SANTA CRUZ, CA -- The cheetah, spotted embodiment of feline grace and power, races across African plains. The pocket gopher, small and squat, scurries among tunnels in the American West. Unbeknownst to either cheetah or gopher, research at the University of California, Santa Cruz, has unveiled a common bond between them -- and has bolstered one side of a hot cheetah debate.
That bond is remarkable genetic similarity between individuals within each species. Many scientists suspect that one reason behind the decline of cheetahs is their lack of genetic variation--a degree of sameness close to that seen in inbred laboratory mice. Key evidence came from a surprising 1985 experiment in which unrelated captive cheetahs accepted skin grafts from each other, a sign that their immune systems were genetically almost identical. However, in recent years some ecologists have questioned the validity of that work.
The UCSC study provides an independent check. Researchers M. A. Sanjayan and Kevin Crooks used pocket gophers in the first attempt to repeat the 1985 experiment on wild animals. Just as with the cheetahs, gophers in populations with little genetic variation accepted skin grafts from one another. However, gophers from a population with higher genetic diversity rejected skin grafts from their neighbors--an experimental control that the 1985 study lacked.
These results led Sanjayan and Crooks, both Ph.D. candidates in biology, to conclude that the cheetah study was accurate. Consequently, cheetahs and other genetically impoverished species, such as California sea otters and elephant seals, may have little variability among their immune systems and thus may be more vulnerable to outbreaks of disease.
"Genetic variation is the stuff of evolution," says Sanjayan. "It allows a population to adapt effectively to changes in the environment. You exist at a price if you have low genetic variation. It's like having lots of clones of yourself."
If animals within a population are similar to each other immunologically, Sanjayan adds, "That doesn't bode well for disease resistance. If a new disease arises, either they'll all be resistant or they'll all be wiped out. You may not get a middle response, where at least some individuals make it. That's something you'd expect with other species."
Sanjayan and Crooks will present their work at a joint meeting of the Ecological Society of America and the Society for Conservation Biology, August 10-14 in Providence, Rhode Island. They also published a synopsis of their findings in the June 13 issue of the journal Nature.
Scientists led by geneticist Stephen O'Brien of the National Cancer Institute began exploring the genetic troubles of cheetahs in the early 1980s. Their skin-graft paper, published in Science on March 22, 1985, greatly impacted the field of conservation biology. The costs of genetic uniformity among cheetahs, the scientists theorized, could include high rates of juvenile mortality, problems with captive breeding, and greater risk of decimation by a virus or other disease.
Some field ecologists doubt parts of O'Brien's analysis. They point to apparent weaknesses in the design of the skin-graft experiment; for instance, the researchers used skin from domestic cats as a control. More profoundly, detractors argue that ecological factors, such as habitat loss and predation, are more vital to cheetahs' survival than genetic diversity. No one has found a direct link, they say, between genetic shortcomings and the decline of cheetahs or, for that matter, any other species.
The UCSC study does not address these latter concerns. It does, however, show that inbred mammals can be so genetically alike that their immune systems distinguish poorly between self and nonself. The proof: The animals accept skin grafts--in effect, organ transplants -- from each other.
"No one had found another wild animal that could tolerate skin transplants from other members of its species, so people disbelieved the cheetah results," says conservation biology pioneer Michael Soule, research professor of environmental studies at UCSC and adviser to Sanjayan and Crooks. "We showed it could happen in a natural population."
The team studied 44 animals from two populations of the pocket gopher, Thomomys bottae, from Humboldt County in northern California and a third group from the Carmel Valley. Gophers within each of the northern populations were so similar genetically that a DNA fingerprinting analysis by coworker David Foran could not separate individuals. The Carmel Valley gophers had much higher genetic variability, more typical of most mammals.
Each Humboldt gopher accepted grafts of small patches of skin from other animals within its own population, whereas the Carmel Valley gophers did not. To test whether the immune systems of the Humboldt gophers functioned at all, the researchers grafted skin from the Carmel Valley gophers onto a few of the Humboldt gophers; the animals rejected them.
Soule notes that the range of genetic variation among
different populations of pocket gophers, from extremely low
to extremely high, makes Thomomys bottae an ideal model
system for exploring the costs of genetic uniformity to
endangered species. His team now is investigating the
effects of low genetic variation among pocket gophers on
disease resistance, physiological fitness, and their
survival in the wild. Team members will present early
results at the August meeting.
GENETICS, July 22, 1996