B. Isler
K. M. Irvin1
The Ohio State University
Department of Animal Sciences
S. M. Neal
The Ohio State University
Agricultural Technical Institute
1 For more information, contact at: The Ohio State University, 110F Animal Science Building, 2029 Fyffe Rd., Columbus, OH 43210; 614-292-6407; Fax: 614-292-2929; e-mail: irvin.3@osu.edu.
The identification of genes or markers associated with reproductive traits in swine is an important area of research because of the large economic impact these discoveries could have on the swine industry. It has previously been reported that one of these genes, the estrogen receptor (ESR) gene, is associated with increased litter size in pigs. At this time, research is lacking in the examination of the association between ESR genotype and the reproductive system itself. This phase of the study investigates the association between ESR genotype and reproductive tract traits in a population of 162 Large White, Yorkshire, and crossbred pigs. The ESR genotypes of all 162 animals were determined using a PCR-RFLP procedure. Out of the larger group of 162 animals, 78 females were selected and mated to Hampshire boars. These animals were allowed to progress to approximately day 75 of gestation, at which time the animals were slaughtered and their reproductive tracts collected. Data collected on the tracts included ovulation rate, horn length, number of fetuses in each horn, fetal mass, uterine mass, number of mummies, fetal sex, and fetal placement. These data will be analyzed for associations between ESR genotype and reproductive tract data.
Reproductive traits play a large role in determining the efficiency of production in livestock species. In the past, selection for reproductive traits has met with limited success due to the low heritability and sex-limited nature of these traits. The development of new techniques in molecular biology (such as genomic maps) allows us to identify quantitative trait loci (QTL) associated with these reproductive traits. These loci can then be used in marker-assisted selection schemes to more efficiently select for these traits.
A promising group of genes that may influence reproductive traits are those genes that are associated with the steroid hormones. The steroid hormones and their associated receptors are known to play a very important role in reproductive processes (O'Malley, 1990). The role of estrogen and the estrogen receptor in reproduction has been especially well studied. It has been shown that mutations in the estrogen receptor gene (ESR) can produce considerable phenotypic changes in the mammalian reproductive system, including cancer (Lehrer et al., 1990) and infertility (Korach, 1994). Based on observations such as these, it was hypothesized that the ESR locus could influence reproductive traits in swine.
Initial studies of the ESR gene in swine utilized Chinese Meishan swine. The Meishan breed is historically known for its large litter sizes (Haley et al., 1992). Studies using Meishan and Meishan crosses discovered variation at the ESR locus in these swine (Rothschild, et al., 1991). Subsequent studies have found an association between a favorable ESR allele and reproductive traits in several breeds of swine. This advantageous allele has a positive additive effect on total number born and number born alive in swine. The effect of this allele has been shown to range from 1.25 pigs per litter in Meishan crosses to 0.4-0.6 pigs per litter in Large White and Large White crosses (Rothschild et al., 1994; Short et al., 1997). Researchers have also tried to find an association between this locus and other traits in swine, such as backfat depth and teat number (Rothschild et al., 1994; Short et al., 1997). One area that has not been studied, however, is the association between the ESR gene and reproductive tract traits. If the ESR gene influences traits such as litter size, it should follow that this gene also influences the reproductive system itself. The purpose of this study is to determine the effect of the ESR gene on several of these reproductive tract traits in swine.
A total of 162 Yorkshire, Large White, and crossbred animals raised at the Western Branch of The Ohio State University's Ohio Agricultural Research and Development Center (OARDC), South Charleston, Ohio, were included in this phase of the study. Animals consisted of related and unrelated males and females of varying ages. Females were divided into five groups depending on their reproductive status (bred gilts, bred sows, gilts bred and recycled, sows bred and recycled, and cycle post breeding). All bred animals were mated to Hampshire boars.
For each animal, DNA was extracted from lymphocytes and the ESR gene was amplified using a polymerase chain reaction protocol. This protocol has been outlined previously (Short et al., 1997). Amplified products were digested with PvuII restriction endonuclease, separated on a 4% agarose gel and visualized under UV light after ethidium bromide staining. Two ESR alleles (A and B) were identified and each animal was classified as either AA, AB, or BB with respect to ESR genotype.
Seventy-eight of the bred animals (from the bred gilt and bred sow groups) were allowed to progress to approximately day 75 of gestation. At this time, these animals were slaughtered and their reproductive tracts collected and analyzed. Data collected on these tracts included ovulation rate, horn length, number of fetuses in each horn, fetal mass, uterine mass, number of mummies, fetal sex, and fetal placement. Also included in this data set was breeding date, slaughter date, parity, breed, and ESR genotype. This data set will be analyzed for associations between ESR genotype and reproductive tract traits.
This project is currently in progress. More animals will be bred and slaughtered in summer 1998. Approximately 700 reproductive tracts will be included in data collection to estimate associations of the ESR genotype.
Haley, C. S., E. D'Agaro, and M. Ellis. 1992. Genetic components of growth and ultrasonic fat depth traits in Meishan and Large White pigs and their reciprocal crosses. Animal Production. 266:105.
Korach, K. S. 1994. Insights from the study of animals lacking functional estrogen receptor. Science. 266:1524.
Lehrer, S., M. Sanchez, H. K. Song, J. Dalton, F. Levine, P. Savoretti, S. N. Thung, and B. Schachter. 1990. Oestrogen receptor ß-region polymorphism and spontaneous abortion in women with breast cancer. Lancet. 335:622.
O'Malley, B. 1990. The steroid receptor superfamily: more excitement predicted for the future. Mol. Endocrinol. 4:363.
Rothschild, M. F., R. Larson, C. Jacobson, and P. Pearson. 1991. PvuII polymorphisms at the porcine oestrogen receptorlocus (ESR). Anim. Genet. 22:448.
Rothschild, M., C. Jacobson, D. Vaske, C. Tuggle, L. Wang, T. Short, G. Eckardt, S. Sasaki, A. Vincent, D. McLaren, O. Southwood, H. van der Steen, A. Mileham, and G. Plastow. 1996. The estrogen receptor locus is associated with a major gene influencing litter size in pigs. Proc. Natl. Acad. Sci. USA. 93:201.
Short, T. H., M. Rothschild, O. Southwood, D. McLaren, A. de Vries, H. van der Steen, G. Eckardt, C. Tuggle, J. Helm, D. Vaske, A. Mileham, and G. Plastow. 1997. Effect of the estrogen receptor locus on reproduction and production traits in four commercial pig lines. J. Anim. Sci. 75:3138.