A study published on Thursday in the Current Biology journal solved the puzzle why a few tigers have a white coat with sepia or dark brown stripes instead of the yellow coat with dark brown stripes. According to the scientists from Peking University, Beijing who undertook the study, an A477V point mutation in a pigmentation-related gene (SLC45A2) causes some tigers to have white coat instead of the normal yellow coat.
Today, white tigers are largely found in zoos and human-induced inbreeding is resorted to to maintain their numbers. Inbreeding causes many health problems and even prematured death.
Shu-Jin Luo from the Peking-Tsinghua Centre for Life Sciences, Peking University, Beijing and one of the authors of the study explained to R. Prasad by email the ways of increasing the number of white tigers without resorting to inbreeding.
You mention that human-induced inbreeding to maintain the recessive trait of the white tiger has led to many health problems. At the same time, you do state that there has been a “dramatic decline of wild tiger.” So how do you think this issue can be resolved?
Tigers in the wild are at the brink of extinction. Rescuing them requires a joint effort, both locally and internationally, from governments, NGOs, and all civil societies. Yet, there are numerous tigers surviving in captivity nowadays (could be over 15,000, five to seven times outnumbering wild tigers), some of which carry pure bred genetic ancestry from their wild cousins and could serve as the genetic ‘stock’ to help breed healthy, outbred white tigers.
Are white tigers still found in the wild? Even if they are present, the numbers would be very few. Under such circumstances, how do you think you would be able to overcome the issue of increasing their numbers without resorting to some kind of inbreeding, even in the wild?
White tigers have not been reported from the wild after 1950s. You are correct that even if they are present, the number would be very few. So the remedy has to come from captivity.
Now we know the A477V mutation at SLC45A2 is responsible for the white tiger coat, even for an orange tiger, we can do a genetic test to know if it carries one copy of the ‘white’ gene. [It is important to note that] a tiger with one copy of ‘white’ gene and one copy of ‘orange’ gene, the so-called heterozygous at the white locus, would appear as orange, because the mutation is recessive trait. Only when both copies of the gene are the ‘white’ ones, so-called homozygous, would it show as white tiger coat.]
Crossing between a wild-type orange tiger and a white tiger will result in all heterozygous orange tigers in their first generation. Crossing two heterozygous orange tigers will have 25 per cent chance of a white tiger offspring. This way of breeding may increase the gene pool diversity of white tigers and still maintain its white coat phenotype. Keep in mind though all the orange tigers used in this strategy should be Bengal tigers so as to avoid mixing tiger bloodlines from different sources.
Will mating of yellow coat tiger and white tiger result in the loss of the white colour trait?
If the orange tiger does not carry any white mutation at the white locus, crossing it with a white tiger will result in heterozygous orange tiger in their first generation. The white colour coat will not appear in looking but still remains in the genome. As said in my previous answer, crossing two heterozygous orange tigers will results in 25 per cent white tiger in their offspring. Crossing a heterozygous orange tiger and a white tiger will result in 50 per cent offspring white tigers.