What Makes a Panda?

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In this week’s issue of Nature, Jian Wang and colleagues from the Beijing Genomics Institute report the first-ever genome sequence of a member of the clade Ursidiae, or the bear family. Wang et al. sequenced all 2.4 billion bases of the panda genome. The panda has 21,001 genes on 22 haploid chromosomes. The fact that a single institute (along with collaborators) was able to complete this task is a testament to how fast and cheap modern DNA sequencing technology is. The $1,000 genome is on the horizon. But enough about DNA sequencing… we all know that pandas are extremely awesome, but the real question is: Why?

Wang et al. found that the panda genome contains sufficient enzymes, such as amylase, protease and lipase, for them to have a carnivorous diet and digestive system. These proteins are especially designed to break down components prominent in muscle tissue and animal fat. However, it is commonly known that pandas only eat bamboo and are strict herbivores, in contrast to most of the bear family. Wang even found that there are no genes specific to the break down of cellulose- the most abundant form of energy contained in plants. They attribute the panda’s ability to eat bamboo to unique bacteria in the panda gut.  Still, if pandas have the ability to eat meat, why do they prefer bamboo?

The panda genome may finally unravel this mystery. It appears that pandas have lost one of their basic senses of taste. For mammals, there are five basic taste sensations: sweetness, saltiness, sourness, bitterness and umami. Everything we taste is a combination of those five.  The first four are familiar, but what is umami? Umami is most concisely described as “savoriness.” It is what you taste when you eat cheese, meats, broth and other protein-heavy foods. For pandas, it turns out the umami receptor is actually the key to their distinct diet. A gene that is present in most mammals, called TIR1, codes for a protein that forms one-half of the taste-bud receptor for umami. In pandas, the TIR1 gene has undergone heavy mutation and is non-functional.

Aside from determining why pandas have a such a bizarre diet (considering they are classified most closely with carnivorous animals) the genome sequence of the panda should help us unravel their odd pigmentation pattern, their very low reproduction rates and- maybe one day- why they are so fun to watch sneeze.

- blog authored by Avi Samelson

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