New Delhi, Sep 15: The enduring mystery behind the 'black tigers' of Similipal in Odisha may finally have been solved with researchers identifying a single mutation in a gene that causes their distinctive stripes to broaden and spread into their tawny pelt, occasionally appearing entirely dark.
Considered mythical for centuries, the 'black tigers' have long been a subject of fascination. Now, a team led by ecologist Uma Ramakrishnan and her student Vinay Sagar from the National Centre for Biological Sciences (NCBS), Bangalore, have discovered that the coat colouration and patterning which make the wild cats appear dark boil down to a single mutation in the Transmembrane Aminopeptidase Q (Taqpep) gene.
"Ours is the first and only study to investigate the genetic basis for this phenotype (look). While the phenotype has been talked about and written about earlier, this is the first time its genetic underpinnings were scientifically investigated," Ramkrishnan, professor at NCBS, told PTI.
The researchers combined genetic analyses of other tiger populations from India and data from computer simulations to show that the Similipal black tigers may have risen from a very small founding population of tigers and are inbred -- providing an answer to the question that had perplexed so many for so long.
The study, published in the journal Proceedings of the National Academy of Sciences on Monday, noted that tigers in the Similipal Tiger Reserve are an isolated population in eastern India, and gene flow between them and other tiger populations is very restricted.
The researchers noted that this has important implications for tiger conservation as such isolated and inbred populations are prone to extinction over even short periods of time.
"They (the black tigers) have not been found in any other places in the wild to the best of our knowledge. Nowhere else in the world," Sagar, a PhD student in Ramakrishan's lab and lead author of the paper, told PTI.
"We used whole genome sequencing from a pedigree (family tree) that includes pseudomelanistic (false coloured) and normally striped individuals to find the mutation responsible for the phenotype," he explained.
The abnormally dark or black coat in such tigers is termed pseudomelanistic or false coloured. The most recent sightings of this rare mutant tiger in Similipal, long considered mythical, was reported in 2017 and 2018.
Since the late 1700s, reports of black tiger sightings and supposed captures in central and northeast India have been recorded by locals and British hunters.
According to the 2018 tiger census, India has an estimated 2,967 tigers. Photos captured from Similipal in 2018 showed eight unique individuals, three of which were 'pseudomelanistic' tigers, characterised by wide, merged stripes.
The researchers at NCBS teamed up with tiger experts nationally and in other countries and found out that pseudomelanistic coat came down to the genes.
They found the black tigers are mutants and are Bengal tigers with a single base mutation.
Different mutations in this gene are known to cause similar changes in coat colour in several other species of cats, including cheetahs.
The drastic change in patterning and colouring of the black tigers' coat is caused by just one change in the genetic material DNA Alphabet from C (Cytosine) to T (Thymine) in position 1360 of the Taqpep gene sequence, the researchers said.
Further genetic analyses and comparisons with a total of 395 captive and wild Indian tiger populations indicates that the mutation in Similipal tigers is very rare.
The only other black tigers outside of Similipal in India exist at the Nandankanan Zoological Park in Bhubaneswar, Ranchi Zoo and Chennai's Arignar Anna Zoological Park where they were born in captivity.
Genetic tracing proved that these captive-born tigers shared a common ancestry with Similipal tigers.
Within Similipal, the mutation is present at a high frequency of 0.58: this means that if you pick any tiger from Similipal, the chance that it carries the mutant gene is almost 60 per cent.
The researchers also carried out investigations to understand why this mutation occurred at such a high frequency in Similipal alone.
One hypothesis is that the darker coat colour of the mutants offers them a selective advantage when hunting in the dense closed-canopy and relatively darker forested areas of Similipal as compared to the open plains of most other tiger habitats.
However, the results of additional genetic analyses coupled with computer simulations indicate that a small founding population and prolonged isolation from other tiger populations in India is likely to be the main reason for the occurrence of these black tigers.
Due to this geographic isolation, genetically related individuals have been mating with each other for many generations in Similipal, leading to inbreeding, the researchers noted.
A combination of these interrelated factors are the likely evolutionary forces that have created Similipal's unique population of black tigers.
"It is amazing that we could find the genetic basis for such a striking pattern phenotype in wild tigers, and even more interesting that this genetic variant is at high frequencies in Similipal," said Ramakrishnan.
"This appears to be a classic example of a founding event, which is followed by a small population that is isolated. As a result, this pseudomelanitic phenotype has become very common here," Sagar added.
The research also included scientists from Stanford University, HudsonAlpha Institute for Biotechnology, both in the US, Indian Institute of Science Education and Research, Tirupati, Center for Cellular & Molecular Biology, Hyderabad , and Wildlife Institute of India, Dehradun.
