Examination of the Relationship Between the Estrogen
Receptor Gene and Reproductive Traits in Swine
B.J. Isler, K.M. Irvin, S.M. Neal, S.J. Moeller, M.E. Davis, and D.L. Meeker
Department of Animal Sciences
The Ohio State University
For many years, scientists and producers have made tremendous improvement in livestock species using traditional methods of genetic selection. New discoveries in the field of molecular genetics now allow the isolation and study of specific regions of the genome that influence important traits. Animals that contain these marker regions can then be selected for inclusion in a marker assisted selection program. This approach has shown special promise for those traits that are of low heritability and act in a sex-limited manner, such as the reproductive traits. Due to the large part reproductive traits play in determining the efficiency of production in livestock species, a great deal of research has focused on the search for genes which influence these traits. Genes that have been shown to be associated with reproductive efficiency in swine include the estrogen receptor (ESR) gene, the follicle stimulating hormone-b subunit gene (Zhao et al., 1998), and the osteopontin gene (Short et al., 1997a). Of these genes, the ESR gene has received special attention.
Initial studies of the ESR gene in swine utilized animals of the Chinese Meishan breed, due to their large litter size. Studies using Meishan and crossbred Meishan animals discovered a polymorphism at the ESR locus (Rothschild et al., 1991). Subsequent studies found this polymorphism to be associated with increased reproductive performance in several breeds of swine other than the Meishan. Specifically, an advantageous allele of the ESR gene (denoted as the B allele) was shown to have a positive additive effect on total number born and number born alive. This effect ranged in magnitude from +0.8 to +1.25 pigs/litter in Meishan crosses to +0.4 to +0.6 pigs/litter in Large White and Large White crosses (Rothschild et al., 1994; Short et al., 1997b). Studies have also attempted to find associations between this locus and other traits in swine, such as backfat depth and teat number (Rothschild et al., 1994; Short et al., 1997b). A group of traits that have not been studied in detail, however, are reproductive tract traits. The objective of this study was to evaluate the effect of the ESR gene on several of these previously uninvestigated reproductive traits.
Materials and Methods
A total of 518 Yorkshire, Large White, and crossbred animals were included in this study. For each animal, DNA was extracted from lymphocytes and the ESR gene amplified using a polymerase chain reaction protocol that was developed by Iowa State University and has been licensed to the Pig Improvement Company (Short et al., 1997b). Amplified products were digested with PvuII restriction endonuclease, separated on a 4% agarose gel, and visualized under UV light following ethidium bromide staining. Two ESR alleles (A and B) were identified and each animal was classified as AA, AB, or BB with respect to ESR genotype.
Of the original 518 animals genotyped, 147 females were included in reproductive tract analysis. Females were of all four breed combinations and varying parities. Females were mated to Hampshire boars and subsequently slaughtered at approximately 75 days of gestation in a commercial slaughter facility. Following slaughter, gravid uterine tracts were collected and analyzed. Data collected on these tracts included ovulation rate, horn length, number of fetuses per horn, fetal space, fetal survival, average fetal weight, total fetal weight, uterine weight, number of mummies, fetal sex, and fetal placement. Also included in this data were; parity, breed, ESR genotype, and (for some traits) uterine horn. All reproductive tract data were analyzed using the General Linear Model Procedure of SAS (1990).
Litter data for 212 dams with a known ESR genotype were included in litter data analysis. Number of animals weaned, litter weight at weaning, number of stillborn animals, number of overlays, number born alive, litter weight born alive, number of animals born, litter weight born, breed of dam, breed of sire, ESR genotype, and parity were recorded and included in the data set. All litter data were also analyzed using the General Linear Model Procedure of SAS (1990).
Results and Discussion
Allele and genotype frequencies for animals included in the reproductive tract analysis are shown in Table 1. The A allele was present more frequently than the B allele in the overall population and in most of the breed group combinations. All breed group combinations were tested for Hardy-Weinberg equilibria. All breed group combinations were found to be in equilibrium; however, the overall population was not in equilibrium. Allele and genotype frequencies for animals included in the litter data analysis were similar to those found in the reproductive tract analysis (data not shown).
Table 1. ESR allele and genotype frequencies for animals included in reproductive tract analysis.
a YHY = Yorkshire sire H Yorkshire dam, YHLW = Yorkshire sire H Large White dam, LWHLW = Large White sire H Large White dam, LWHY = Large White sire H Yorkshire dam
b P-values from the chi-square analysis for determination of Hardy-Weinberg equilibria, where H0 = Hardy-Weinberg equilibrium.
ESR genotype was found to be a significant for number of fetuses per horn (P = .04). This trait also showed a consistent trend with respect to number of B alleles present (Table 2). Animals with the AA genotype were found to have a lower number of fetuses per horn than animals with either the AB (P = .02) and BB (P = .06) genotypes. Other traits that displayed notable trends with respect to ESR genotype are displayed in Table 2. Note however, that in these cases, all trends were non-significant in nature and the P-values for the effect of ESR genotype in all models were generally high, ranging from .16 to .8.
Table 2. Least-squares means and standard errors for all reproductive traits that showed notable trends with respect to ESR genotype.
a UTLTH = combined length of both uterine horns (cm), TFETWT = total fetal weight (g) per uterus, TNOMUM = total number of mummies per uterus, TAVWT = average fetal weight (g) per uterus, NOFET = number of fetuses per horn, NOMUM = number of mummified animals per horn, WTNBA = total litter weight of animals born alive (kg), WTTNB = total litter weight of animals born (kg), NOSTILL = number of stillborn animals at birth, NWN = number of piglets alive at weaning, WTWN = total litter weight at weaning (kg)
b Significance level of effect of ESR genotype on specified trait
Two-way interactions involving ESR genotype were also found for several traits. An ESR genotype ´ breed group combination interaction was found for number of fetuses per horn (P = .02), in which LW ´ LW animals with the BB genotype were found to have a higher number of fetuses per horn than other breed combinations. An ESR genotype ´ parity interaction was found for horn length (P = .01), with BB ´ parity 3 animals having a lower horn length than animals of other combinations.
From this study, it appears that the ESR gene is associated with several previously uninvestigated reproductive traits. We can now begin to construct a preliminary picture of how the ESR gene positively influences the reproductive performance of the female pig. Adding copies of the ESR B allele appears to increase uterine length, number of fetuses per horn, and average weight of each fetus in the pregnant female. The overall effect of the B allele would therefore be an increase in reproductive efficiency and performance. The future addition of more animals to this study will allow further investigation of the true association between the ESR gene and these reproductive traits.
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