Results and Discussion
A total of 142 animals were genotyped for the PRLR polymorphism (Table 1). PRLR genotype was found to be a significant contributor to the model for several of the studied traits (Table 2). Animals with the 22 genotype were observed to have a greater average fetal weight, total fetal weight, fetal weight per horn, number of fetuses per horn, and average fetal weight per horn than animals with the 11 or 12 genotype. For most traits, allele 1 appeared to be dominant over allele 2 and the PRLR gene acted in a dominant fashion. For several other traits, notable, but statistically nonsignificant trends with respect to PRLR genotype were observed (Table 3). Animals with additional copies of allele 2 have less fetal space, less fetal space per horn, and a greater fetal survival. It would be expected that animals with a greater number of fetuses and a greater fetal weight would have less space available per fetus in the uterus. Also, perhaps the greater number of fetuses per horn present in 22 animals is due to the greater survival rate of ova in these animals as is observed for fetal survival in Table 3. This could be one area in which the PRLR gene exerts its influence on the reproductive physiology of the female.
The previous study by Vincent et al. (1998) found somewhat different results from those outlined here. Previously, the A allele (allele 1) was found to be the favorable PRLR allele. The conflicting results between this study and the current study are probably due to several things. First, the breed composition of the populations used in the 1998 study is very different from the one used in our current study. Secondly, the number of records utilized in the study by Vincent et al. is much different than the number utilized here (2714 versus 142). Thirdly, the PRLR RFLP used in these studies may be closely linked to the mutation that causes the reproductive performance advantage and may not be the causative mutation itself. Recombination between the PRLR RFLP and the causative mutation could have occurred and allowed either PRLR allele to be associated with increased performance. In different populations of swine, these loci could be linked in opposite phases and different PRLR alleles would appear to be advantageous, depending on the population being studied. Linkage disequilibrium could therefore also explain the conflicting nature of which allele was preferred in different swine populations.