Today, fewer than 4,000 free-ranging tigers survive in the wild, covering only 7% of their historical range. Efforts to protect these animals have been stymied by uncertainty about whether they comprise six, five, or two subspecies. Now, researchers who’ve analyzed the complete genomes of 32 representative specimens confirm that tigers indeed fall into six genetically distinct groups: the Bengal tiger (Panthera tigris tigris), the Amur tiger (P.t. altaica), the South China tiger (P.t. amoyensis), the Sumatran tiger (P.t. sumatrae), the Indochinese tiger (P.t. corbetti), and the Malayan tiger (P.t. jacksoni).
“The lack of consensus over the number of tiger subspecies has partially hindered the global effort to recover the species from the brink of extinction, as both captive breeding and landscape intervention of wild populations increasingly requires an explicit delineation of the conservation management units,” said study senior author Dr. Shu-Jin Luo, a researcher at Peking University in China.
“This study is the first to reveal the tiger’s natural history from a whole-genomic perspective. It provides robust, genome-wide evidence for the origin and evolution of this charismatic megafauna species.”
“Tigers are rapidly headed for extinction in the wild, and the loss seen over the past few decades is increasing and is happening in spite of intense conservation interest, surveillance, legal protection, and expenditure,” said Dr. Stephen O’Brien, a scientist at Nova Southeastern University’s Halmos College of Natural Sciences and Oceanography.
“The tiger depletion stands as one of conservation’s most poignant morality tales of loss.”
Dr. Luo, Dr. O’Brien and co-authors set out to expand their earlier genetic evidence on tiger’s evolutionary history and population structure using a whole-genome approach.
They realized that genome-wide screening was also the only way to look for signals that distinct groups of tigers have undergone natural selection to adapt to the environments found in the distinct geographic regions they inhabit.
“Fossil evidence shows that tigers go back two to three million years,” they said.
“But, the genomic evidence shows that all living tigers only trace back to a time about 110,000 years ago, when tigers suffered a historic population bottleneck. It also shows that there is very little gene flow among tiger populations.”
“Despite the tiger’s low genetic diversity, the pattern across groups is highly structured, offering evidence that tiger subspecies each have a unique evolutionary history.”
“That’s quite unique among the big cats. Several other species, such as the jaguar, have shown much more evidence of intermixing across whole continents.”
Tiger subspecies have distinct features. For example, Amur tigers are large with pale orange fur, while Sumatran tigers in the Sunda Islands tend to be smaller with darker, thickly striped fur.
In fact, despite the very recent common ancestor of all living tigers, the team was able to detect evidence of natural selection.
“In the end, we were quite amazed that, by performing a stepwise genome-wide scan, seven regions including 14 genes stood out as the potential regions subject for selection,” Dr. Luo noted.
The strongest signal of selection the study authors found was in the Sumatran tiger, across a genomic region that contains the body-size-related ADH7 gene.
They suggest that the Sumatran tiger might have been selected for smaller size to reduce its energy demands, allowing it to survive on the island’s smaller prey animals, such as wild pigs and muntjac, a small deer.
The new findings provide the strongest genetic evidence yet for subspecies delineation in tigers.
“Tigers are not all alike. Siberian tigers are evolutionarily distinct from those from India. Even tigers from Malaysia and Indonesia are different,” Dr. Luo said.
“The origin of the South China tiger remains unresolved since only one specimen from captivity was used in this study,” the scientists said.
“Unfortunately, this subspecies has gone extinct in the wild. We plan to study old specimens with known origin from all over China to fill in the missing pieces of living tigers’ evolutionary history.”
The team is also retrieving genomic information from historical specimens, including those representing three extinct subspecies: the Javan tiger (Panthera tigris sondaica), the Bali tiger (P.t. balica) and the Caspian tiger (P.t. virgata).
“Our study brings important context and conclusions to recovery and management strategies for a treasured endangered species, and included subspecies, at high extinction risk,” Dr. O’Brien said.
“This new report represents a genome-wide sequence analyses that has direct bearing on recovery and management strategies for a treasured endangered species and subspecies at high extinction risk.”
“We recommend the recognition, classification and management of the six living tiger subspecies as separate conservation units deserving individual protection planning.”
The team’s results are published in the journal Current Biology.
Yue-Chen Liu et al. Genome-Wide Evolutionary Analysis of Natural History and Adaptation in the World’s Tigers. Current Biology, published online October 25, 2018; doi: 10.1016/j.cub.2018.09.019