A Blog by Ed Yong

Tiger, Tiger, Burning Bright, Just One Gene To Make It White

White tigers were first recorded in India in the 1500s, but the last wild one was shot in 1958. Still, this spectral animal thrives in captivity. Its captivating white coat and blue eyes have made it a popular mainstay of zoos, and a small number of individuals have been repeatedly bred with each other to boost captive numbers. There were just a few dozen in the 1970s. Now, there are hundreds.

The white tiger isn’t a species in its own right, or even a subspecies. Instead, it’s a mutant version of the Bengal tiger, whose orange coat has whitened thanks to an extremely rare recessive gene. If a tiger inherits two copies of this recessive variant, one from each parent, it’s white. If it has even one normal copy, it’s orange.

Back in the 1970s, Roy Robinson suggested that the gene in question was tyrosinase (TYR). It’s involved in making melanin—a pigment responsible for black, brown, red and yellow colours. If individuals have faulty versions of TYR, they are born without melanin and have pale hair, skin and eyes—they’re called albinos.

The white tiger isn’t a true albino since it still has black pigment in its stripes and eyes. Instead, Robinson thought that it carries chinchilla—a version of the TYR gene that only removes the type of melanin behind yellow and red colours. Without this, the orange coat becomes white, but the black bits stay black. Mystery solved.

You’ll still find this explanation all over the internet, but Xiao Xu from Peking University showed that Robinson was wrong. White tigers have the same version of TYR as orange ones. They also carry identical variants of four other genes that affect the colour of mammal coats. These include MC1R, the gene responsible the white coats of “snow coyotes” and “spirit bears”.

To find the real culprit behind the white coats, Xu’s team compared the DNA of 7 white tigers and 9 orange ones, living in China’s Chimelong Safari Park. They’re all related, and you can see their family tree below. The team sequenced the entire genomes of the three parents, identified more than 170,000 places where their DNA varied between individuals, and sequenced these locations in the rest of the animals.

Family tree of tigers involved in this study. Credit: Xu et al, 2013. Current Biology, Cell Press
Family tree of tigers involved in this study. Credit: Xu et al, 2013. Current Biology, Cell Press

Gradually, they homed in on seven genes that consistently differed in the white and orange animals. And by looking at these genes in 130 more tigers, from several unrelated sources, the team narrowed their list down to just one.

It has the tremendously catchy name of SLC45A2. It’s also involved in making melanin, although no one is entirely sure how. Variations in the gene have been linked to lighter skin or hair in mice, horses, chickens, medaka fish and humans. It’s associated with light skin colour in modern Europeans, as well as one type of albinism.

The SLC45A2 gene makes a protein of the same name, which consists of 560 amino acids. A single mutation in the gene—a change in just one DNA letter—switches one of those 560 amino acids from an alanine to a valine. This distorts the protein’s shape, and potentially prevents it from taking part in the creation of red-yellow melanin. Every white tiger has two copies of this mutated gene, and can only make the distorted protein. That’s all it takes to change their coats from orange to white.

Greg Barsh from the HudsonAlpha Institute of Biotechnology thinks that Xu’s team have found the right gene, and their results might eventually help to explain exactly what SLC45A2 does. In other species, mutations in the gene usually interfere with both the red-yellow and brown-black types of melanin. But in the tigers, they just disrupt the red-yellow pigments. Mutations in the TYR gene can sometimes do the same—remember chinchilla?—so even though Robinson was wrong about the gene behind the white coats, it’s still possible that SLC45A2 somehow interacts with TYR.

Some people have suggested that the genes behind the white coat also cause other defects, which have become more prominent because the captive animals are so inbred. These include club feet, crossed eyes, cleft palates, and hip or spine problems.

But Xu’s team argue that the white coat is the result of a pigmentation problem, and nothing more. After all, white tigers did once exist in the wild, and those that were captured or shot were often mature adults. This suggests that they’re capable of surviving in the wild despite their mutation—possibly because they hunt colour-blind prey.

Barsh disagrees. “Many humans and other animals with SLC45A2 mutations have severe visual problems,” he says, and he notes that previous studies have found abnormal visual connections between the tigers’ eyes and brains. This might explain the crossed eyes of the captive animals, and probably means that the mutation did affect the white tigers’ survival in the wild.

