The effect of estrogen receptor genotype on litter size and placental traits at term in F2 crossbred gilts

The effect of estrogen receptor (ESR) genotype (two alleles, A and B) on litter size of 275 Large White x Meishan F2 crossbred gilts (73 AA, 126 AB and 76 BB gilts) was tested. In addition, for 63 of these gilts (18 AA, 24 AB, and 21 BB) the effect of ESR genotype on average placental traits at term was tested, since individual placental information was available for 88% of the 628 liveborn piglets. Without affecting average birth weight of the piglets, ESR genotype significantly affected litter size, i.e. AB gilts had larger litters than BB gilts (P < 0.05). Total number born was 11.38+/-0.38, 11.88+/-0.28, and 10.68+/-0.35, while number born alive was 10.45+/-0.39, 11.07+/-0.29, and 9.85+/-0.36 for AA, AB and BB gilts, respectively. Since the B allele in previous research was associated with largest litters, the hypothesis that ESR is a marker rather than the major gene itself is discussed. Average placental length, surface area, and weight including and excluding amnion were not affected by ESR genotype. However, placentae of AB gilts had a significantly lower number of areolae per placenta than BB gilts and had a lower number of areolae/cm2 placenta than AA and BB gilts. Number of areolae was 8945+/-663, 7240+/-619, and 9694+/-633, for AA, AB and BB gilts, respectively. Although the reason for the low number of areolae on placentae in AB gilts is not yet known, the results suggest that the ESR linked major gene for litter size might be involved in the development and activity of endometrial glands.