Scientists Solve Mystery of Brown Giant Pandas

Brown-and-white giant pandas are distinct coat color mutants found exclusively in the Qinling Mountains of China.

“Coat color variation has substantial adaptive and cultural value in mammals,” said study’s first author Dr. Dengfeng Guan from the Key Laboratory of Animal Ecology and Conservation Biology at the Chinese Academy of Sciences and colleagues.

“This trait is directly determined by the ratio of eumelanin to pheomelanin, as well as the density and distribution of melanosomes in hair.”

“These factors are under complex regulation by hundreds of genes that influence various aspects of melanogenesis, including melanocyte proliferation and migration, melanin synthesis, and melanosome biogenesis and transfer.”

The giant panda (Ailuropoda melanoleuca), one of the most charismatic flagship species, is characterized by its striking black-and-white coat color.

However, the discovery of brown pandas (Ailuropoda melanoleuca qinlingensis) in the Qinling Mountains, Shaanxi province, China, challenges the conventional belief that the panda can never have a color photo.

Recognized as a subspecies of the giant panda, these brown pandas are exceedingly rare, which has undoubtedly made them the treasures of national treasures.

Since the first brown panda was found in 1985, eleven records have been reported by official news or personal communications, of which seven cases have been confirmed with photographs or entities: three from Foping County, two from Yangxian, one from Taibai, and one from Zhouzhi.

All brown pandas were found exclusively in the Qinling Mountains, indicating that they are endemic to this region.

“The first recorded brown panda, a female named Dan Dan, was rescued to Xi’an Zoo from the Foping Nature Reserve in 1985,” the biologists explained.

“She later mated with a black panda, Wan Wan, and gave birth to a male black panda, Qin Qin, in captivity.”

“Dan Dan passed away in 2000, while Qin Qin followed in 2006, leaving no offspring behind.”

“In 2009, a male brown panda cub, Qi Zai, was rescued from the Foping Nature Reserve and is currently the only brown panda living in captivity.”

Dr. Guan and co-authors established two family trios associated with the brown panda Qi Zai and sequenced their genomes.

They analyzed ecological and genetic data from their long-term study of wild giant pandas in the Foping Nature Reserve.

Furthermore, they explored the cellular mechanisms underlying the brown coat color based on the analysis of microscopy and transmission electron microscopy.

They identified the candidate mutation, a 25-bp deletion in Bace2 gene, as the most likely genetic basis of brown pandas.

They validated this deletion through sequencing of an additional cohort of 192 black pandas and CRISPR-Cas9 knockout mice.

Their findings not only provide insights into the genetic basis of coat color variation in brown pandas and even in wild animals but also would guide scientific breeding of the rare brown pandas.

“The extremely low number of brown pandas and the nature of the frameshift deletion mutation suggest that this mutation could be a neutral or weakly deleterious mutation,” the authors said.

“Notably, two brown pandas (Qi Zai and Dan Dan) have exhibited normal growth and reproduction while the Bace2 knockout mice are viable, fertile, and do not have noticeable physical abnormalities, indicating that this mutation does not have obviously negative impacts on the fitness of these individuals.”

“However, other physiological impacts of this mutation on the brown pandas remain unclear, as Bace2 is known to be involved in the Alzheimer’s disease pathway.”

“Considering the small population size of the Qinling giant panda population, weakly deleterious mutations might be fixed due to the genetic drift effect.”

“Thus, further studies on the brown panda and the knockout mouse model will provide valuable insights into the functional effects of this mutation.”

“For the rare coat color mutant of giant panda with great scientific and ornamental values, our findings would offer guidance on the scientific breeding of the brown pandas.”

The study was published online this week in the Proceedings of the National Academy of Sciences.

_____

Dengfeng Guan et al. 2024. Taking a color photo: A homozygous 25-bp deletion in Bace2 may cause brown-and-white coat color in giant pandas. PNAS 121 (11): e2317430121; doi: 10.1073/pnas.2317430121