All of this feeds into a longstanding debate about the role that these white beasts should play in tiger conservation. Writing in Slate, Jackson Landers argues that white tigers should play no role in breeding programmes or reintroduction efforts, and should be allowed to “disappear into memory”. Every zoo enclosure that houses one is an enclosure that isn’t preserving one of the genuinely endangered tiger subspecies, whose numbers and genetic diversity are dwindling.

Xu’s team argues, based on their results, that the white tiger “should be considered a part of the genetic diversity of tigers that is worth conserving”. They argue that both white and orange tigers should be used to boost the Bengal population, and that reintroductions are possible.

It’s hard to see how their results address that issue, though. Given the past existence of wild white tigers, it’s clear that the white mutation was indeed a naturally occurring one—we just know which gene it affects now. Identifying SLC45A2 doesn’t change the fact that white tigers do suffer from several abnormalities, thanks to generations of inbreeding.

And with fewer than 3,200 tigers left in the wild, it’s perhaps a distraction to worry about conserving this one mutant gene. As John Seidensticker form the Smithsonian National Zoological Park bluntly puts it, “We have much more pressing tiger conservation problems.”

Reference: Xu, Dong, Hu, Miao, Zhang, Zhang, Yang, Zhang, Zou, Zhang, Zhuang, Bhak, Cho, Dai, Jiang, Xie, Li & Luo. 2013. The Genetic Basis of White Tigers. Current Biology. http://dx.doi.org/10.1016/j.cub.2013.04.054

More on cat genes:

9 thoughts on “Tiger, Tiger, Burning Bright, Just One Gene To Make It White

  1. Nice summary of the results with your title–probably the best functional and memorable title I’ve seen (or at least remember seeing). [Doffs hat].

  2. The title makes me think of William Blake!

    BUT IN REALITY WHATS MOST IMPORTANT IS ABOUT BRINGING THE NUMBER OF Bengal Tigers Up National Geographic should study how the worlds population is rising that the only places in a 100 years from now these animals will only be seen in zoos. I always loved the idea to breed endagered species to than get released back to their natural environment but we cant fully protect them from being hunted that is what hurts me the most. I loved watching national explorer since I was a kid watching these episodes of these animals in their natural environment and that today are in extinction. I want to see updated episodes which actually shows the world where these animals stand and live up to survive, the point isnt about seeing a white bangel tiger in the wild but what we have done to creat this white bangel tiger is wrong we had inbreed to be able to creat this color when in the wild this has happened naturally by coincidence.

  3. Greg Barsh needs to chill. The fact that white tigers did once exist in the wild, and those that were captured (or shot) were often mature adults is enough for me. Not to mention the fact that they are stunningly beautiful animals. The White Tiger is absolutely worth conserving! Save the White Tiger!

  4. While I do not think white tigers specifically need to be bred to be white, I do think they should be used in breeding programs because they do offer a unique genetic diversity and isn’t that what we want? To preserve as much genetic diversity as possible for the over all health of a species. Yes, we might see less white tigers over all considering it’s a recessive gene, but just like two brown eyed parents giving birth to a blue eyed daughter I think we’ll see orange tigers giving us white tigers if we breed carefully. I could be wrong though it’s been a long time since I took a genetics course.

  5. I feel certain that Blake would have been certain either 1) that white tigers are a tormented abomination created by Man and must be allowed the final freedom of release from club feet, crossed eyes, cleft palates, and especially from zoos OR 2) that white tigers were brought to Albion under a yellow moon lit by the fires of Urthona, and so must be preserved and venerated for all the generations of Man until the end of days.

    So it’s no good asking Blake.

    My theory is that, in our world, problems can never really be solved, because every time an answer is found for one problem, another three or four problems instantly spring up.

    Conclusion: find a way to let the white tigers decide on their own what is to become of them. We certainly cannot know what is right for them better than they do.

  6. I am surprised that the article doesn’t mention how highly marketable the white tigers are. In other words, I’m pretty sure the reason that zoos make such a point of keeping and breeding this variant is that it attracts visitors and donations, and helps sell merch.
    So, is that good or bad for tigers? Are white tigers displacing common tigers from limited conservation opportunities, or are they carrying conservation programs by their PR value?

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