Submitted to: Animal Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/9/2004
Publication Date: 1/20/2005
Citation: Vallet, J.L., Freking, B.A., Leymaster, K.A., Christenson, R.K. 2005. Allelic variation in the erythropoietin receptor gene is associated with uterine capacity and litter size in swine. Animal Genetics. 36(2):97-103. Interpretive Summary: An increase in the average litter size in pigs of just one extra piglet per litter would be worth about $200 million a year to swine producers in this country, because it would reduce the number of sows required to produce a given number of pigs, and would save the cost of maintaining the sows. Litter size is influenced by the number of eggs shed per reproductive cycle, the percentage of those eggs fertilized by sperm, the percentage of embryos that die during early pregnancy, and the ability of the uterus to carry pig fetuses to term. This last factor has been termed uterine capacity, and it currently represents the factor that most limits litter size. Uterine capacity is influenced by both the environment created by the uterus and the ability of the fetuses to survive a crowded intrauterine environment. We discovered a single base change in the DNA sequence of the erythropoietin receptor (EPOR) gene which controls red blood cell development in pigs. The difference in the sequence of the EPOR gene affected the uterine capacity of gilts. We further demonstrated that this difference was associated with a 2.5 pig difference in litter size. Although the presence of sequence variation in the mother and the piglet are somewhat confounded, our results indicated that the effect on litter size was due to EPOR sequence variation in the piglet rather than the mother. A genetic marker based on this difference in the DNA sequence of the pig EPOR gene could be useful for marker assisted selection of gilts for litter size.
Technical Abstract: A single nucleotide polymorphism (SNP; C versus T) that creates an extra GATA-1 site (T allele) in intron 4 of the swine erythropoietin receptor (EPOR) gene was discovered and a genotyping assay for this SNP was developed. Four hundred and two gilts from lines selected either at random (control), for ovulation rate (OR) or for uterine capacity (UC) for 11 generations were unilaterally hysterectomized-ovariectomized (UHO) at 160 d of age, mated at approximately 250 d of age and slaughtered at 105 d of pregnancy. Blood samples and spleens were collected from each fetus and the numbers of CL and live fetuses, the weights of each fetus and placenta, and each fetal hematocrit were recorded. In addition, intact gilts from the OR line or from a Yorkshire, Landrace, Duroc, crossbred line (BX) were mated and farrowed. At farrowing, the numbers of fully-formed and live piglets were recorded for each litter. Genomic DNAs were isolated for both the UHO and intact gilts, from fetuses from the UHO gilts that were heterozygous for the EPOR SNP, and from the boars from the BX line and were then used to determine EPOR SNP genotypes. Presence of the EPOR T allele was associated (P < 0.01) with increased uterine capacity in UHO gilts from the three selected lines. The number of heterozygous and homozygous fetuses did not differ within UHO litters, nor did EPOR genotype influence fetal hematocrit. In intact gilts from the OR line, litter size was significantly associated (P < 0.05) with EPOR SNP genotype. Finally, results from intact gilts of the BX line, in which both the gilt and the boar genotypes were known, allowed an analysis to determine the effect of the gilt and/or the fetal genotype on litter size. This analysis indicated that the predicted fetal genotype (adjusted for gilt genotype) was associated with litter size (an increase of 2.6 +/ 1.0 piglets born alive predicted for homozygous T litters compared to homozygous C litters, P < 0.01) and the effect of the gilt genotype (adjusted for fetal genotype) on litter size was not significant. These results indicate that the EPOR SNP is associated with uterine capacity and litter size in two distinct populations and could be useful in increasing litter size in swine that are not limited in ovulation rate.