Effects of Neonatal Litter Size and Age on Ovarian Gene Expression and Follicular Development in Gilts

Abstract Text: Gilts raised in small litters have greater ovulation rate, stay in the herd longer and produce more pigs.  The objective was to understand how neonatal litter size affects gilt development.  The hypothesis is that gilts reared in smaller litters have greater ovarian follicular development.  Within 24 h of birth, Yorkshire-Landrace crossbred pigs were cross fostered into normal (13.6 ± 0.2 pigs) or small (8 ± 0.2 pigs) litters.  Pigs were cross-fostered between dam pairs such that piglets from each dam were reared in each litter size. Pigs were weaned at 4 wk of age, placed in a nursery building until 8 wk of age, and then moved to a finishing building.  Ovaries were collected from necropsied pigs at weaning, 60, 100, and 140 d of age (n = 8 per treatment at each age). For each animal, one ovary was snap-frozen for RNA extraction and one was placed in fixative for histological sectioning.  Three representative histological sections from each ovary were examined and the number of primordial, primary, secondary, and antral follicles was quantified.  Relative abundance of mRNA in the ovarian cortex was determined using quantitative PCR and analyzed by ANOVA using litter size and age as fixed effects with sire as a random effect.  Target genes were chosen for known function in primordial follicle formation or as candidate genes from a GWAS for ovulation rate.  There was no effect of neonatal litter size on the number of follicles in ovaries during development. The number of primordial and primary follicles decreased (P < 0.0001) whereas the number of antral follicles increased (P < 0.001) with age. The number of secondary follicles in the ovary was greatest (P < 0.01) at 60 and 100 d of age.  Expression of mRNA for AMH and HSD17β4 was greatest (P < 0.04) at 100 d of age.  Abundance of ZFYVE9, BMP4, GDF9, and DDX9 mRNA was greatest (P < 0.02) at weaning and least at 140 d of age.  Expression of ADAMTS19, ESR1, POU5F1, BMP7, and BCL2 mRNA was greater (P < 0.01) at weaning and 60 d of age compared with 100 and 140 d of age. There was no effect of neonatal litter size on ovarian gene expression.  These data indicate that increased ovulation rate of gilts raised in smaller litters is not a result of neonatal programming causing greater primordial follicle formation. USDA is an equal opportunity provider and employer.

Keywords: pig, ovary, litter size, development, follicle, gene expression