The dopamine D2 receptor gene polymorphisms associated with chicken broodiness

Chicken broodiness is a polygenic trait controlled by autosomal genes. Prolactin gene is a candidate of great interest in molecular studies of broodiness. However, another candidate dopamine D2 receptor (DRD2) gene has not been studied extensively. The objective of this study was to analyze the genetic effects of the DRD2 gene on chicken broodiness through linkage disequilibrium analyses, tag SNP selection, genetic diversity observation, 2-tailed test, and association analyses. In this study, we assayed 27 variations of this gene in 456 individuals from 6 chicken populations to observe linkage disequilibrium pattern, the tag SNP, and genetic diversity. Among the 6 populations, Taihe Silkies exhibited no characteristic between the square of the correlation coefficient of gene frequencies (r(2)) and physical distance. The other populations including Red Jungle Fowls, Xinghua chickens, Ningdu Sanhuang chickens (NDH), Baier Huang chickens, and Leghorn layers exhibited conspicuous characteristic of decreasing r(2) value over physical distance. Linkage disequilibrium decayed more rapidly in Red Jungle Fowls, Xinghua, and NDH than in Baier Huang and Leghorn layers. Allelic frequencies and genotype distributions in the 5 populations showed that A-38600G, I-38463D, T-32751C, A-16105G, A-6543G, C-6539T, and A+2794G were possibly associated with broodiness. Besides the above 7 sites, another 2 sites that might be associated with broodiness were screened by 2-tailed test. All 9 sites were used for association analyses with broodiness in 644 NDH chickens. A significant association (P < 0.05) was found between A-16105G and broody frequency (%), and the T+619C in intron 1 was significantly associated with duration of broodiness (P < 0.05). These findings suggested that the DRD2 gene should be included in future genetic studies of chicken broodiness and 2 SNP of A-16105G and T+619C might be markers for breeding against broodiness.