yukon. Mountain lion, catamount, cougar, painter, panther—these are just a few of its common names. Although there are a couple of characteristics that have been used to distinguish Florida panthers from other pumas—Roman nose, white flecks on the neck and shoulders (lately blamed on ticks, not genetics), dorsal cowlick, and crooked tail—before it became isolated in the swamps of South Florida, its range was contiguous with that of the Texas cougar. Historical gene flow, evidence that all North American pumas descended from a common ancestor 10,000 years ago, supported the genetic restoration of the panther. The North American puma, like the saber-toothed tiger, American lion, and cheetah, probably went extinct after Pleistocene glaciers covered the continent, bringing cold temperatures—and humans—over the Beringia Land Bridge. With most of the other large carnivores gone after the ice receded, a small band of pumas moved north from Central America. Their descendants would become North America’s dominant feline.
Big cats and wilderness are deeply intertwined in the human psyche. Wilderness has the same Norse roots as “will” or “willful,” uncontrollable. “Wild” conveyed the idea of being lost or disordered. One of the earliest uses dates to the eighth century: in Beowulf, “wildor” refers to the savage beasts of the dismal forests, cliffs, and crags. In the U.S., the Wilderness Act of 1964 defined wilderness as “an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain.”
Yet the paradox we now face is that in order to protect our notion of wilderness, we must meddle in it more and more, from the landscape level all the way down to the nucleotides that form the DNA of each species. The transfer of Texas cougars to Florida was controversial, but at least they were pumas, closely related members of the same species. In the race to restore rare plants, molecular biologists have gone even further. They have started moving genes between species and even directly from the pathogens themselves into their hosts.
Until the nineteenth century, one of every four trees in the Appalachians was a chestnut. The tree defined the region, turning the mountains into white caps when its creamy-colored catkins bloomed in spring; in fall, the trees showered the forest floor with nuts as red as fire coals, forming a bed four inches deep. Turkeys, passenger pigeons, elk, and bear thrived on the mast. The nut crop was so thick farmers didn’t need pasture land—they notched the ears of their cattle and hogs before turning them loose in the forest to feed. Chestnut-flavored mountain pork hung in smokehouses throughout the region, esteemed by nineteenth-century foodies. After an autumn storm, families raced out to beat the hogs to the crop—an entire winter’s worth of food could be gathered in a month. Sacks of chestnuts were hung near the kitchen all winter to be baked over coals. Some were sold for “shoe money,” the earnings spent on children’s winter footwear.
Commercial timber harvest almost ruined these mountains. At its peak, about 4 billion board-feet were cut from the region, enough to build a plank road 30 feet wide around the equator—but it took a microbe to reduce this mighty tree, the tallest of the eastern hardwoods, to a rare understory shrub. Around 1900, a blight hitchhiked on chestnut trees imported from Japan. Spores were spread by raindrops and squirrels, the fungus entering the trees through any wounds present, killing the cambium between the bark and heartwood and girdling the tree. The disease was first observed at the Bronx Zoo, where the oldest trees were reduced to stumps. It spread south to Mississippi, north to Maine, throughout the chestnut’s range. At least seven native moths have gone extinct since the tree disappeared.
With its tall, vase-like shape, the American elm was the perfect street tree, creating a beautifully dappled canopy in cities and suburbs of the East and Midwest—until 1928, when another Asian fungus, this one spread by bark beetles, reached the U.S. Dutch elm disease attacked the sapwood of these once-common trees, killing more than 70 million. The elm survives in isolated pockets and along the edge of its original range, in Canada and Florida, in Central Park, and in Tompkins Square Park. Manhattan’s steel-and-concrete walls formed battlements, protecting the species from attack.
Can genetic engineering restore the chestnut, the elm, and other species wiped out by pathogens? It’s worked in fruit trees. On Oahu in the 1950s, ring-spot virus devastated papaya trees, reaching the Big Island by the 1990s. Broad leaves were reduced to shoestrings, stunting growth. Infected young trees never produced fruit. Genetic researchers injected into a line of trees the gene for a protein in the virus’s coat. The new lineage proved resistant to the virus while producing high-quality fruit. Transgenic fruit trees, controversial in some circles, are one thing; transferring DNA between species to restore a lost ecosystem quite another. The American Chestnut Foundation has interbred American chestnuts with Japanese and Chinese species, the original (and blight-resistant) sources of the disease. Early attempts failed: the trees looked too Chinese—the leaves too thick and oval, the nuts too large and not sweet enough. The goal is to get a tree that looks, and tastes, like the Appalachian native but acts as if it had battled the blight for generations—and then to release it back into the mountains. The depleted surface mines of the Appalachian coal region are a logical place to start.
More contentious is the introduction of bacteria into these trees to help them defend against foreign microbes. Agrobacterium can cause tumor-like galls in plants by transferring its own DNA into their roots, but it has also been used to deliver genes into the germ lines of soybeans, sugar beets, and other genetically modified food. Can this pathogen save the chestnut and the elms? An antimicrobial peptide effective against bacteria, fungi, and viruses has been inserted into the DNA of the American elm. (It’s designed to break down in the mammalian digestive system and have little effect on plant growth.) Field tests are now comparing these transgenic trees to Liberty elms, which are resistant to Dutch elm disease, and to wild types. The trees with the greatest resistance will win.
We have, in effect, become pastoralists of the wild and arborists of the forests. Speciation in the bigger vertebrates is over—our largest protected areas are just too small to foster new species of elephants, rhinoceroses, apes, bears, and cats. So we throw a few new alleles into the genetic monotony of these isolated populations.
The species we’ve chosen to save bear our mark, just as domestic animals do. Lions and other large mammals are regularly moved among large fenced reserves in South Africa. The Amur leopard, down to about 50 individuals, will probably require a genetic rescue if it is to avoid extinction in the wild. I think of hand shadows on the wall: an elephant’s trunk, made with middle and ring finger. The fingered wings of a bird in flight. The back of a hand arched into a tortoise shell.
What are we conserving when we add new genes? Sure, we’re rebalancing an ecosystem, but the Florida cats are now part panther and part in situ genetics experiment. (When biologists noted that Texas cougar 101 was more fertile than expected, having raised eight kittens that survived to maturity, they administered a long-term contraceptive to prevent the female from becoming “genetically overrepresented.”) If their numbers don’t increase, it’s probable that inbreeding will once again take its toll on the panthers. More cougars will be needed to restore the genetic health of the population, further eroding its uniqueness. If a panther is a puma is a cougar, then perhaps it doesn’t matter very much. Perhaps we should be glad that even a few strands of homegrown Florida DNA from individuals such as Panther 59—battered in youth, victorious in maturity—have persisted. The eastern cougar disappeared from Appalachia and New England more than a century ago. Whatever unique genetic traits it had are gone forever.
–Joe Roman is a conservation biologist and author at the Gund Institute for Ecological Economics at the University of Vermont. His latest book, Listed: Dispatches from America’s Endangered Species Act (from which this article was adapted) is available from Harvard University Press. His science and nature writing has appeared in The New York Times, New Scientist, Audubon, Conservation, and other venues.
Adapted from Listed: Dispatches from America’s Endangered Species Act by Joe Roman, published in May 2011 by Harvard University Press. Copyright ©2011 by the President and Fellows of Harvard College. All rights reserved.
Illustrations by David Badders