Why does the giant panda eat bamboo? A comparative analysis of appetite-reward-related genes among mammals

Abstract

Background: The giant panda has an interesting bamboo diet unlike the other species in the order of Carnivora. The umami taste receptor gene T1R1 has been identified as a pseudogene during its genome sequencing project and confirmed using a different giant panda sample. The estimated mutation time for this gene is about 4.2 Myr. Such mutation coincided with the giant panda's dietary change and also reinforced its herbivorous life style. However, as this gene is preserved in herbivores such as cow and horse, we need to look for other reasons behind the giant panda's diet switch.

Methodology/principal findings: Since taste is part of the reward properties of food related to its energy and nutrition contents, we did a systematic analysis on those genes involved in the appetite-reward system for the giant panda. We extracted the giant panda sequence information for those genes and compared with the human sequence first and then with seven other species including chimpanzee, mouse, rat, dog, cat, horse, and cow. Orthologs in panda were further analyzed based on the coding region, Kozak consensus sequence, and potential microRNA binding of those genes.

Conclusions/significance: Our results revealed an interesting dopamine metabolic involvement in the panda's food choice. This finding suggests a new direction for molecular evolution studies behind the panda's dietary switch.

Figure 1. Panda and dog COMT protein structural simulation.

The simulated panda or dog proteins were indicated with pink color for backbone and red color for special amino acid residues. Human proteins were indicated with blue color for backbone and yellow color for special amino acid residues. The side chain of COMT catecholamine substrate binding sites (Lys₁₄₄, Asn₁₇₀, Glu₁₉₉) and SAM binding sites (Val₄₂, Ser₇₂, Glu₉₀, Asp₁₄₁) were shown in ball and stick model. (A) The simulated panda COMT structure compared with human COMT. The α4 helix in blue square has turned into a loop in panda. (B) The simulated dog COMT structure compared with human COMT.

Figure 2. Sequence alignment of COMT from all nine species.

The number for amino acids is based on human soluble COMT. The conserved amino acids were highlighted. The regions for alpha helix and beta sheet were marked at the bottom of the alignment (wave for alpha helix and arrow for beta sheet). The region with missing amino acids in panda is highlighted with purple square box.

Figure 3. Part of the panda COMT 3′UTR and predicted miRNA-199a-5p binding.

COMT 3′UTR secondary structure was predicted by Mfold and part of it was shown on the left. The possible miRNA-199a-5p target with panda COMT gene and the calculated free energy was shown on the right side.