Updated: September 18, 2021 12:44:55 pm
A team of scientists has resolved the genetic mystery of Simlipal’s so-called black tigers. Led by Uma Ramakrishnan and her student Vinay Sagar from the National Centre for Biological Sciences (NCBS), Bangalore, the study found that a single genetic mutation in these tigers caused black stripes to broaden or spread into the tawny background.
Tigers have a distinctive dark stripe pattern on a light background of white or golden. A rare pattern variant, distinguished by stripes that are broadened and fused together, is also observed in both wild and captive populations. This is known as pseudo-melanism, which is different from true melanism, a condition characterised by unusually high deposition of melanin, a dark pigment.
While truly melanistic tigers are yet to be recorded, pseudo-melanistic ones have been camera-trapped repeatedly, and only, in Simlipal, a 2,750-km tiger reserve in Odisha, since 2007. Launched in 2017, the study was the first attempt to investigate the genetic basis for this unusual phenotype (appearance). Through whole-genome data and pedigree-based association analyses from zoo tigers, the study found that pseudo-melanism is linked to a single mutation in Transmembrane Aminopeptidase Q (Taqpep), a gene responsible for similar traits in other cat species.
Why black tigers are rare
Mutants are genetic variations which may occur spontaneously, but not frequently, in nature. Black tiger sightings have been claimed sporadically at least since 1773 when artist James Forbes painted a watercolour of one shot in Kerala. Similar claims were made from Myanmar (1913) and China (1950s). In 1993, a confiscated black tiger skin of unknown origin was displayed at Delhi’s National Museum of Natural History.
Unfortunately, individuals of unusual appearance were sought out as a novelty by trophy hunters until recently, and probably few survived long enough to establish bloodlines.
Besides, pseudo-melanism is caused by a recessive (hidden) gene. A cub gets two copies of each gene from both parents, and a recessive gene can show up only in the absence of the dominant one. So, two normal-pattern tigers carrying the recessive pseudo-melanism gene will have to breed together for a one-in-four probability of giving birth to a black cub.
But recessive genes are rare and it is unlikely that two unrelated tigers will carry the same one and pass it on together to a cub. In an ideal tiger world, where far-ranging individuals are never short of choices for partners, that makes succession of black tigers a rarity.
Under exceptional circumstances, a black tiger may succeed as part of a very small (say, up to five individuals) founding population that is forced to inbreed in isolation for generations, offering the recessive gene frequent chances to show up. As it turned out, that is what happened at Simlipal.
The Simlipal mutants
Long before three black tigers were camera-trapped in 2007, Simlipal furnished the first confirmed record of the mutant in 1993 when a tribal youth killed a pseudo-melanistic tigress in self-defence. In 2018, three of Simlipal’s eight tigers turned out black.
Pseudo-melanistic tigers are also present in three zoos in India — Nandankanan (Bhubaneswar), Arignar Anna Zoological Park (Chennai) and Bhagwan Birsa Biological Park (Ranchi) — where they were born in captivity. All of them have ancestral links to one individual from Simlipal.
The closest breeding tiger population to Simlipal is around 800 km away, a distance much longer than the average home range of Bengal tigers (20-110 km) and their average dispersal distance (78-124 km). Dispersals longer than 500 km have been documented, the study noted, but they are very rare.
While previous studies detected three major genetic clusters within Indian tigers—Central India, South India, and Northwest India — the present one found that Simlipal tigers are genetically distinct from other central Indian populations and disconnected at a dispersal threshold of 200 km.
The conclusion: Simlipal’s small and isolated tiger population led to inbreeding and the anomalous phenotype characterised by wide, merged stripes. The loss of genetic diversity is evident from the low heterozygosity — chances of inheriting different forms of a particular gene from each parent — in Simlipal (28%) compared to Central India (36%). Consequently, mean relatedness between Simlipal individuals (38%) is higher than those in Central (9%) or South India (13%).
What about natural selection
Natural selection eliminates the weakest from a gene pool, and the traits of the more successful get passed on. Niche modelling, the study said, shows higher frequency of melanistic leopards in darker tropical and subtropical forests than in drier open habitats. Likewise, darker coats may confer a selective advantage in both hunting and avoiding hunters in Simlipal’s tropical moist deciduous and semi-evergreen closed-canopy forest, with a relatively darker understory.
While the present study did not have enough data to test the hypothesis, it observed that the disappearance of black tigers from across India, where populations may be larger and hence selection more effective, backed the possible deleterious effects of the mutation.
Simlipal is not the only example of “intense founding bottlenecks” in endangered tiger populations due to human-induced habitat fragmentation. While the dangers of this isolated, depleting population manifest dramatically in phenotypic evolution, invisible time bombs may be ticking in other island reserves.
“While tigers have recovered in some parts of India, several populations remain small and isolated. We predict that such populations will be subject to genetic drift (chance events), inbreeding and inbreeding depression (decreased survival). Overall, such populations have a high chance of extinction. It has been a fascinating journey to understand pseudo-melanism in Simlipal. Hopefully, such studies and the insights they provide will help safeguard the futures of endangered species,” said Ramakrishnan of NCBS.
Fortunately, introduction of fresh genes in an isolated pool, depending on the number of migrants, frequency of influx and population size, can reverse the damage over time. While airlifting tigers appears to be the solution of convenience these days, there is no alternative to restoring or maintaining natural connectivity between tiger forests in the long term.
Meanwhile, India’s northwestern tiger population shows higher mean relatedness between individuals (46%) and lower heterozygosity (22%) than even Simlipal (38% and 28%). While Ranthambhore has been a genetic island for decades, tiger siblings were handpicked from this inbred population for repopulating Sariska. That is another study in the offing.
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