Periovulatory hormone profiles and components of litter size in gilts with different estrogen receptor (ESR) genotypes

Estrus, endocrine changes during the periovulatory period, and components of litter size at Day 35/36 of pregnancy were studied in gilts with estrogen receptor genotype AA (AA gilts) or BB (BB gilts), in which the B allele is associated with a larger litter size. Neither estrus length nor estrous cycle length was affected by estrogen receptor genotype. No differences in periovulatory plasma LH, estrogen or progesterone profiles between the AA and BB gilts were detected. Furthermore, temporal aspects of these profiles were not different for both genotypes. Although the B allele is associated with a larger litter size, no differences in number of corpora lutea or number and percentage of vital Day 35/36 embryos were found in this study. This indicates that the difference in litter size is not due to differences in oocyte maturation, fertilization, implantation or embryonic survival, but is likely caused by a difference in fetal survival. Thus, uterine capacity might be different for both genotypes. The available uterine space per embryo seems to be the same for both genotypes, as is endometrial folding of uterine surface area. However, a difference in placental size was found. Embryos of BB gilts had significantly longer placentae than embryos of AA gilts. These results suggest a higher chance for placental insufficiency in AA gilts, leading to the expected higher fetal mortality compared with the BB gilts. The difference in placental size might have been related to a difference in the timing of embryonic mortality.