The first species to have its genome decoded by ‘next-generation-sequencing’ (NGS) machines is the giant panda (Ailuropoda melanoleuca). The individual animal was known previously to the world as the mascot of the 2008 Beijing Olympic Games. Scientists have been excited by the report because the NGS approach is significantly cheaper and faster than other methods. [. . .]
The estimated size of the giant panda genome is said to be 2.40 Gb (compared with 2.45 Gb for the dog genome and 3.0 Gb for humans) making up about 21,000 genes (similar to humans). “Overall, we found that the quality of the predicted panda genes was comparable to that of other well-annotated mammalian genes.” Although the panda eats only bamboo leaves, genes associated with carnivory are present in the panda:
“Of interest, our analysis of genes potentially involved in the evolution of the panda’s reliance on bamboo in its diet showed that the panda seems to have maintained the genetic requirements for being purely carnivorous even though its diet is primarily herbivorous.”
There was no trace of genes that encode enzymes for digesting cellulose, raising questions about how the panda can possibly survive on bamboo. The hypothesis proposed is that the bamboo diet “may instead be more dependent on its gut microbiome”. Confirmation of this will require further work. A related dietary factor concerns the sense of taste. The authors refer to the five components of taste: sweetness, saltiness, sourness, bitterness and umami. The giant panda has lost the capability of sensing umami, which means that meat has become unappetizing.
“Umami is sensed through the T1R family. In the panda genome, T1R2 and T1R3 are in an intact form, but T1R1 has become a pseudogene – we found that [. . .] two panda T1R1 exons contain transcript errors.”
“Two frameshift mutations occurred in the third and sixth exons of the panda T1R1 gene. The third exon contained a 2-bp (‘GG’) insertion; the sixth exon contained a 4-bp (‘GTGT’) deletion.”
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Some have considered whether the panda genome helps resolve the animal’s taxonomic status. Although most place the panda in the bear family (Ursidae), a case has been made that it belongs elsewhere – in the raccoon family (Ailuridae). Since we do not have the genomes for any of these possible relatives, there is little more that can be said on the matter. However, even if other genomes were sequenced, does the “genome” tell us much about what makes a bear differ from a raccoon or a dog or a human? The genome can be described as the repository of housekeeping genes; it provides the materials needed for the organism to function – but something much more than this is needed to inform taxonomy. The ENCODE project (along with many others) has revealed rich functionality in the non-coding DNA (alias ‘junk DNA’). Consequently, it is probable that the gene sequencers are just scratching at the surface of genetic information.
If the giant panda is correctly assigned to the Ursidae, the new research contributes significantly to the way we understand the speciation of this animal. Before genome sequencing, we could say that it has diversified significantly from ancestral Ursidae stock. It has a reduced number of chromosomes, 42, whereas most bears have 74. It has a wholly vegetarian diet and it has a modified sesamoid bone which it uses to strip bamboo leaves from stems. The panda genome findings provide the background for understanding herbivory: the panda still retains the genes for carnivory but mutations have destroyed the taste trigger for it to eat meat. Although the panda cannot make enzymes for digesting plant food, communities of gut microbes are the most likely explanation of its continuing survival. The reproduction problems experienced by giant pandas may also be linked to a mutation affecting follicle stimulation.
The overall picture is one of speciation/diversification linked to genetic degradation. Natural selection, which has often been portrayed as all-powerful and capable of building exquisitely complex structures, has failed to provide the giant panda with any enzymes for digesting plant food. We do not know whether the modified sesamoid bone is an evolutionary innovation, a part of the degradation story or information neutral. The News & Views essay that accompanies the research paper calls the panda China’s “national treasure” – and so it is. However, from the perspective of genetics, the giant panda is not in a healthy state. Whatever else may be relevant, this case has strong affinities with speciation by gene pool reduction. From the perspective of Darwinism, the giant panda genome testifies to the failure of Darwinian mechanisms to overcome problems caused by mutations. From the perspective of design, we have a story of how a superbly designed carnivore has managed to survive the effects of genetic degradation. From a conservation perspective, without human intervention, the chances of long-term survival are slender.
There is also the finding that Jingjing’s genome has a high degree of genetic diversity, but she is unlikely to be representative of the panda population taken as a whole. It is more prudent to assume that the relatively isolated panda enclaves harbour problems of inbreeding and that Jingjing is an example of the benefits of breeding across enclaves – further supporting the case for human intervention.
The research paper: is:
Li, R. et al., The sequence and de novo assembly of the giant panda genome, Nature 463, 311-317 (21 January 2010) | doi:10.1038/nature08696
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