Identification Of Candidate Genes Affecting The Trait Of Thick-to-thin Albumen Ratio In Layer

The chicken egg is one of the high quality sources of animal protein.With the development of economy and the improvement of living standard,much attention has been paid to the internal quality of eggs.One of the major factors contributing to interior egg quality is the thick egg albumen.High quality eggs have a firm jelly-like structure of albumen,which keeps the yolk in the centre of the egg,and a large proportion of thick egg white is also desirable in the food industry.In this study,the characteristics and molecular mechanism of thick-to-thin albumen ratio trait were investigated in Rhode Island white layers and other local chicken breeds.Eggs were collected from D3 strain,R2 strain,D1 strain,Hy-line Brown,Hoddan and Huainan chickens at 40 weeks of age,to measure the content of thick albumen and relative egg composition traits,for analyzing the effect of breed(strain)on the ratio of thick to thin albumen and relative egg composition traits.Eggs were collected from the D3 and R2 strains at 30,40 and 50 weeks of age to to investigate the effect of age on the ratio of thick to thin albumen.The results showed thatthe content of egg composition were different between each breed(strain).The thick albumen proportion of eggs was highest in R2 strain whose ratio of thick to thin albumen was 1.60,whereas the lowest proportion of thick albumen was found in Hoddan whose ratio of thick to thin albumen was 0.91.Egg weight,albumen height,and Haugh unit of eggs in D3 strain were significantly(P< 0.05)higher than those in R2 strain at every experimental week,whereas the yolk percentage and the thick to thin white ratio of eggs were significantly(P< 0.05)lower in D3 as compared to R2.Furthermore,albumen height and Haugh unit significantly decreased(P< 0.05)from 30 to 50 weeks of age in D3 and R2 strains.Additionally,the thick to thin white ratio was positively correlated with egg weight(0.125~0.326)and negatively correlated(-0.041~-0.220)with yolk proportion in both two strains.Significant(P< 0.05)correlations of the thick to thin albumen ratio were also found among 30,40,and 50 weeks of age.The results indicated that the thick-to-thin white ratio was a relative stable trait and might be used as a promising index for evaluating egg composition in the future chicken breeding.REML was applied to estimate heritabilities and genetic correlations for the egg quality traits at 40 wks age in Rhode Island white layers.Heritabilities of egg weight,yolk weight,albumen height,HU,yolk%,thick albumen%,thin albumen% and the ratio of thick to thin albumen were 0.32,0.62,0.28,0.51,0.61,0.39,0.31 and 0.45,respectively.The genetic correlations and phenotypic correlations between egg weight and other traits were high,whereas those for yolk weight with albumen height and HU were low.Positive genetic correlations were found between thick albumen% and other traits except yolk weight.Negative genetic correlations were found between the ratio of thick to thin albumen with HU and yolk%.However,genetic parameters of the thick to thin white ratio should be further investigated larger samples to determine their correlation with other egg quality traits.Using RNA sequencing,we explored the magnum transcriptome in 9 Rhode Island white layers: four layers with phenotypes of extremely high ratios of thick to thin albumen(high thick albumen,HTA)and five with extremely low ratios(low thick albumen,LTA).A total of 220 genes were differentially expressed,among which 150 genes were up-regulated and 70 were down-regulated in the HTA group compared with the LTA group.Gene Ontology(GO)analysis revealed that the upregulated genes in HTA were mainly involved in a wide range of regulatory functions(cellular and organismal developmental processes).In addition,a large number of these genes were related to glycosphingolipid biosynthesis,focal adhesion,ECM-receptor interactions and cytokine-cytokine receptor interactions.Based on functional analysis,ST3GAL4,FUT4,ITGA2,SDC3,PRLR,CDH4 and GALNT9 were identified as promising candidate genes for thick albumen synthesis and metabolism during egg formation.By comparing the mRNA expression trends of candidate genes in the F1 progenies,we foud that the relative fold change of gene expression of FUT4 and CDH4 was consistent between the RNA-Seq and qRT-PCR data.