Abstract
The eggshell blueness is an interesting object for chicken genetic studies and blue-shelled chicken industry, especially after the discovery of the causative mutation of chicken blue eggshell. In the present study, genome wide association study (GWAS) was conducted in Chinese Dongxiang blue-shelled chicken underlying four traits of blue eggshell pigments: quantity of biliverdin (QB), quantity of protoporphyrin (QP), quantity of total pigment (QT), and color density trait (CD). A total of 139 individuals were randomly collected for GWAS. We detected two SNPs in genome-wise significance and 35 in suggestive significance, 24 out of the 37 SNP were located either within intron/exon or near 15 genes in a range of ~1.17 Mb on GGA21. For further confirmation of the identified SNP loci by GWAS, the follow-up replication studies were performed in two populations. A total of 146 individuals of the second generation derived from the former GWAS population, as well as 280 individuals from an alternative independent population were employed for genotyping by MALDI-TOF MS in a genotype-phenotype association study. Eighteen SNPs evenly distributed on the GGA21 significant region were successfully genotyped in the two populations, of which 4 and 6 SNP loci were shown significantly associated with QB, QT and QP in the two repeat populations, respectively. Further, the SNPs were narrowed down to a region of ~ 653.819 Kb on GGA21 that harbors five candidate genes: AJAP1, TNFRSF9, C1ORF174, CAMTA1, and CEP104. Shell gland of chickens laying dark and light blue eggshell was chosen for detection of mRNA expression of the five candidate genes. The results showed differential expression levels of these genes in the two groups. The specific function of these genes has not yet been defined clearly in chickens and further in-depth studies are needed to explore the new functional role in chicken eggshell blueness.
Highlights
Eggshell color is considered one of the essential aspects of egg quality in many countries
Before inclusion PCA components as covariates in the association model, the inflation factor calculated for the genomic control analysis was more than 2 for all traits, indicating that population stratification is detected in our GWA data
Our GWA study revealed two genome-wise significant SNPs and 35 chromosome-wise significant SNPs besides 29 genes associated with eggshell color intensity traits, of which 24 SNPs
Summary
Eggshell color is considered one of the essential aspects of egg quality in many countries. The distribution of eggshell pigments on the two types of eggs (blue and brown eggshell) have been studied by Zhao et al [7] and Wang et al [8], they found that the eggshell from blue-shelled chickens contains a remarkable greater level of biliverdin and a lower level of protoporphyrin than the eggshell from brown-shelled chickens. Recent studies demonstrated that the blue eggshell phenotype in chickens is caused by an endogenous avian retroviral (EAVHP) insertion in the 5’ flanking region of the SLCO1B3 gene This retrovirus was shown to be associated with the over-expression of the SLCO1B3 gene, proposing it to be the causative mutation of the oocyan phenotype in blue-shelled chickens [10, 11]
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