| Four different models including wood incomplete gamma function model were used to select the best fitting model to provide the basis for accurate prediction of the 305-d milk yield, and provide a reference basis for the study of production performance and lactation curves of Holstein cows in the future. Meanwhile, Single nucleotide polymorphisms were detected by the methods of DNA pooling and PCR-SSCP, combining with HRM technique to identify all genotypes to study the correlation between genetic variation and milk yield and fat percentage. Besides, mRNA expression and methylation status of the AGPAT6 gene were analyzed to provide a theoretical basis for revealing the molecular mechanism affecting milk production traits. The results are as follows:1. Fitting analysis of Holstein lactation curveLactation curves of 305-d milk yield were analyzed using curve estimation and nonlinear procedure in SPSS 17.0. The results showed that polynomial regression model (six times) were the best lactation curve for Holstein cows, its fitting results for each parity lactation curve were ideal. Models showed significant effect on the fitness of lactation curve (P<0.01). The parity showed no significant effect on the fitness of lactation curve (P>0.05).2. AGPAT6 gene polymorphisms and their correlation with milk production traits(1) Six SNP s from six loci(P1, P2, P3, P6, P7 and P8)were detected from eight loci in AGPAT6 gene, no polymorphism was found in P4 and P5 locus. There were three genotypes for each SNP. The Holstein cows at the six loci were all in moderate polymorphism (0.25<PIC<0.5).(2) The correlation analysis between different genotypes of the six SNPs and milk production traits were analyzed using the least square linear fitting method in SPSS 17.0. The results showed that individuals with different genotypes of P1 locus showed significant effect on milk composition(P<0.05); Individuals with different genotypes of P2, P3, P6 and P8 locus showed significant effect on milk yield and milk composition(P<0.05); Individuals with different genotypes of P7 locus showed significant effect on milk yield, fat percentage and non-fat solids(P<0.05), and there was no significant effect on protein percentage(P>0.05).3. ABCA1 gene polymorphisms and their correlation with milk production traitsOnly a SNP was detected among three exons in ABCA1 gene. There were also three genotypes in the mutation:TT, TC and CC. The Holstein cows at the six loci were all in moderate polymorphism (0.25<PIC<0.5) and not in the agreement of Hardy-Weinberg equilibrium. The correlation analysis between different genotypes of the SNP and milk production traits was also analyzed. The results showed that individuals with TT and TC genotypes showed significant effect on milk yield than CC genotype (P<0.05). However, there was no significant effect on fat percentage and protein percentage (P>0.05).4. Analysis of mRNA expression and promoter methylation of AGPAT6 gene(1) Total RNA was extracted from somatic cells to study the individual expression of different genotypes of AGPAT6 gene P2 locus under the condition of the same parity and milk yield. The results showed that mRNA levels of individuals with TT and TC genotypes were significantly higher than CC genotype (P<0.05), and there was positive correlation between mRNA levels and fat percentage.(2) The methylation status of individuals with different genotypes of AGPAT6 gene P2 locus were analyzed by methylation special PCR. The results showed that the methylation of individuals with TT, TC and CC genotype were not agree with the fluctuating trend of gene expression. Thus, mRNA expression of individuals with different genotypes may not be regulated by AGPAT6 gene promoter methylation process. |
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