An ancient positively selected BMPRIB missense variant increases litter size of Mongolian sheep populations following latitudinal gradient.

New gene mutation origination is a driving force for the evolution of organisms. The effect of FecB mutation in BMPRIB gene on the litter size of sheep has been well known for a long time, each copy of the mutant allele increases litter size by 0.4-0.5. However, the origin and adaptive evolution mechanism of FecB mutation are still unclear. Here we carried on the thorough analysis on evolutionary features of BMPRIB gene and found that 150 species as a whole is under purifying selection while sheep lineage shows evidence of positive selection. The results of allele age estimation revealed that the FecB mutation in Mongolian sheep of China originated in Mongolian Plateau at about 5000 years ago. Due the two shape drops in temperature subsequently, Mongolian sheep migrated from north to south following the northern nomadic people. Accordingly, the FecB mutant allele frequency increased, with the lowest in sheep locating at Mongolian plateau (0.01) and the highest in sheep locating at Yangtze River valley (0.96). In conclusion, the FecB mutation in Mongolian sheep of China originated in Mongolian Plateau at about 5000 years ago, and the differentiated litter size of Mongolian sheep might be the result of adaptation to various environments during the migration following latitudinal gradient. This study may well exemplify selection on an ancient variation triggered by drastic ecological shifts, and is also helpful to analyze the adaptive evolution mechanism of economic traits of domestic animals and identify major genes and molecular markers.