Black Tiger In Odisha's Simlipal National Park Genetic Mystery Solved

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A team of scientists has resolved the genetic mystery of Simlipal’s so-called black tigers.
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).

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.

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.

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.

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 —

where they were born in captivity. All of them have ancestral links to one individual from Simlipal.

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.

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%).

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.
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.

Question:

The term M-STRIPES’ is sometimes seen in the news in the context of-

(a)Captive breeding of Wild Fauna

(b)Maintenance of Tiger Reserves

(c)Indigenous Satellite Navigation System

(d)Security of National Highways