Young Scholar Presentation: Follicle Dynamics and Hormonal Environment in a Novel High Fecundity Genotype in Cattle

A high fecundity bovine genotype has been recently discovered. Carriers of this allele have multiple ovulations (MO), while half-sibling, non-carriers have single ovulations (SO). In sheep, high fecundity genotypes have mutations in pathways involving oocyte-derived TGF-β members, BMP-15/GDF-9. Consistent with this mechanism, MO cattle exhibit 6.6-fold greater expression of SMAD6 compared to SO (P<5x10^-5). SMAD6 is an inhibitor of BMP-15 transduction pathways that regulate granulosa cell proliferation and differentiation. Thus, we hypothesized that MO carriers would have reduced follicle growth rate and earlier differentiation than SO. In experiment 1, a synchronized follicular wave was induced with follicle growth in a controlled progesterone (P4) environment (intravaginal P4 implant for 5d). In experiment 2, a complete interovulatory interval was evaluated. Circulating FSH, P4, and estradiol (E2) were evaluated and size of follicles and CL were determined by ultrasound. In experiment 1, number of ovulations was greater for MO than SO (Table). Mean ovulatory follicle size was greater for SO than MO, however total ovulatory follicle volume was not different. Interestingly, follicle growth rate (volume basis) was 3.2-fold greater for SO than MO cattle. Peak FSH was similar (P=0.65), for MO and SO, with declining but similar FSH during the next 2 d. However, nadir FSH (ng/ml) concentrations were greater for MO (0.25±0.02) than SO (0.17±0.02; P=0.02) cattle. In experiment 2, individual CL volume on d7 was greater for SO than MO (Table), however total luteal tissue volume and circulating progesterone was not different. In experiment 3, size for attaining ovulatory capacity was evaluated by means of a challenge with 200μg of GnRH at different intervals after wave emergence. In MO cattle, 100% (34/34) of follicles ≥6mm ovulated, while in SO cattle, follicles <7.5mm did not ovulate (0/44) but 100% (16/16) of follicles ≥9mm ovulated to the GnRH challenge. Thus, MO cows have reduced rate of follicle growth in spite of greater nadir FSH concentrations, with smaller individual follicle volume but similar total follicle volume. In addition, follicles in MO cattle attain dominance and ovulatory capacity at smaller follicle size.

Study	Endpoint	MO (n=9)	SO (n=5)
Experiment 1	Ovulations	4.1±0.4A	1.6±0.2B
	Mean ovulatory follicle (mm)	9.43±0.7A	15.5±0.9B
	Mean ovulatory follicle (mm3)	508±127A	2033±321B
	Total ovulatory follicle (mm3)	2100±312	3153±487
	Follicle growth rate (mm3)	69.9±18A	225 ± 25B
Experiment 2	Ovulations	4.0±0.5A	1.2±0.2B
	Maximal P4 (ng/ml)	8.1±0.7	8.7±1.2
	Mean CL (mm3)	1300±353A	4742±668B
	Total luteal volume (mm3)	4337±605	4742±668

^ABIndicates differences within row (P<0.05).