Discussion
The ESR B allele has been shown in multiple studies to have a significant effect on reproductive efficiency in pigs. As a result, this allele is being utilized by a commercial breeding program in a marker assisted selection scheme. It has been speculated and is logical that the origin of the allele seen in Large White and Yorkshire populations (and not in other domestic breeds) is from interbreeding of English pigs that were to become the Large White breed with Chinese pigs prior to 1800. Porter (cited in Jones, 1998) suggested this practice as a `fad' that resulted in the decrease of age at slaughter of the English pigs from 18 mo to 9 mo or less. While Chinese pigs are reproductively superior to European pigs, they are notably inferior in regards to production and carcass traits (Haley et al., 1992). White et al. (1995) reported that Meishan barrows compared to Yorkshire barrows at the same age (123 days) had a decreased ADG of 0.313 lb, an increased BF thickness of 0.331 inches, and a decreased LMA of 2.58 sq. inches. Because of these characteristics and the likelihood that Chinese germplasm is the origin of the ESR B allele, it is also of interest to researchers to determine if any pleiotropic effects with respect to production and carcass traits are associated with this allele.
Previous association studies by Rothschild et al. (1996) and Short et al. (1997) included production and carcass traits such as ADG, average daily feed intake, feed efficiency, and BF. Rothschild et al. (1996), utilizing nearly 2,000 records, found a non-significant antagonistic trend of the ESR B allele with increased backfat thickness and a non-significant beneficial trend of the ESR B allele with increased ADG. However, in the study by Short et al. (1997) utilizing nearly 12 times as much data, a significant favorable pleiotropic effect of the ESR B allele was observed for decreasing BF. While a significant reduction in average daily feed intake was observed in animals with the B allele, there was no significant effect of the allele on ADG or feed efficiency.
The current study was designed to determine any associations between the ESR B allele and production and carcass traits in our population. Also included in the analyses were IBW and LMA, which were yet to be investigated. Our finding of a small association between the ESR B allele and IBW is logical given the role of estrogen during pregnancy in pigs. In addition, Ying et al. (2000) reported that the ESR-a gene is expressed in pig embryos during preimplantation development. ESR mRNA was detected using RT-PCR at the one-cell, two-cell, and four-cell stages but then became undetectable at the five- to eight-cell stages and the morula stages. ESR protein was detected using an immunocytochemistry technique at the one-cell and four-cell stages and at a reduced intensity at the five- to eight-cell stages. ESR mRNA, but not the protein, became detectable again at the blastocyst stage. Sensitivity of the protein detection method was a limiting factor in the study and may not detect protein that was actually present at the blastocyst stage. Results of the study show that the ESR mRNA reappearance at the blastocyst stage is the result of embryonic RNA synthesis and thus a switch in the control of development from the maternal genome to the zygotic genome. Although the ESR polymorphism utilized in the current study is intronic and does not alter the receptor protein, there may be a direct effect on receptor expression, ligand binding, turnover, and/or DNA binding.
The association seen in this study between the ESR locus and BF, when considered simultaneously with previously published reports, gives strong evidence of linkage between the ESR PvuII polymorphism and a QTL affecting BF. The effect on BF due to incorporation of the ESR B allele in a marker-assisted selection program for litter size seems to be herd (line) dependent. Thus, due to linkage disequilibrium between populations, selection for this allele may be accompanied by an increase or decrease in BF depending upon the linkage phase of the founder population. Future QTL studies with increased marker densities will likely resolve the position of the QTL and possibly the underlying gene affecting BF.