Physiological aspects of two candidate genes for litter size in pigs : ESR and PRLR

In the present thesis, physiological aspects of two candidate genes for litter size, estrogen receptor (ESR) and prolactin receptor (PRLR) gene were investigated in gilts at D 35 of pregnancy and at term. For both genes two alleles (A and B) were described. The results clearly indicate that the two polymorphisms affect different components of litter size that are expressed at different stages of pregnancy, i.e. ovulation rate (PRLR gene) and fetal survival (through an effect on placental size: ESR gene). This implies there are favorable and unfavorable PRLR/ESR genotype combinations for litter size. The favorable PRLR/ESR genotype combination is AA/BB for L93 Meishan Synthetic gilts, and AA/AB for Large White x Meishan F2 crossbred gilts. In contrast with the general opinion, there actually seem to be traits and relations between traits that are dependent on both maternal and fetal ESR genotype, which might lead to a difference in Iitter size. In ESR AA gilts, for example, the fraction of ESR AA fetuses that are growth retarded at D35 of pregnancy is larger than the fraction of their AB littermates. Therefore a distortion in genotype ratio of liveborn piglets was predicted. Combining the results with information from literature has resulted in the hypothesis that the ESR gene is a marker for litter size, while the possibility of PRLR gene being a major gene rather than a marker for a closely linked major gene for litter size can not be excluded. Since other genes (e.g. ESR gene) and also environmental factors might change the effect caused by the PRLR polymorphism within the 112 days to parturition, however, at present it is preferable to state that PRLR gene is a major gene for ovulation rate rather than for litter size. Like every marker assisted selection, selection on the favorable genotype combination will lead to an enlargement of litter size once-only. Furthermore, the present thesis shows examples of marker alleles having positive and negative effects at the same time, making it difficult to use the marker for selection. The favorable PRLR allele for litter size for example, appears to be the unfavorable allele for age at first estrus and litter average of teat number of the piglets, while the favorable ESR allele for litter size appears to be the unfavorable allele for growth until weaning. This problem seems to be a biological reality animal scientists will have to live with. It clearly demonstrates the importance of physiological research parallel to and coherent with the search for QTLs and markers for any trait.