Chinese researchers have found the gene responsible for brown fur seen rarely in a tiny group of giant pandas instead of normal black fur
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Brown-and-cream pandas possess a distinct coat color seen only rarely in a tiny, isolated population of pandas found at elevations between 1,300–3,000 metres (4,300–9,800 ft) in the Qinling Mountains of Shaanxi, China. This population, originally discovered in 1959, was later classified as the first — and so far the only — known subspecies of panda in 2005 (ref). This study found that Qinling pandas may have genetically separated from Sichuan pandas around 300,000 years ago.
The basis for this striking coat coloration has attracted a lot of speculation about its origin but, like almost all rare coat colors in animals, it is likely the result of inbreeding. Only seven brown-and-cream pandas have ever been documented, and all were from the Qinling population. The population of Qinling pandas is tiny, currently numbering only 200-300 individuals — up from just 100 individuals in 2001 — and is genetically isolated, which has allowed the coat color variation to persist in the population.
A team of Chinese researchers wanted to find the source of this peculiar coat color. They just published their elegant study where they used a combination of field work, population genomic data, and a CRISPR–Cas9 knockout mouse lab model to identify the genetic basis for this coat color variation.
The team started by sequencing the genome of the only known living brown panda, a captive individual known as Qi Zai, and resequencing the genomes of two more brown pandas and their black-and-white parents, along with 29 other black-and-white pandas. They uncovered a 25-bp deletion mutation in a gene called Bace2, which is related to pigmentation. This 25-bp deletion mutation created a stop codon early in the first coding region, which results in a truncated message that is not translated into a viable protein. In humans, this same gene encodes a protein involved in the development of Alzheimer’s disease.
Based on their studies, the researchers think that brown pandas have two identical copies of the mutated Bace2 gene, one from each parent, which led them to identify this 25-bp deletion as the most likely genetic basis for the brown-and-cream coat color.
The team validated this Bace2 deletion mutation by comparing it with another 192 black-and-white giant pandas and finding that none of them carried it. To further confirm this Bace2 gene deletion as the source of brown coat hair, the team created several CRISPR-Cas9 edited knockout mice that carried this Bace2 mutation. All test mice grew pale coat hair.
The team also investigated other traits associated with this mutation and found the number and size of melanosomes in the hairs of the knockout mice were fewer and smaller than in normal mice, as were those in the brown panda hairs when compared to the hairs of black pandas. Melanosomes are intracellular organelles that are uniquely generated by pigment cells in animal cells, where they function to synthesize and store melanin pigments.
Other notable physical features of brown pandas include a smaller skull along with a smaller overall physical size compared to black pandas. The brown eye spots are located under the eyes, rather like big brown teardrops beginning to slide down the cheeks, instead of surrounding the eyes like sunglasses, as in black pandas.
How does this deletion mutation lead to changes in the size and number of melanosomes in brown pandas? This is a really great question and already, the team are investigating.
Source:
Dengfeng Guan, Shuyan Sun, Lingyun Song, Pengpeng Zhao, Yonggang Nie, Xin Huang, Wenliang Zhou, Li Yan, Yinghu Lei, Yibo Hu, and Fuwen Wei (2024). Taking a color photo: A homozygous 25-bp deletion in Bace2 may cause brown-and-white coat color in giant pandas, Proceedings of the National Academy of Sciences 121(11):e2317430121 | doi:10.1073/pnas.2317430121
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