Introduction
Pituitary hormones are responsible for the control and regulation of many aspects of mammalian physiology, including many of the economically important traits in common livestock species. Of these hormones, prolactin has been especially well studied due to its influence on growth, lactation, and mothering ability. Prolactin exerts its physiological effect via the PRLR, a member of the class 1 cytokine receptor family of membrane receptors. Transgenic mice homozygous for a null mutation in the PRLR gene have reduced fertility, lactation, and mothering ability (Bole-Feysot et al., 1998). In swine, prolactin and PRLR levels have been associated with nursing performance (King et al., 1996) and mammary gland development (Farmer et al., 2000). Polymorphisms in the PRLR gene have similarly been associated with increased litter size in several breeds of swine. In 1998, Vincent et al. reported the discovery of a restriction fragment length polymorphism (RFLP) within the PRLR gene that was associated with an additive effect of +0.10 to +0.51 pigs born per litter in Large White, Meishan, and Landrace synthetic lines. One area that has not been studied, however, is the association between the PRLR gene and reproductive tract traits. If the PRLR gene influences litter size, it should follow that this gene also influences the underlying reproductive system directly. The purpose of this study was to determine the effect of the PRLR gene on several of these reproductive tract traits in swine.