Two strange mammals illuminate the process of natural selection

CONVERGENT evolution—the arrival, independently, by different species at the same answer to a question posed by nature—is a topic of great interest to biologists. One aspect of the phenomenon which has not yet been much looked at, however, is its underlying genetics. In particular, an issue not previously addressed is how often such changes arise from similar mutations in the two convergent lines, and how often they have different genetic causes that happen to have similar effects on the organisms’ forms and functions. That has now been rectified by an examination of two creatures which, though only distantly related, share an unusual feeding habit, an unusual anatomical feature and an unusual name: panda.

The giant panda is a black and white bear. The red panda (pictured above) is related to weasels, raccoons and skunks. Their habitats—mountainous areas of southern China and its neighbours—overlap, but their last common ancestor lived 43m years ago. They do, though, share a limited kinship, for both are members (along with dogs, cats, hyenas, mongooses, seals and so on) of the mammalian order Carnivora. Which is curious, because both are vegetarian.

More intriguing still, both subsist almost entirely on bamboo (some etymologists think their mutual name is derived from the Nepali phrase nigalya ponya, meaning “bamboo-eater”). And most curiously of all, both have a sixth digit on their forepaws—a kind of ancillary thumb derived from one of the bones of the wrist that helps them hold bamboo stalks for consumption. These common features led Hu Yibo of the Institute of Zoology, in Beijing, and his colleagues, to wonder if the two vegetarian carnivores also shared genetic modifications that might explain those features. And, as Dr Hu observes in a paper just published in the Proceedings of the National Academy of Sciences, they do.

To search for such genetic commonalities, Dr Hu and his colleagues compared the DNA of the two pandas with that of four other members of the Carnivora: polar bears (close relatives of giant pandas), ferrets (close relatives of red pandas), and tigers and dogs (close relatives of neither). If pandas share features of their DNA with each other, but not with the four comparison species, he reasoned, then it is likely that those features encode recent adaptations common to the two of them.

Altogether, the team identified 70 genes (out of the 20,000 or so that mammals have) which sport bits of DNA that seemed to be shared. They also found ten genes which have been disabled by crippling mutations in both of the pandas, but not in the other four species. Not all of the shared genetic features obviously tied in with the shared peculiarities of pandas, but some did. These fell into three categories: genes affecting anatomy, genes affecting appetite and genes affecting the digestion and metabolism of nutrients.

The anatomy-related adaptations were those that seemed to control the development of the pandas’ second thumbs. Two genes in particular, DYNC2H1 and pericentrin, have mutations that cause identical changes in each type of panda in the proteins encoded by these genes. In mice, mutations in these genes are known to encourage extra digits to grow, so it is not unreasonable to suspect that they are also the cause of this in pandas.

The appetite-modifying change involved one of the disabled genes. When working, this gene encodes a protein which forms part of the tongue’s taste buds for umami, a savoury “meatiness” separate from the other four tastes of sweetness, sourness, bitterness and saltiness. Umami perception is stimulated by glutamic acid, one of the 20 amino acids that make up proteins—but one that is more characteristic of animal proteins than plant ones. Sensitivity to umami is valuable to a carnivore but unnecessary (and possibly harmful) to a herbivore.

The other genetic convergences Dr Hu recorded were related to the digestion and metabolism of substances scarce in, or absent from, bamboo. He and his colleagues noted parallel changes in the genes for three digestive enzymes whose job is to liberate two particular amino acids, lysine and arginine, from proteins. Both lysine and arginine are abundant in meat, but in short supply in bamboo. The team also noted parallel changes in four genes that encode proteins involved in the metabolism of two vitamins, A and B12, and of arachidonic acid, a lipid essential for bodily function. All of these are scarce or non-existent in bamboo as well. Exactly how the genetic changes seen alter the effectiveness of the proteins involved remains to be determined. But the prediction would be that they enhance the availability of the nutrients in question.

The upshot is that the two pandas do indeed seem to have similar genetics underlying their similar peculiarities. Such similarities are, admittedly, easier to find than different genetic routes to the same outcome would be. But Dr Hu has established that, at least in the case of pandas, natural selection has often taken the same paths to arrive at the same outcomes.