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There are less than 4,000 tigers left in the wild. New research aims to give environmental advocates a better understanding of their genetics in order to save them.
After years of debate, scientists report in Current Biology that tigers include six unique subspecies. One of these subspecies, the South China Tiger, only survives in captivity.
"The results presented in this paper are important because they contradict the currently accepted international tiger conservation classifications," said Uma Ramakrishnan, a molecular ecologist at the National Center for Biological Sciences in Bangalore, India, who did not participate. at the study.
A system recently proposed by some scientists that would classify the world's tigers into two subspecies would harm the world's remaining tigers rather than their profit, said Shu-Jin Luo, a geneticist at Peking University. To preserve the remaining genetic diversity of tigers, it will be necessary to ensure that all remaining subspecies are taken into account, she explains.
"If you think that all tigers are genetically homogeneous, you could say that if you lose the Love Tiger, you still have the Bengal Tiger – and that's OK because they are very similar," said Luo. "But it's not right, because now we know that tigers are not all the same."
Luo hopes that their new discoveries put an end to a debate about whether tigers are six, five or two subspecies. In 2004, she and her colleagues presented for the first time a research on tigers constituting six living subspecies, based on partial genomic analyzes. But other researchers quickly challenged the results.
The latest analysis confirmed the existence of six living subspecies: Bengal, Love, South China, Sumatra, Indochina and Malayan. Scientists also believe that three other subspecies – the Caspian, Javan and Bali tigers, described in the 1930s – have already disappeared from extinction.
The distinction between species and subspecies is sometimes unclear. Although two different subspecies can reproduce and produce viable offspring, subspecies are often separated by distinct habitats, different environmental adaptations, and unique genetic and morphological characteristics. Many researchers believe that subspecies are the intermediate stages of evolution on the path of fully formed species.
Armed with more affordable and robust genomic technologies, Luo and his colleagues reinforced their initial discoveries. Previously, only one complete tiger genome had been sequenced. In the new study, researchers undertook a complete genome sequence analysis of 32 preserved wild tiger specimens from around the world. A statistical analysis of 1.8 million DNA variants belonging to tiger genomes of specimens made it possible to finalize the decomposition into six distinct subspecies.
"While genomic methods have been widely applied to humans and model organisms, their use for studying endangered species remains underutilized," Ramakrishnan said.
The analysis also offered a window on the evolutionary history of tigers. Fossil evidence indicates that the predator evolved about 2 million years ago in Asia. The new study revealed that the last tiger subspecies last shared a common ancestor about 110,000 years ago, probably in Southeast Asia and southern China. When the climate changed, the tigers spread west to India, north to China and Siberia, and south to Indonesia and Malaysia.
Over the past tens of thousands of years, tigers have evolved into a subspecies with distinct genomic signatures. For example, the researchers found that Sumatran tigers, which live on one island in Indonesia and were the first to diverge from other subspecies, have genes associated with smaller bodies than most tigers on the continent. . This corresponds to morphological observations and ecological expectations, said Luo.
"In India and Siberia, tigers attack big ungulates, but in Sumatra, they are more dependent on wild boar and smaller deer," she said. "It makes sense that smaller prey exerts selection pressure for smaller tigers."
A century ago, about 100,000 tigers roamed the forests, swamps and meadows of Asia. But poaching, habitat destruction and reprisal killings have reduced their numbers. Today, more tigers survive in captivity than in the wild. Tigers have already disappeared from Cambodia and Vietnam, and only a few isolated and wandering individuals survive in the wild in Laos and China.
Ullas Karanth, scientific director of the Wildlife Conservation Society for Asia, praised the new study to advance scientific understanding of tiger genetics, but questioned the feasibility of saving the six subspecies. "The numbers in all subspecies except Indian and Russian are just too small," he said. Tiger populations with a reasonable prospect of survival occupy only about 10% of their potential habitat, and the pressures are only increasing, Karanth said. Rather than focusing on genetics, efforts to maintain and recover tigers should prioritize sites where conservation seems the most realistic and promising, he said.
The authors of the study argue, however, that saving tigers from extinction also means preserving their genetic diversity.
"Preserving such genomic signatures, it's preserving the uniqueness of the evolution accumulated by tigers over the course of thousands of years," Luo said. "We must respect this specificity by maximizing our efforts for all subspecies of tigers."
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