Genome-wide re-sequencing and transcriptome analysis reveal candidate genes associated with the pendulous comb phenotype in domestic chickens

To determine the causative variations associated with two chicken comb phenotypes, pendulous comb (PC) or upright comb (UC), two pooled genomic DNA samples from PC and UC chickens were re-sequenced by Next-Generation Sequencer, and genome-wide Single nucleotide polymorphisms (SNPs) were detected. Using three selective sweep approaches, F-ST, theta pi, and Tajima's D, with top 5% window values serving as the threshold, a total of 84 positively selective genes (PSGs) were identified. There were no SNPs in exons of the PSGs with significant differences in allele frequencies between the two comb phenotype groups. Then, 515 differentially expressed genes (DEGs) between the PC and UC were identified by RNA-seq. Three genes including CD36 (CD36 molecule), ADAMTSL3 (ADAMTS-like 3), and AOX1 (aldehyde oxidases 1) are overlapped between PSGs and DEGs. After genotyping seven candidate SNPs in the regulatory regions of the three overlapping genes in 120 chickens from two other breeds, two variants (rs14607046 and rs731818051) in the regulatory regions of AOX1 and ADAMTSL3 were found to have significant differences in allele frequency between the PC and UC, suggesting that the two variants may be causative mutations for PC. Overall, our study shed light on the genetic basis underlying the PC phenotype in chickens.