Nearly half a century after tribals of Similipal in Mayurbhanj district of Odisha claimed they had spotted a rare black tiger, researchers have finally solved the mystery of this unique feature. Researchers from the National Centre for Biological Sciences (NCBS) have zeroed in on a rare mutation that results in large, merged stripes that cause the tiger to appear black. These Odisha tigers essentially have stripes that are larger than found in other tigers. And, these stripes merge among one another, causing the tigers to appear as though they have black-coloured skin.
The rare mutation behind this has been found in one gene, Transmembrane Aminopeptidase Q or Taqpep. The findings of the research published in the journal Proceedings of the National Academy of Sciences state that genome sequencing across the tiger range helped find the presence of this unique mutation in the Similipal Tiger Reserve.
The research team, which was led by ecologist Dr Uma Ramakrishnan, said that most endangered species exist today in small populations, many of which are isolated. Evolution in such populations is largely governed by genetic drift. They found that approximately 37 per cent of tigers in the Similipal Tiger Reserve are pseudomelanistic (a term used to describe the morphing of a pattern), characterised by wide, merged stripes.
The most recent sightings of this rare mutant tiger in Similipal, long considered mythical, were reported in 2017 and 2018.
A GENETIC MUTATION
Researchers investigated the genetic basis for the pseudomelanism in these tigers and examined the role of genetic drift. They identified a coding alteration in Transmembrane Aminopeptidase Q (Taqpep) and sampling of the tigers in the reserves revealed a high frequency of the Taqpep p.H454Y mutation.
Distribution of the genotyped individuals. A total of 428 individuals were genotyped. (Photo: PNAS)
The research compared the findings with genetic analyses of other tiger populations from India and data from computer simulations to show that the Similipal black tigers may have come from a very small founding population of tigers and are likely inbred.
“Tigers have a distinctive dark stripe pattern on a light background, which can appear in several colour shades—white, golden, and snow white. Segregation of these colour variants in captive tiger populations has permitted their genetic and molecular characterisation,” the paper said adding that pseudomelanistic tigers are present in three populations in India: Nandankanan Biological Park, Bhubaneswar, Arignar Anna Zoological Park, Chennai, and Bhagwan Birsa Biological Park, Ranchi.
UNIQUE COLOUR PATTERNS
Researchers said that distinct processes are involved in implementing mammalian colour patterns that occur during recurring hair cycles. The patterns take colour due to direct engagement with pigment cells to regulate light or dark pigment production. “Taqpep mutations in the domestic cat (Felis catus) and the cheetah (Acinonyx jubatus) alter pattern formation in a manner that is strikingly similar to pseudomelanistic tigers.”
The shift in pattern is caused by a change in the genetic material DNA Alphabet from C (Cytosine) to T (Thymine) in position 1360 of the Taqpep gene sequence, the researchers said. “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,” Dr Ramakrishnan told PTI.
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. Researchers worked with tiger experts nationally to look for unique traits.
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