The use of peripubertal donors in embryo transfer (ET) programs has been studied, and no differences in TNO or NTE have been found among heifers that were older than 10 months of age. Furthermore, reproductive and lactational traits of heifers subjected to embryo transfer do not differ from their untreated full siblings. Thus, the heifers in this study can be used as donors in embryo transfer programs without compromising their subsequent reproductive or lactational performance. Hayakawa et al. (2009) compared the outcomes of superovulation and embryo transfer with sexed and unsexed sperm and observed no difference between the two groups for superovulation traits or pregnancy rates. These observations are consistent with results of our previous study. For AI in our study, sexed frozen-thawed sperm from different bulls was used randomly to inseminate heifers.
Heifers with the GG genotype had significantly higher TNO and NTE than those with the TT genotype, indicating that the G allele had a favorable, positive effect on these two superovulation traits. Three SNPs (ss52050737, ss52050738, and ss52050739) in exon 11 of LHCGR and their significant associations with variations in cattle fertility were described, and our results further demonstrated the SNP (ss52050737), in particular, was associated with a positive response to superovulation.
As a response to superovulatory treatment in cattle, more ovarian follicles initiate growth, granulosa cells proliferate and acquire follicle stimulating hormone receptors, and LHRs are induced, permitting the granulosa cells to later respond to LH. Subsequently, several events occur: resumption of oocyte meiosis, transformation of the steroid enzyme complex from production of estrogen to production of progesterone, follicular rupture, and finally corpus luteum formation when serum LH levels increase during the preovulatory LH surge. In cattle, moreover, LH may promote cytoplasmic maturation or competence of the oocyte as well as embryonic development after fertilization, both in vivo and in vitro[4, 5, 27, 28]. The presence of LHRs in bovine oocytes, embryos, and blastocysts has been reported. Bovine oviducts also express LHRs, and their activation results in increased synthesis of oviductal glycoprotein, which binds to embryos to enhance their development[30–34]. Thus, LH may enhance embryonic development through both direct and indirect mechanisms after binding to its receptor. As a G protein–coupled receptor, binding of the LHCGR allows dissociation of membrane-bound cognate G proteins that regulate phospholipase C, adenylyl cyclase, and ion channels, which in turn control cellular inositol phosphates, cAMP, Ca2+, and other secondary messengers. Exon 11, in which SNP ss52050737 is located, encodes the intracellular domain of the LHCGR protein. Therefore, a missense mutation may change the structure of the intracellular region of the LHCGR and reduce the effect of LH to explain the functional relevance of this mutation to variation in response to superovulation and phenotype. However, the biological mechanism by which the mutation influences the function of LHCGR requires further investigation.