Exploring Genetic Mechanisms Underlying Eggshell Color Of Duck Eggs And Relationship Between Eggshell Color And Quality

Eggshell color is one of the indicators that impacts the quality of eggs. Blue shell is prevalent in poultry eggshell color due to its unique color and rich nutrition, and blue-shelled eggs are loved by consumers. However, there still exists two problems on the study of blueshelled eggs:(1) the blue-shelled egg is caused by biliverdin(a kind of bile pigment) which is deposited on the eggshell. If this eggshell pigment affect the eggshell quality?(2) blue-shelled eggs of chicken is caused by a genetic mutation of SLCO1B3, but further study found that duck blue-shelled egg is not caused by the same mutation. Consequently, what forms the genetic mechanisms that caused duck blue-shell phenotype?The first study was conducted to analyze the relationship between duck eggshell color and eggshell thickness and strength using 272 blue-shelled eggs and 315 white-shelled eggs of the Jinding duck as the research objects. The results demonstrated that the eggshell strength and thickness of blue-shelled eggs were significantly higher than white-shelled eggs(P <0.05), and there were no significant correlation between eggshell thickness and eggshell strength in blue-shelled eggs. Moreover, the blue eggshell color was not correlated with the eggshell strength and thickness of blue-shelled eggs.The second study on the formation mechanism of blue-shelled duck eggs mainly adopted the idea of ? ? reverse genetics which could explore the possible underlying molecular mechanisms based on detecting the differently expressed genes. This experiment selected the eggshell gland which is the key position for eggshell pigment synthesis and secretion of three white-shelled ducks and three blue-shelled ducks for transcriptome sequencing using next-generation sequencing technology. The results showed that there were 1438 differently expressed genes in eggshell gland between blue-shelled ducks and white-shelled duck(P<0.05, FDR q < 0.05), among which 677 up-regulated expressed genes and 761 down-regulated expressed genes of blue-shelled ducks compared with white-shelled ducks. Functional enrichment analysis revealed that the differentially expressed genes were mainly involved in the cell membrane formation and the transporting process of some substance.Organic anion transporting polypeptide(OATP) are a variety of membrane protein which could mediate the cell membrane transportation, one of the transporting substrates among which is biliverdin that mainly color the eggs blue. The transcriptome sequencing results andOATP gene family features showed that the members of OATP gene family as the important candidate genes could be used for exploring the formation mechanism of blue-shelled poultry eggs. In this study, RT-PCR technology was applied for determining the gene expression of 10 genes of OATP gene family in the shell gland and liver. The results found that SLCO2B1 was normally expressed in the eggshell gland of blue-shelled duck eggs, but was not expressed in blue-shelled duck eggs. Other 9 genes were not expressed in the two groups, or no significant different expression in this two groups. In order to clarify the reasons why SLCO2B1 differentially expressed, RACE and RT-PCR technologies were used in this study to clone the complete transcript sequence of SLCO2B1. Sequence alignment showed that the homology of SLCO2B1 between blue-shelled and white-shelled duck was 98.64 %, which indicated that the different gene expression of SLCO2B1 was not caused by the abnormal primers combination.Meanwhile, this study carried out a resequencing in the regions of upstream 5 kb sequence and the first six exons of SLCO2B1, which demonstrated that there existed many structural variations that were significantly correlated with the trait of blue eggshell. This result supports that differential expression of SLCO2B1 are more likely to be caused by the mutations in cis-elements.In summary, the results in this study support the existing biological relationship between eggshell color and quality. In view of the advantages of eggshell thickness and strength in blue-shelled duck eggs, blue-shelled duck breeding is of positive significance for improving duck eggshell quality and decresing the rate of broken eggs. Transcriptome squencing demonstrated that blue duck eggshell may be related to the cell membrane structure or abromal cell membrane transportation. Further study on the OATP gene family members showed that there was a bigger difference of SLCO2B1 expression between blue-shell and white-shell ducks, which suggested SLCO2B1 as an important candidate gene to explore the genetic basis of the blue eggshell pheno of duck. Further identification of the mutations regulation of SLCO2B1 would help expound the genetic mechanism underlying blue eggshell color of duck, thereby providing a method for blue-shell duck breeding based on molecular detection.