For its role in the mediation of growth hormone (GH) galactopoietic effect, growth hormone receptor (GHR) was considered a functional candidate gene for milk performance in cattle. However, its genetic variation and potential effect have not been investigated in Egyptian buffaloes. This study aimed to screen GHR for polymorphisms and study their associations with milk traits in Egyptian buffaloes. Polymerase chain reaction, single-strand conformation polymorphism, and sequencing were used to identify mutations in 4 exons (E4–E6 and E8) of the GHR gene in 400 Egyptian buffaloes. No polymorphisms were found in E4, while 2 SNPs (c.380G>A/p.Arg127Lys and c.387C>T/p.Gly129) in E5, one silent mutation (c.435A>G/p.Pro145) in E6, and another missense mutation (c.836T>A/p.Phe279Tyr) in E8 were detected. The c.380G>A SNP in the extracellular domain was associated with milk yield, fat %, protein %, and 305-day milk, fat and protein yield, with higher levels in animals carrying the mutant A allele. The c.836T>A SNP in the transmembrane domain was associated with milk yield, fat %, protein %, and 305-day milk, fat and protein yield, with higher milk yield and lower fat %, protein %, fat and protein yield in the mutant A allele-animals. Interestingly, animals with the two mutant AA alleles produced higher milk yield, fat %, protein %, fat and protein yield, accompanied with upregulated expressions of GHR, GH, insulin-like growth factor 1 (IGF1), prolactin (PRL), prolactin receptor (PRLR), β-casein (encoded by CSN2 gene), and diacylglycerol acyltransferase-1 (DGAT1) genes and proteins in milk somatic cells. Therefore, selection of Egyptian buffaloes with mutant AA haplotypes for the novel c.380G>A SNP and the well-known c.836T>A SNP could improve milk yield and quality in buffaloes.

Prolactin (PRL) and its receptor (PRLR) were considered as potential genetic markers for milk production and quality traits in cattle. However, little information is available regarding PRLR genetic diversity and association studies with milk traits in Egyptian water buffaloes. Therefore, the present study was conducted to search for mutations in PRLR and determine their associations with milk performance in these animals. Exon3 (E3) and E10 of PRLR were screened for polymorphisms using single strand conformation polymorphism (SSCP) and sequencing in 400 buffaloes. The associations between haplotypes and milk production (fat%, protein%, lactose%, and solid%) traits as well as mRNA and protein levels of PRL and PRLR were studied. Two single nucleotide polymorphisms (SNPs) in E10 were detected: g.11685G>A (p.Ala494Thr) and g.11773T>C (p.Val523Aal). The G and T alleles were wild (ancestral) alleles, while the A and C alleles were mutant alleles. These SNPs resulted in four haplotypes; AC, AT, GC, and GT. Buffaloes with wild GT haplotypes showed significantly higher milk yield, fat% and protein%, mRNA and protein levels of PRL and PRLR in milk somatic cells than other animals. Animals carrying mutant AC haplotype had inferior milk traits and lowest levels of associated mRNAs and proteins. With these results, we could conclude that the selection of buffaloes with wild GT haplotypes for g.11685G>A and g.11773T>C SNPs of the PRLR gene might improve the milk production traits of Egyptian water buffaloes.