In 1985 the first brown panda was observed. It was so unique that everything was thought to be: that it was a fake, that it had stained fur, that it was a strange case of albinism, that it had a genetic alteration. In the end, they classified it as a subspecies of the panda bear Ailuropoda melanoleuca which they called the Qinling panda, because it was in this mountain range where they found it. Now, the analysis of the fur of two of them and the genome of 227 pandas, all black and white except these two, has identified a mutation in a gene that intervenes in the pigmentation of these ursids that they inherited from their parents. It is estimated that there are only a hundred of these animals in their last refuge.
The first recorded sighting of a brown panda killed the bear, which they named Dandan (something like single red, due to its chestnut color) in the zoo of Xi'an, capital of the Chinese province of Shaanxi, famous for the archaeological treasure of thousands of terracotta warriors. For years, they tried to get Dandan pregnant. But she aborted, or her offspring died within a few months. Only one came out ahead, Qinqin, a black and white panda bear, like her father, another specimen from the zoo. In the end, the brown panda died in 2000 from skin cancer and, shortly after, her offspring did, ending her lineage. But since then sightings in the bamboo forests of the Qinling Mountains have multiplied. It's not that they saw hundreds, but they did see a dozen brown pandas. And in 2009 luck smiled on scientists. In the same area where they found the Dandan, they found a young male Qizai, the only coffee with milk that lives in captivity. The detailed study of both, the results of which have just been published in the scientific journal PNAShas revealed the mystery of its color.
Studying the fur of both animals and comparing it with that of the black-and-white pandas showed that they had fewer melanosomes in their hair. These are organelles within the cells of the epidermis that store and synthesize melanin, the pigment present in almost all living beings and which in humans is concentrated mainly in skin and hair. The melanosomes of brown pandas are also 55% smaller.
To determine the genetic origin of the brown, the researchers sequenced the genome of all known relatives of Qizai and Dandan and another dozen specimens from the Qinling Mountains and compared them with the genetic data of almost 200 animals from other areas of China, all with the usual color pattern. They were looking for something that would make them different and they found it: “Based on the pedigree, we conclude that brown coat color is governed by autosomal recessive mutations,” the authors write in the results of their research. These types of mutations follow Mendelian patterns of inheritance. For a panda to be born brown-white, the mutation must be in both parents, even if they are black-white.
The next thing was to find the mutation responsible for this very special phenotype. Comparing all the sequences, they found that the deletion (loss or deletion of one or more letters or bases from the DNA) of 25 base pairs at the start of the gene Bacce2 was the possible cause. The data is this: none of the almost 200 pandas in the other mountain ranges where they still remain, most concentrated in Sichuan, had the mutation. However, several of the specimens from the Qinling Mountains and a hybrid of both populations had the variation. Some were heterozygous, that is, they carried the allele or variation inherited from only one of their parents. Only Qinqin and Dandan were homozygous, meaning they had inherited the mutation from both their mother and father. Hence the character of a recessive mutation and its scarcity.
Once the responsible gene was found, its responsibility had to be confirmed. To do this, they modified a series of 78 homozygous laboratory mice, all black, with the CRISPR genetic editing technique to delete that part of their Bacce2. Mice have a somewhat particular hair cycle, with a phase of growth, another of regression, and a third of rest, and starting again. When compared to other rodents as a control group, they saw that the mutants began to grow brown fur. “These results support the idea that the deletion in Bacce2 “It is probably the genetic basis of the brown color of pandas' fur,” say the authors, from the Institute of Zoology of the Chinese Academy of Sciences.
This gene expresses a protein present in the cell membrane, with various enzymatic functions. Recently, in humans, it has been linked to Alzheimer's. The two pandas have shown normal growth and reproduction, while the mutant mice were also found to be viable, fertile and have no notable physical abnormalities, which would indicate that this mutation has no obvious negative impacts. However, the authors conclude, “other physiological impacts of this mutation are still unclear and it is already known that Bacce2 “is involved in the course of Alzheimer's disease.” Any deleterious effect could put an end to this oddity of nature.
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