The single gene mutation (change) explains why white tigers do not have the orange coloration of other tigers but have the dark stripes.
analyzed the genomes of 16 related orange and white tigers in China's Chimelong Safari Park in Panyu, Guangzhou Province. They also sequenced the full genomes of three parent tigers and compared the results to 130 unrelated tigers. They found that a mutation in one pigment gene called the SLC45A2
causes the white coats of the tigers. A similar mutation is responsible for light coloring in Caucasians, horses, chickens, fish and mice.
The researchers Luo, Xiao Xu, Ruiqiang Li, and their colleagues, in a paper titled "The Genetic Basis of White Tigers,"
found that an amino acid change (A477V) in the transporter protein SLC45A2 of white tigers inhibits the production of red and yellow pigments but does not affect the black stripes of the animals.
White tigers were first discovered in the Indian jungle as a variant of Bengal tigers (Panthera tigris tigris
). White tigers are now extinct in nature. Several generations of captive white tigers have been inbred to preserve the white coat trait controlled by a recessive gene (that is a gene that expresses itself only in the absence of its "normal" variant).
Some have suggested that the white coat gene is a genetic defect intensified by inbreeding in captivity. But others have pointed out that the mutation affects only the cat's color and does not affect any other trait. They argued that white tigers are healthy variants of orange colored tigers and pointed out that the genetic abnormalities observed in captive white tigers, such as club feet, cleft palates and crossed eyes are the result of inbreeding by humans.
Some experts also pointed out that the fact that many white tigers were found and hunted as mature adults suggests they are able to survive in their natural habitats. They noted the related fact that their favorite prey, such as the deer, are color blind.
Research author Shu-Jin Luo of China's Peking University, said in a statement
: "The white tiger represents part of the natural genetic diversity of the tiger that is worth conserving, but is now seen only in captivity."
The researchers recommended a captive management program to maintain the population of both white and orange Bengal tigers and a carefully managed conservation program for re-introduction of white tigers to the wild.
According to Luo and his colleagues
, available records of white tigers in the Indian subcontinent which date back to the 1500s indicate that the last free-ranging white tiger was shot in 1958. The researchers note that before the last one was shot, there had been "sporadic sightings in India."
The authors write
: "Reasons for the extinction of wild white tigers were likely the same as those accounting for the dramatic decline in wild tigers in general: uncontrolled trophy hunting, habitat loss, and habitat fragmentation. However, the fact that many white tigers captured or shot in the wild were mature adults suggests that a white tiger in the wild is able to survive without its ﬁtness being substantially compromised."
Now that scientists have identified the gene that causes white coats in Bengal tigers, they plan to investigate the evolutionary factors that drove the emergence and maintenance of the population of white tigers in the wild before they were hunted to extinction.