Captive Tigers Harbor Rare "Purebred" Genes

Nearly half of tested captive tigers are “purebred” members of an endangered subspecies, raising the possibility they could bolster conservation efforts, a new genetic analysis suggests.

Similar screening of some of the thousands of tigers with unknown heritage held on farms and by private owners would considerably increase the number of animals useful for captive breeding programs, the scientists say.

The news comes at a dire time for wild tigers. As few as 3,000 individuals remain where more than 100,000 roamed just a century ago. Three of the eight subspecies have become extinct, and a fourth, the South China tiger, persists only in zoos.

(Related: “India’s Tigers Number Half as Many as Thought” [August 7, 2007].)

The number of captive tigers, on the other hand, has boomed. Zoos, farms, circuses, and private owners hold an estimated 15,000 to 20,000 tigers. Only a small fraction of these are part of breeding programs oriented toward conservation.

“The captive population of these wild animals has been justified based on the principle that they are the genetic representation of their natural counterparts,” said study leader Shu-Jin Luo, who studies genetic variation at the National Cancer Institute in Frederick, Maryland.

“They can act as insurance against extinction in the wild.”

Genes Match Geography

But many owners have no knowledge of their cats’ ancestry.

To tease apart their heritage, Luo and her colleagues developed a test based on variations in 30 locations on the tiger genome that had originally been identified in domestic cats.

In an earlier study, Luo and her colleagues found that they could separate wild tigers into groups that corresponded to the recognized subspecies based on how many versions of these genetic markers the animals shared.

In the new study, the scientists screened DNA samples from 104 captive tigers living in 14 different countries. Of those, 49 could be confidently assigned to a particular subspecies, they report in Current Biology today.

“These are fairly closely related lineages that they’re trying to sort out,” said Michael Russello, a conservation geneticist at the University of British Columbia in Kelowna who was not involved in the study. “I think it is remarkable that they were able to find individuals from unmanaged populations that actually are purebreds of a given subspecies.”

Genetic Surprises

The numbers of purebred tigers the researchers found likely overestimate the proportion of unmanaged animals with pure lineages, because many of the owners who sent samples to the team had some idea of their tiger’s ancestry.

Of the 50 tigers without any pedigree, 7 could be assigned to a particular lineage based on the new genetic test.

Luo thinks that 15 to 23 percent of tigers not part of conservation breeding programs are likely to be potentially useful for preserving genetic variation unique to endangered subspecies.

Given the size of the captive population, that would mean thousands of additional useful animals.

The test can also reveal mixed ancestry. Owners of 11 tigers who thought they had purebreds turned out to own mutts instead.

“It’s extremely important to know those individuals that have a hybrid origin,” Russello said. Those animals should be excluded from breeding programs.

In addition, the researchers found that the captive tigers harbored at least 46 new genetic patterns that have not been found in wild animals so far.

Some of these occurred only in mixed-lineage tigers, which are currently thrown out of breeding programs when discovered.

While the findings come as welcome news, most conservationists agree that breeding programs are a last-ditch resort and that efforts should focus on protecting existing wild populations.

Tigers once ranged from the Indian subcontinent to Siberia.

Today that swath has been reduced to small remnant habitats, often depleted of prey. (Related: “Tiger Habitat Plummeted 40 Percent in 10 Years, Survey Finds” [July 20, 2006].)

“We are not at that stage where we are looking to reintroduce tigers from captive populations,” said Mahendra Shrestha, who directs the Save the Tiger program based in Washington, D.C.

“The main challenge we are facing now is providing good quality habitat.”

http://news.nationalgeographic.com/news/
2008/04/080418-extinct-tigers_2.html