By Meera Bhardwaj:
With sightings of Black Tigers at Similipal Tiger Reserve (TR) in Odisha recorded, scientific studies have now revealed how small, isolated, inbred and endangered population of Bengal tigers (with reduced connectivity) are facing genetic mutation with the cubs being born black.
The rare pseudo-melanistic tigers were camera trapped at Similipal in 2017-18.
Scientists have unraveled the genetic basis for this phenotype (an observable trait) and found that the black coat coloration and pattern is all due to a Single Mutation in the Taqpep gene.
About 37 per cent of tigers in Similipal are pseudo-melanistic, characterized by wide, merged stripes. They investigated the genetic basis for pseudo-melanism and examined the role of ‘drift’ in driving this phenotype’s frequency. Population genetic analyses confirmed few (minimal number) tigers in Similipal and its genetic isolation with poor geneflow.
PHOTO CREDIT: G VEERESH AT BHADRA TR
Normally, Tigers have a distinctive dark stripe pattern on a light background which can appear in several colour shades -white, golden, and snow white. However, the royal Bengal tigers in Similipal were found with pelts that had a black background with white abdominal stripes and tawny dorsal stripes.
The closest source population for Similipal is 800 Kms away, a distance much larger than the average home range of Bengal tigers which is 20-110 sq kms as also the average dispersal rate of 78-112 sq kms. So, the possibility of closest source of dispersers reaching Similipal tiger population is indeed very low.
PHOTO CREDIT: SOUMEN BAJPAI @ NANDANKANAN BIOLOGICAL PARK
Researchers add that small and isolated tiger populations show strong genomic signatures of inbreeding and high mutation load. However, future studies incorporating whole genomes or genome-wide data from Similipal should help confirm the role of genetic drift versus selection. Presently, genetic drift appears to be the most parsimonious explanation for the observed frequency of the variant.
Recently, camera trap images from across global range have identified pseudo-melanistic tigers from only one population – Similipal TR which is a 2,750 square km PA in eastern India. These rare tigers are also present at Nandankanan Biological Park, Bhubaneswar, Arignar Anna Zoological Park, Chennai, and Bhagwan Birsa Biological Park, Ranchi but all in captivity.
Uma Ramakrishnan, Molecular Ecologist and associate professor at National Centre for Biological Sciences (NCBS), Bangalore and Ph.D. student Vinay Sagar collaborated with national and global tiger experts and found the reasons for this change was all due to genetics. And this was due to mutation of a single gene in a very small founding tiger population in this tiger reserve. With restrictions in gene flow between the Simplipal tigers and other tiger populations, the result has been black tigers. In fact, inbreeding and isolation of such populations result in extinction over a short period of time.
Genome sequencing and extensive genotyping of non-invasive samples across tiger range revealed unique spatial presence of this allele in the Similipal. Population genetic analyses confirmed that Similipal is a small and isolated population. Simulations suggest that intense founding bottlenecks could result in the observed patterns, implicating drift. “Our study highlights ongoing phenotypic evolution, potentially from human-induced fragmentation in endangered large carnivore populations.”
The researchers state in their study published recently in the Journal Proceedings of the National Academy of Sciences, “We highlight an unusual evolutionary trajectory in a small and isolated population of an endangered species. While tigers may have recovered in India overall, some populations remain small and isolated. Small and isolated populations have low genetic variation and a high probability of fixation of deleterious alleles because of inbreeding, demographic stochasticity and random genetic drift, making them prone to extinction.”
In their study paper, the scientists have confirmed that a Taqpep missense mutational variant as the genetic basis for this rare phenotype in tigers. They used a whole-genome sequence data and known pedigrees of captive tigers that included these black individuals to determine. “Our analyses of the genomes of eight captive tigers (three pseudo-melanistic) from NKB revealed that all black tigers were homozygous for a single nucleotide variant predicted to alter a conserved and functionally important residue in the Taqpep gene.
PHOTO CREDIT: SOUMEN BAJPAI @ NANDANKANAN BIOLOGICAL PARK
Contrastingly, the nature of the mutation suggests that it could be deleterious. Moreover, while these tigers occur at a high frequency in Similipal, they have disappeared from across India, where populations may be larger (and hence selection more effective). This lends support to the possible deleterious effects of Taqpep gene, the researchers concluded.
Conservation practice recommends the genetic rescue of Similipal populations that are small and isolated, with potentially related and inbred individuals. They present a rare case of rapid evolutionary change, with this allele possibly on its way to fixation. Wildlife managers are faced with a choice of fixation of the mutant allele and a need for genetic rescue strategies. Finally, the high frequency of black tigers in Similipal and its apparent absence everywhere else suggests strong stochastic (random probability) effects and inbreeding operating locally in this population.