December 15, 2009
An international group of scientists, reporting in Nature, has produced a draft genome sequence for the panda (Ailuropoda melanoleura), giving them a first look at the animal’s genes and confirming that, yes, the panda is one weird creature.
Giant pandas have been a recognized oddity for decades. Were they really bears, or were they more closely related to raccoons? (They’re bears.) They eat bamboo, grasping shoots with a pseudo thumb. They don’t breed easily, even in the wild. And their numbers, and habitat, are dwindling; there are now only about 2,500 to 3,000 pandas left in the wild, confined to a few small mountain regions in Western China.
To get more insight into panda biology (and try out a new, more economical type of genome sequencing), the 120 scientists picked a 3-year-old female giant panda from China’s panda breeding center in Chengdu, producing a draft sequence of about 94 percent of the panda’s genome and comparing it with the human genome and the dog genome, the only other carnivore genome now known.
Though pandas aren’t carnivorous, their genes show that they definitely belong in the Carnivora order. Giant pandas have genes for several digestive enzymes—such as amylase, cellulase and maltase—that are necessary for a carnivore diet. On the other hand, they lack genes for enzymes that would help them digest bamboo; the scientists think that the pandas rely on their gut bacteria to help them digest their woody diet.
Why are they eating bamboo, then, instead of meat? The researchers say that it’s probably due to taste. Pandas have a mutation in both copies of a gene called T1R1, which is the one that lets the tongue detect umami. Umami, sometimes known as the “fifth taste,” is the one that makes meats, cheese and other protein-heavy foods (as well as tomatoes and mushrooms) taste good. Pandas probably don’t find meat all that yummy.
The researchers also found that the sequenced genome had a high rate of heterozygosity (that is, the panda who had her genome sequenced had a high number of genes with two different copies instead of genes with two identical copies). This could indicate a lack of inbreeding and a high level of genetic diversity in the panda population, which would help in the survival of the species, despite the small size of the panda population. However, because the panda whose genome was sequenced has a mixed genetic background (from two different regions of wild Chinese pandas), they will have to sequence other panda genomes to see if the species really is genetically diverse.
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