Science Literature

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. It is not the purpose of this blog to assess the robustness of the method, but it is important to be aware that the reported sequence utilizes previously determined genomes as a reference platform: dog and human genomes in this case.

"Using evidence-based gene prediction, the human and dog genes [. . .] were projected onto the panda genome, and the gene loci were defined by using both sequence similarity and whole-genome synteny information."

The iconic giant panda's genetic makeup reveals degradation (Source here)

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."

A possible genetic factor affecting the giant panda's low fecundity rate was identified. Nearly all of the mammalian reproduction genes were mapped, and "a putative pseudo follicle-stimulating hormone (FSH) [beta]-subunit gene (giant panda-FSHB2)" was noted. The authors comment:

"At this stage, whether the pseudo FSHB2 gene contributes to the reproduction features of the giant panda remains to be determined."

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 sequence and de novo assembly of the giant panda genome
Li, R. et al.
Nature 463, 311-317 (21 January 2010) | doi:10.1038/nature08696

Abstract: Using next-generation sequencing technology alone, we have successfully generated and assembled a draft sequence of the giant panda genome. The assembled contigs (2.25 gigabases (Gb)) cover approximately 94% of the whole genome, and the remaining gaps (0.05 Gb) seem to contain carnivore-specific repeats and tandem repeats. Comparisons with the dog and human showed that the panda genome has a lower divergence rate. The assessment of panda genes potentially underlying some of its unique traits indicated that its bamboo diet might be more dependent on its gut microbiome than its own genetic composition. We also identified more than 2.7 million heterozygous single nucleotide polymorphisms in the diploid genome. Our data and analyses provide a foundation for promoting mammalian genetic research, and demonstrate the feasibility for using next-generation sequencing technologies for accurate, cost-effective and rapid de novo assembly of large eukaryotic genomes.

See also:

Worley, K.C. and Gibbs, R.A. Decoding a national treasure, Nature 463, 303-304 (21 January 2010) | doi:10.1038/463303a

Qiu, J., Genome reveals panda's carnivorous side, 13 December 2009, Nature News | doi:10.1038/news.2009.1141