Tigers Pass the Purity Test

17 April 2008 (All day)

Rupak De Chowdhuri/Reuters/Corbis

Spare tiger.
Purebred captive cats could be a valuable genetic backup for a wild population in decline.

Captive tigers are no longer the mutts of the conservation world. Kept by farms, zoos, circuses, and private owners, these caged cats often come with a dubious ancestry. But scientists have found that a significant number bear the genetic imprint of a single subspecies, raising their status from uncertain to purebred. The discovery could be a boon to conservation programs that breed captive subspecies as an insurance policy against extinction.

Wild tiger numbers have plunged in the last century, with an estimated 3000 left of the more than 100,000 that used to roam their natural habitat. Meanwhile, their comrades in captivity have continued to multiply, numbering 15,000 to 20,000 at last count. A small fraction belong to breeding programs that track their lineage and could potentially be reintroduced into the wild if a subspecies disappears. But many lack detailed records and are suspected to be hybrids of various subspecies, which may not be as well adapted to survive in specific environments. People assumed these "generic" tigers wouldn't be useful for conservation efforts, says Stephen O'Brien, a geneticist at the National Cancer Institute (NCI) in Frederick, Maryland.

O'Brien and colleagues decided to find out if the "generic" tigers had hidden value. They analyzed 20 years' worth of DNA samples from 105 captive tigers around the world. Using data from tigers of known ancestry as a reference, the team looked for telltale genetic patterns that would suggest a match with one of five subspecies. The scientists relied on both mitochondrial DNA, inherited only from the mother, and microsatellites, or sets of repeating sequence that differ in number between subspecies, to determine each tiger's lineage.

Almost half the tigers could be assigned to one subspecies, whereas the rest were of mixed heritage, the team reports online today in Current Biology. The proportion of purebreds in the overall captive population is probably lower, they note, because some of the analyzed tigers already had tentative subspecies identifications or belonged to managed breeding programs. A more reasonable estimate is 14% to 23% "purity" for unknown and unmanaged tigers, says team member Shu-Jin Luo, also a geneticist at NCI. She says that although traveling circus animals were usually mixed, tigers bred in localized regions had a higher chance of staying pure. The team also found new mitochondrial DNA and microsatellite features not yet recorded in wild tigers, suggesting that the captive population harbors genetic diversity that may have been lost in the wild.

The work is "a major step forward" because it will allow zoos to identify genetically valuable animals from private owners, says John Seidensticker, a tiger conservation expert at the Smithsonian's National Zoological Park in Washington, D.C. But Eric Wikramanayake, a conservation scientist at the World Wildlife Fund in Washington, D.C., cautions that releasing captive tigers into the wild may not work because tigers are highly territorial. Efforts should focus on habitat restoration instead, he says, so that wild populations can rebound on their own.

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