Since having been tamed and domesticated some ten thousand years ago, pigs have been getting longer. Researchers from the universities of Wageningen, Uppsala, Copenhagen and Edinburgh are publishing details of the genetic origins of this remarkable change in an article in PNAS (Proceedings of the National Academy of Science), which will be published simultaneously with their Nature article about the sequencing of the pig genome on 15 November.
The European research team discovered the genes used by the very first farmers to select pigs while domesticating them from wild boar, and for selection during the following thousand years. According to the team led by Prof. Leif Andersson (University of Uppsala) and Prof. Martien Groenen (Wageningen University, part of Wageningen UR), the most striking revelation is that pigs have become longer. Early farmers chose pigs with more vertebrae. They varied between 21 and 23, unlike wild boar which have 19 vertebrae. Analysis of the pig genome points towards interaction between three variants of the NR6A1, PLAG1 and LCORL genes. These variants originate from European wild boar.
Charles Darwin also noticed this change in the length of pigs, giving a detailed account in a book published in 1868 and entitled The variation of animals and plants under domestication. However, the underlying genetic variation was not discovered until recently.
Genetic variation can be restricted to a change in a single base pair (SNPs such as the variants in the three genes mentioned above), but can also involve larger parts of the genome (duplications and/or deletions of thousands to hundreds of thousands of base pairs). A duplication of 400,000 base pairs containing the KIT gene is a good example. The KIT gene plays a part in the development of melanoblasts, cells that affect pigment formation. In 1996, scientists discovered that this gene influences skin colour, but genomic research showed that stringent selection can lead to a whole range of variants of the gene and therefore an array of different skin colours (white, dark mottled and even animals with a white band around their girth just behind the front legs). This latest research into the KIT gene shows variations in colour could be caused by a complex combination of one or several copies of the gene and duplications of two other DNA segments around the gene that possibly control the way the gene manifests itself. According to the research, a combination of mutations can have a very specific effect on the function of a gene, a phenomenon that may arise more often than scientists had realised. The results are therefore highly relevant to genetic research into complex diseases affecting human beings.
Carl-Johan Rubin, Hendrik-Jan Megens, Alvaro Martinez-Barrio, Khurram Maqbool, Shumaila Sayyab, Doreen Schwochow, Chao Wang, Örjan Carlborg, Patric Jern, Claus B. Jörgensen, Alan A. Archibald, Merete Fredholm, Martien A. Groenen, Leif Andersson. Strong signatures of selection in the domestic pig genome. Proc. Natl. Acad. Sci. USA, 15 November 2012)