At a wildlife park in eastern India, tigers feature a variety of coat fashions. Most notably, some of the cats have very thick black stripes. Now, a team of geneticists in India and the United States have identified a genetic mutation in the tiger troupe that explains why some of them are so dark.
The tigers are pseudomelanistic, meaning they have wide, merged stripes along their body. (That’s not to be confused with leucistic tigers, which are white with black stripes due to a different uncommon trait, leucism.) From some angles, the pseudomelanistic tigers can look mostly black, hence their nickname of “black” tigers. Over one-third of the tigers in the Similipal Tiger Reserve are pseudomelanistic. Led by Vinay Sagar of the Tata Institute of Fundamental Research, the research team took a genetic survey of 85 tigers across four subspecies to figure out what was different, at a molecular level, about those animals. Their findings were published this month in the Proceedings of the National Academy of Sciences.
“The pseudomelanistic tigers have a mutation in their Taqpep gene — like humans, tigers have two copies of every gene, and both copies are mutated in pseudomelanistic tigers — and without any Taqpep the process of pattern establishment is defective, leading to a widening and occasional fusion of stripes,” co-author Greg Barsh, a geneticist at Stanford University and the HudsonAlpha Institute for Biotechnology, said in an email to Gizmodo.
The team’s analysis found that the pseudomelanistic cats all had a single nucleotide variant in their genetic code, which appeared to alter a specific gene. That gene is called Transmembrane Aminopeptidase Q (Taqpep for short), and it’s the same gene that is responsible for the blotches and stripe patterns in tabby cats and cheetahs, as members of the current research team discovered back in 2012.
In the tigers, a lone cytosine nucleotide was swapped for a thymine, which altered how the Taqpep gene behaved. Just as the recessively inherited versions of the genes whorled tabby markings and made king cheetahs, wonky Taqpeps in tigers appear to make the animals more black than orange.
There are eight subspecies of tiger known, but three have been declared extinct, according to the US Fish & Wildlife Service. All the extant subspecies are endangered, and the few tigers in captivity struggle with the problems that arise when you have a small number of animals trying to sustain the genetic diversity of an entire species. That’s why conservation isn’t as simple as breeding as many endangered animals as possible.
“Totalling numbers for a species is not enough,” said Uma Ramakrishnan, a scientist at the Tata Institute of Fundamental Research, in an email to Gizmodo. “Overall, the numbers of tigers have increased. But many populations of tigers across their range remain small and isolated, and hence are subject to genetic drift or chance changes in allele frequency and inbreeding. We are still learning about the futures of such populations.”
The recessive Taqpep phenotypes were present among more than half of the tigers living in the Similipal reserve. Even white tigers can have the trait, leaving some of the animals looking a bit like marble bread. The tiger population appears to be inbred, which could explain the presence of the trait in so many animals.
“Most colour mutations tend to affect the entire body, like albinism or melanism, so mutations that affect colour pattern are especially interesting from a scientific perspective because they help us to understand more about developmental biology,” Barsh said.
The wide-striped trait isn’t necessarily deleterious: The research team said that if the trait wasn’t simply due to inbreeding, it could be because of some evolutionary benefit. They reference the case of melanistic leopards, which crop up with greater frequency in dark, dense tropical forests than in drier, open environments. If a similar situation is the case for the Similipal tigers, they may lose some of their orange to better blend into a jungly understory.
Whatever evolutionary logic may be underpinning the tigers’ stripes, it’s a reminder that animal coats should never be seen as skin-deep.