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Updated: May 24, 2013 15:23 IST

White Bengal tiger enigma solved

R. Prasad
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Bety, a female white tiger, holds one of her three 45 day-old cubs at the Buenos Aires Zoo in this file photo. Photo: AP
Bety, a female white tiger, holds one of her three 45 day-old cubs at the Buenos Aires Zoo in this file photo. Photo: AP

The colour of the fur, stripes and eye of the tiger is determined independently by two types of melanin — pheomelanin and eumelanin.

A change in a single amino acid (A477V) in one pigmentation-related gene (SLC45A2) causes some tigers to have white fur with dark or sepia brown stripes, scientists from Peking University, Beijing, have found. They studied 16 captive white tigers from three parents. The results were published on Thursday in the Current Biology journal.

The colour of the fur, stripes and eye of the tiger is determined independently by two types of melanin — pheomelanin and eumelanin. In the case of white tigers, only the pheomelanin that produces the red to yellow colour is affected. Eumelanin gives the black to brown colour and is unaffected, the reason why the eye and hair in the stripes are dark or sepia brown.

The scientists found that the point mutation in the amino acid partially blocks a particular channel, as a result of which the yellow pigment-forming process gets affected. Incidentally, mutations in the same pigmentation-related gene (SLC45A2) causes light skin colour in modern Europeans, as well. Mutations in the same gene causes skin lightening in some mouse, horse, and chicken, the scientists point out.

The point mutation has “evolved only once and its frequency is probably never high,” they write. Though white tigers were found in the wild once, their decline was probably due to mindless killing by humans. The last known white tiger was killed in 1958, they note.

To maintain and increase the number of white tigers in zoos, humans often force them to inbreed. But inbreeding, as seen in the case of humans, causes many health problems. In the case of white tigers, the human-induced inbreeding has resulted in “premature death, stillbirth and deformities.” Since the mutation affects only the pigmentation process, it probably has no role in causing deaths.

They dismiss the notion that white tiger trait is a genetic deformity. That matured white tiger adults have been sighted in the wild negates this notion.

Despite its low frequency, they emphasise that the mutation is a naturally occurring one and should be considered as a “part of genetic diversity of tigers that is worth conserving.”

Keywords: Bengal tiger

As this article mentioned, Xiao Xu et al published ‘The Genetic of White Tigers’ where they observe the effect of melanin in color variation of human and other species. It is well known that lack of melanin, eumelanin and pheomelanin pigments, production leads to albino and over production of melanin to darker skin color. The point mutation resulting in color variation is not an uphill evolution from microbes to moral homosapiens.

from:  M Chandrasekaran
Posted on: May 24, 2013 at 08:58 IST

How is Peking University able to maintain 16 tigers for studying? It is
so difficult for universities to maintain a basic animal house for mice,
rabbits and other general model organisms for studies. This directly
shows tiger farms are thriving in China and they are being exploited for
the so called scientific investigation.

from:  Nandini
Posted on: May 24, 2013 at 08:31 IST
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