Don't call it a comeback, but the regal Caspian tiger--thought to have gone extinct nearly 40 years ago--lives on in a closely related subspecies, a new genetic analysis reveals. Conservationists say they can use these relatives to help reestablish the Caspian tiger in Central Asia, parts of which are no longer inhabited by people and have plenty of suitable prey.
Once among the most widespread animals in Asia, tigers are now gone from more than 90% of their habitat. Biologists broke the original population down into eight subspecies, based on looks and geography, from the relatively small and dark Indochinese tiger of southern continental Asia to the massive Siberian tiger of the Russian far east. In 2004, researchers for the first time applied DNA analysis to the tiger family tree and confirmed the existence of five extant subspecies.
But the Caspian tiger, the last of which was shot in Turkey in 1970, remained an enigma. These tigers were isolated from other tiger populations by the Tibetan Plateau, leaving their origins a mystery. So researchers from the University of Oxford in the United Kingdom and the U.S. National Cancer Institute (NCI) Laboratory of Genomic Diversity in Frederick, Maryland, again turned to DNA. Oxford's Carlos Driscoll and his colleagues collected tissue from 20 of the 23 known Caspian tiger specimens kept in museums across Eurasia. NCI's Stephen O'Brien and colleagues then sequenced parts of five mitochondrial genes, which evolve rapidly and thus accumulate changes that can help distinguish subspecies. "That's allowed them to develop a picture of the distribution of tigers in a way that we haven't had before," says John Seidensticker, a tiger conservation biologist at the Smithsonian National Zoological Park in Washington, D.C.
Caspian tiger DNA was readily distinguishable from most other tigers' DNA. But when the team compared the genome of the Caspian tiger with that of the Siberian, or Amur, tiger, only one letter of genetic code separated them. Thus, the two subspecies are really one, with the supposed Siberian tiger splitting off from the Caspian tiger in the past century, Driscoll and his colleagues report today in PLoS ONE. The genetic data indicate that the ancestor of the Caspian tiger spread westward from China along what became the Silk Road. Then these tigers expanded northward and eastward, establishing what is now the range of the Siberian tiger. The researchers suggest that through the early 1900s, Caspian and Siberian tiger populations intermingled, but hunters subsequently isolated the two groups.
The find is good news for tiger conservation. "This is very exciting," says Seidensticker. "[We've] just lost part of a subspecies as opposed to a whole subspecies." Furthermore, he says, "we now know what source population to use to restore tigers in the Caspian Range." And because zoo-breeding programs for Siberian tigers are so successful, recovery efforts can use zoo stock.
That leaves two other extinct tigers to sort out: the Bali tigers and the Javan tigers, both from Indonesia. Efforts are under way to collect DNA and pin down their place on the tiger family tree.