White tigers - 'freaks' or just different?
A white tiger is a rare Bengal tiger (Panthera tigris tigris) with a natural mutation in the colour gene. Unfortunately, nowadays the white tiger can only be seen in captivity. Historical records of white tigers on the Indian subcontinent date back to the 15th century, but the last known free-ranging white tiger was shot in 1958.
Captive breeding of white tigers caused some health problems in those animals, the most common being crossed eyes. These prompted some to advocate that white tiger mutation is perhaps a genetic defect and therefore white tigers should not be bred in captivity at all. The first person to speak out against the displaying of white tigers was William Conway, director of the NY Zoological Association, who became famous for his anti-white tiger statement at the Bronx zoo: "White tigers are freaks. It's not the role of a zoo to show two headed calves and white tigers."
Recently researchers from Peking University in China have mapped the full genomes of a family of 16 Bengal tigers living in Chimelong Safari Park including both white and orange individuals. The comparison between the white and orange tigers led the to discovery of a single mutation in a pigment gene called SLC45A2 in white tigers. SLC45A2 has already been associated with lighter coloration in animals including horses, chickens and fish. White lions are relatively common. In fact the same gene has also been found in modern Caucasians. In humans this gene is responsible for production of protein called melanoma antigen AIM1, also referred to as ‘solute carrier family 45 member 2’ (SLC54A2) protein or Membrane-associated transporter protein - the protein that mediates myelin synthesis. White tigers are not albino. True albino tigers would not have black stripes and even those white tigers which appear to be ‘stripeless’ in reality have very pale stripes.
The SLC45A2 gene found in white tigers primarily inhibits the synthesis of red and yellow pigment with little or no effect on black. Hence the black stripes on white tigers. This single mutation in the SLC45A2 gene is recessive, therefore to get white tiger offspring both parents need to have the mutated SLC45A2 gene (though both parents might be orange because the gene is expressed on only one strand of DNA). With only a small pool of white tigers available, these tigers were heavily inbred in captivity. The authors of the study believe that this interbreeding is the cause of some of the medical problems seen in captive white tigers rather than the mutated gene itself. Therefore the researchers advise that changes in the breeding program of white tigers rather than eliminating the tigers from zoos should be the way to go.
The authors go further and even suggest that it might even be worth considering the reintroduction of white tigers into their wild habitat.
- Xiao Xu, Gui-Xin Dong, Xue-Song Hu, Lin Miao, Xue-Li Zhang, De-Lu Zhang, Han-Dong Yang, Tian-You Zhang, Zheng-Ting Zou, Ting-Ting Zhang, Yan Zhuang, Jong Bhak, Yun Sung Cho, Wen-Tao Dai, Tai-Jiao Jiang, Can Xie, Ruiqiang Li, Shu-Jin Luo. The Genetic Basis of White Tigers. Current Biology, 2013. http://www.cell.com/current-biology/retrieve/pii/S0960982213004958