THE MYSTERIOUS BLACK TIGER

The Similipal Tiger reserve is popular for  the numerous sightings of pseudo-melanistic tigers.

Genetic mutants that spot unusually wide and merged stripes, were extremely rare even when tigers were plentiful centuries ago.

India’s National Center for Biological Sciences sequenced  the genomes of three zoo-born black tigers and their typical-coated parents and they succeeded in tracking the reason for the pattern . 

The change in colour & pattern of stripes is due to the single base mutation in the gene Transmembrane Aminopeptidase Q (Taqpep). Altered taqpep genes were already known to cause blotched tabby patterns in cats. But such patterns are so rare because they occur only when genes from both parents have matching mutations. 

Tigers surveyed outside the  reserve had not even one copy of the mutation.This suggests that the Similipal tigers are so isolated that they never breed with tigers outside that range and that the group has begun to maintain genetic changes over generations. This drastic change in patterning and colouring of the black tigers’ coat is caused by just one change in the DNA alphabet from C (Cytosine) to T (Thymine) in position 1360 of the Taqpep gene sequence. This causes a ‘missense mutation’ in the Taqpep protein, where one amino acid in position 454 of the protein sequence is changed from a histidine to a tyrosine residue.

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. 

Geographically, the closest tiger population to Similipal is nearly 500 km away. The median dispersal distance of tigers is only 148 km led to the inbreeding.  

In addition, the area’s small tiger population makes it prone to ‘genetic drift’, a condition where a mutation may end up occurring in very high frequencies or being lost due to chance alone. 

A combination of these four interrelated factors – a small founding population, isolation, inbreeding, and genetic drift – are the likely evolutionary forces that have created Similipal’s unique population of black tigers.An estimated 37% of Panthera tigris in the Similipal Tiger Reserve (in eastern India) are pseudomelanistic, characterised by wide, merged stripes.

With shrinking habitats, the tiger population becomes increasingly isolated. This causes inbreeding, resulting in a lack of genetic variation, making them prone to extinction.