White Bengal tiger enigma solved

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In the case of white tigers, only the pheomelanin that produces the red to yellow colour is affected. – Photo: Wikimedia Commons

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.”

Published in The Hindu on May 24, 2013

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