Fewer than 4,000 tigers remain in the wild. New research aims to give conservationists a better understanding of their genetics in order to save them.
After years of debate, scientists in the journal Current Biology report that tigers consist of six unique subspecies. One of these subspecies, the southern Chinese tiger, survives only in captivity.
"The results presented in this article are important because they contradict the currently accepted international conservation ratings for tigers," said Uma Ramakrishnan, molecular ecologist at the National Center for Biological Sciences in Bangalore, India, who was not involved in the study.
A system recently proposed by some scientists that would classify the tigers of the world into two subspecies would harm the world's remaining tigers instead of benefiting them. Jin Luo, a geneticist at Peking University, who led the study. To preserve what is left of the genetic diversity of tigers, all other subspecies need to be considered, they and their co-authors argue.
"If you think that all tigers are genetically homogeneous, you could say that if you have the Amur Tiger, you still have the Bengal Tiger ̵
Luo hopes that her new insights will eclipse a decade-long debate over whether tigers are made up of six, five or two subspecies. In 2004, she and her colleagues presented research results for the first time, according to which tigers form six living subspecies on the basis of partial genome analyzes. But other researchers soon challenged the results.
The last analysis confirmed six living subspecies: Bengal, Amur, South China, Sumatra, Indochinese and Malay. Scientists also believe that three other subspecies – Caspian, Javan and Bali tigers, which were described in the 1930s – were already threatened with extinction.
The distinction between species and subspecies is sometimes unclear. Although two distinct subspecies can mate and form viable offspring, subspecies are often separated by different habitats, different environmental adaptations, and unique genetic and morphological traits. Subspecies, many researchers believe, are the intermediate stages of evolution on the way to fully-formed species.
Armed with more affordable and robust genomic technologies, Luo and her colleagues bolstered their original insights. Previously, only a complete tiger genome had been sequenced. In the new study, the researchers performed a total genome sequencing analysis of 32 preserved wild tiger samples from around the world. A statistical analysis of 1.8 million DNA variants across the animal genome genome completed the breakdown into six distinct subspecies.
"While genomic methods have been extensively applied to humans and model organisms, their use to study endangered species remains unused." Ramakrishnan said.
The analysis also offered a window into the evolutionary history of the tigers. Fossil evidence indicates that the predator developed in Asia about 2 million years ago. The new study revealed that Tiger subspecies had a common ancestor about 110,000 years ago, probably in today's Southeast Asia and southern China. As the climate changed, the tigers spread to India, north to China and Siberia, and south to Indonesia and Malaysia.
Over the tens of thousands of years, tigers have become subspecies with pronounced genomic signatures. For example, the researchers found that Sumatran tigers, which live on a single island in Indonesia and were the first to deviate from the other subspecies, have genes associated with smaller body sizes compared to most tigers on the mainland , "It's about morphological observations and environmental expectations," says Luo.
"In India and Siberia, tigers hunt large ungulates, but in Sumatra they are more reliant on wild boars and smaller deer," she said. "It makes sense that smaller prey animals would exercise selective pressure for smaller tigers."
An estimated 100,000 tigers roamed Asia's forests, swamps, and grasslands a century ago. But poaching, habitat destruction and retaliatory strikes have thrown their numbers down; Today more tigers survive in captivity than in the wild. Tigers have already disappeared from Cambodia and Vietnam, and only a few stray, isolated individuals are said to survive in the wild in Laos and China.
Ullas Karanth, Asia's Science Director of the Wildlife Conservation Society, praised the new study to promote the scientific understanding of Tiger genetics, but he doubted the possibility of saving all six subspecies. "The numbers in all but the Indian and Russian subspecies are just too small," he said. Tiger populations with a reasonable chance of survival occupy only about 10 percent of their potential habitat, and the pressure only increases, Karanth said. Instead of focusing on genetics, efforts to preserve and regain tigers should give priority to places where preservation seems most realistic and promising.
The authors of the study, however, argue that rescuing tigers also saves their genetic diversity
"Preserving such genomic signatures requires preserving the evolutionary uniqueness that tigers have accumulated over millennia," Luo said. "We must respect this uniqueness by maximizing our efforts for all Tiger subspecies."