Pigs evolved bright coat colours rapidly after domestication thanks to the human a penchant for novelty, a new gene analysis suggests.
Farmers selected and bred the brightly coloured pigs to distinguish them from their brown and black wild cousins and probably also because they preferred the unusual colours.
At the other extreme, the gene analysis shows that wild pigs today are evolving through natural selection to maintain camouflage colours to escape detection by predators.
"Every time a gene mutation arose in the wild causing coat colour to change, it was eliminated immediately," says Greger Larson of Durham University, UK, and joint leader of the analysis with Leif Andersson of Uppsala University in Sweden. "So if a black piglet showed up, that was the one picked off by a predator."
Pig a colour
Domestication overrode natural selection with artificial selection from around 10,000 years ago, when humans began to domesticate pigs and other animals such as dogs, favouring animals with mutations resulting in brightly coloured coats. "What it comes down to is the real human penchant for novelty," says Larson.
To establish how colours might have arisen in pigs, the researchers analysed DNA from 68 domestic pigs of 51 breeds, and 15 wild boar. All samples were from animals in Europe and Asia.
In each sample, they examined variations in the gene melanocortin receptor-1 (MC1R) in melanocyte skin cells, which orchestrates the manufacture of melanin pigments. In each species, the gene governs coat colour by dictating the balance between production of dark coloured eumelanin and red-yellow coloured pheomelanin.
They found about 10 mutations in the domestic and wild pigs. But the mutations in the wild pigs were all "silent", insofar as they had no physical effect on the protein produced and therefore on the colour of the animal's coat. This shows that in the wild, colour change was selected against to avoid losing camouflage. "When you mess with the gene, you get over-expression of dark or light melanins, which alters the colour," Larson explains.
By contrast, all the mutations in the domestic pigs altered coat colour. Black pigs overproduce eumelanin, for example, and pink pigs stop making melanin altogether, resulting in a "default" pink colour.
Some of the domesticated pigs had as many as three mutations in their MC1R, each new mutation adding something that couldn't have arisen without the previous ones. For example, in pigs which are pink with black spots, for example, three mutations are needed, and the mutation causing the black spots had to have come last, following on from mutations which gave the pink background colour.
This provided the clinching evidence that the coat colours were selected for after domestication, says Larson, because pink pigs wouldn't have survived long enough in the wild to have allowed the third mutation to arise. "It shows there was a big difference in the selection regimes practiced by Mother Nature and by humans," says Larson.
The analysis also revealed that black pigs in Europe owe their blackness to different mutations from the black pigs in Asia. "It proves independent domestication of pigs on two continents," says Larson.
David Fisher, who studies melanocytes as director of the melanoma programme at Massachusetts General Hospital in Boston, says the study was sound. "It's not difficult to imagine the potential advantages during animal domestication of being able to have an easily recognisable body feature, such as coat colour," he says.
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