Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2004
Publication Date: 1/1/2005
Citation: Yen, H., Ford, J.J., Zimmerman, D.R., Johnson, R.K. 2005. Follicular development and maturation in gilts selected for an index of high ovulation rate and high prenatal survival. Journal of Animal Science. 83(1):130-135. Interpretive Summary: Females from a line of pigs that was produced by multiple generations of selection based on the combined influence of ovulation rate and litter size was evaluated for the pattern of development of ovarian follicles during the period of selection of those follicles that were destined to ovulate. This pattern of development was compared to that observed in the control line of females that are maintained along with the selected line. Females were slaughtered on days 0, 2, 3, 4, and 5 after an injection of prostaglandin which caused corpora lutea to regress and a new group of follicles to grow and subsequently ovulate. The results of this comparison indicate that ovarian follicles in females from the selected line develop at a slower rate than follicles of females in the control line. These results are useful in the design of subsequent studies as we search to identify the genetic basis for differences in ovulation rate among these two lines of pigs. This collaborative study was conducted with a line of pigs developed at the University of Nebraska.
Technical Abstract: Fifty-nine 10th generation gilts from White Line-1 (WL-1, randomly selected control line) and White Line-2 (WL-2, selected for an index of ovulation rate and prenatal survival rate) were used to compare the pattern of follicular development and atresia during the follicular phase of the estrous cycle. Gilts were assigned randomly within line and sire for ovary recovery on d 0, d 2, d 3, d 4 and d 5 following prostaglandin F2alpha induced luteolysis on d 13 (d 0) of the estrous cycle. Ovaries were evaluated for numbers of corpora albicantia (CA) and small (S, 2 to 2.9 mm), medium (M1, 3 to 4.9 mm; M2, 5 to 6.9 mm) and large (L, greater than or equal to 7 mm) follicles. Concentration of estradiol-17beta (E) in follicular fluid was used to classify individual M2 and L follicles as healthy (greater than or equal to 100 ng E/mL) or atretic (< 100 ng E/mL). The WL-2 had a greater ovulation rate than WL-1 gilts at the pretreatment estrus (20.5 vs 13.9 CA, P < 0.01). The S and M1 follicle populations declined rapidly in both lines over time (P < 0.05). The M2 follicle population increased in both lines between d 0 to d 4 and then declined on d 5. However, the rate of increase and decrease in percentage of M2 follicles differed between the two lines (line x day, P < 0.05). Large follicles developed earlier and accounted for a greater relative percentage of all follicles in WL-1 gilts to d 4 but WL-2 gilts showed a more rapid increase in percentage of L follicles between d 4 and d 5 (line x day, P < 0.06) and achieved their major advantage in ovulatory follicles by d 5. Mean E in M2 follicles increased in both genetic lines over time (P < 0.02). The percentage of healthy M2 follicles was greater numerically but not statistically in WL-1 gilts on d 2 and d 3 (P > 0.1). The percentage of healthy M2 follicles increased rapidly in WL-2 gilts from d 3 to d 4 and d 5 (P < 0.05) but was unchanged in WL-1 gilts after d 3. All L follicles were estrogen active (>100 ngE/mL) and classified as healthy in both lines. White Line-2 gilts maintained a larger pool of healthy M2 follicles to d 4 of the follicular phase and rapidly selected and matured these follicles into ovulatory follicles to achieve their ovulation rate advantage. Selection of an additional six ovulatory follicles from the M2 follicle pool after d 5 was required in both lines to achieve their projected ovulation rate.