Follicular dynamics and dominance in Booroola × Finnish Landrace and Booroola × Suffolk ewes heterozygous for the F gene

Summary. To study the influence of the F gene on follicular dynamics and dominance, 2-year-old Booroola × Finnish Landrace (BFL, N = 17) and Booroola × Suffolk (BS, N = 18) ewes were compared with contemporary purebred Finn (FL, N = 18) and Suffolk (S, N = 18) ewes. In Exp. 1, oestrous cycles of ewes were synchronized during the breeding season with progestagen-impregnated sponges. At sponge removal (Day 0), 14 days after insertion, ewes of each of the 4 genetic groups were assigned to Group 1 in which all follicles visible on both ovaries were destroyed by electrocauterization except for the largest (F1) which was marked, Group 2 in which all visible follicles on both ovaries were destroyed, or Group 3 in which the 3 largest follicles of both ovaries were identified as F1, F2 and F3 and marked. At 48 h after treatment (Day 2), follicular growth was evaluated. At Day 0, the mean number of small follicles (1–3 mm) was higher (P < 0·05) for BS, S and BFL (35·8, 35·1 and 32·9) than FL (24·9) ewes. Large follicles (≥4 mm) were more numerous (P < 0·05) in FL (3·5) than in BS (2·1) ewes, BFL and S ewes being intermediate. Diameter of the F1 follicle was larger (P < 0·05) for S (7·6 mm) than FL, BS and BFL (5·8, 5·1 and 5·1 mm) ewes. In Group 1, all F1 follicles marked at Day 0 ovulated at oestrus after sponge removal for BFL, BS and S ewes while in FL ewes, 2 of 6 F1 follicles regressed. In ewes ovulating, only the Fl follicle ovulated except for one S ewe which shed one more ovum. In Group 2, there were no follicles ≥4 mm at Day 2 and no ewes ovulated after treatment. In Group 3, the proportion of marked follicles that ovulated was higher for S ewes than in those of the prolific genotypes. The number of follicles not marked at Day 0 but ovulating (compared to the total number of ovulations) was higher in BFL, BS and FL (8/11, 9/13 and 9/13) than S (3/10) ewes. In Exp. 2, prolific (BFL + BS) and non-prolific (S) ewes were compared following destruction of follicles ≥3 mm with the F1 left intact (Treatment 1) or destroyed (Treatment 2), 12 days after sponge insertion. At 4 days after treatment (at sponge removal), the numbers of 3–4 mm follicles were not different between the 2 treatments and a very high proportion of the F1 follicles left intact at Day 0 had regressed at Day 4 in both genotypes. From these results, it can be concluded that the higher ovulation rate in Booroola-crossed ewes is not associated with a higher number of recruitable follicles but involves late follicular-phase recruitment and selection combined with smaller preovulatory follicle diameters. Dominance by the largest follicle is not present in ewes of prolific or non-prolific genotypes. In contrast, it is likely that small sized follicles exert a negative effect on growth of large follicles.