Despite centuries of domestication, the genetic variation of pigs remains high. The gene pool is still large because domesticated pigs were often crossed with their wild counterparts. This was the conclusion of researchers from Wageningen University and several international colleagues in an article that was published online in Nature Genetics on 31 August.
Pigs were initially domesticated about 9000 years ago at two locations: in Anatolia, now the Asian part of Turkey, and in the Mekong Valley in Vietnam. The researchers from Wageningen University started with two questions: what has changed in the genome of domesticated pigs relative to that of wild pigs, and how did those changes come about?
For the study, the complete genomes from 104 wild and domestic pigs were mapped out and compared with each other. These 104 animals were chosen from a collection of 4000 pigs from all over the world whose genomes were previously characterised using a chip with 60,000 SNPs (single nucleotide polymorphisms: variations in a single base pair of DNA).
Behaviour and body type
The genome sequences revealed that domestic pigs have been strongly selected over many centuries for their behaviour and body type. Pigs became calmer, their skulls became shorter and the animals became smaller – although in recent centuries they have again become taller and larger. However, two distinct subgroups can still be distinguished, related to their region of origin (Asia or Europe), says Martien Groenen, Professor in Animal Genomics and a co-author of the article. “The network of genes that was used for selection in Asia is similar to that of European pigs, but the specific locations on the genome are often different.”
In addition, the researchers showed that genes from feral pigs have continuously intermixed with genes of domestic swine. “In the Middle Ages, farmers let their pigs range free in the forest, so the genetic exchange with their wild counterparts remained strong.” This mixing of wild genes is good news for the breeding sector, says Groenen. “As a result, pig breeders still have access to much genetic variation.”
For breeders, it is useful that the genomes of commercial pigs were also sequenced in the study. In this way, the researchers compiled a catalogue that shows what is specific for wild and domestic pigs. This could be useful information for the development of new breeding lines. Groenen: “Our extensive selection of genome sequences provides more insight into potentially interesting genetic variants as well as undesirable variants.”
Laurent A.F. Frantz et all, Evidence of long-term gene flow and selection during domestication from analyses of Eurasian wild and domestic pig genomes, Nature Genetics, 31 augustus 2015 online first, doi:10.1038/ng.3394.