|Spicer Leon J, - OKLAHOMA STATE UNIVERSITY|
|Wong E A, - VIRGINIA POLYTECH INST.|
|Hamilton, D - VIRGINIA POLYTECH UNIVER|
|Vernon, R - OKLAHOMA STATE UNIVERSITY|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 23, 1994
Publication Date: N/A
Interpretive Summary: Increasing lambing rate in large-type mutton sheep would enhance meat production significantly, but the biological basis for genetic differences in litter size among breeds of sheep is poorly understood. Insulin-like growth factors (IGF-I, primarily of liver origin) and follicle stimulating hormone (FSH, pituitary origin) have been shown to stimulate the development of follicles in the ovaries, increase ovulation rate (i.e., number of eggs released from the ovaries) and increase the numbers of offspring born per female in some species. The objective was to determine whether genetic differences in ovulation rate, and subsequently litter size in sheep, were related to concentrations of IGF-I, FSH or steroids (e.g., estradiol or progesterone) in the blood or the production of IGF-I in the ovaries. Five genetic populations evaluated were: Finn (F), Composite III (C, Columbia x Suffolk x Hampshire), 1/2 F x 1/2 C (FXC), 1/2 Booroola Merino (B) x 1/2 F (B X F) and 1/2 B x 1/2 C (B X C) with respective ovulation rates of 3.1, 1.6, 2.9, 3.8 and 2.9. Concentrations of IGF-I, FSH and steroids in blood or fluid from the follicles did not differ among the populations. Production of IGF-I and estradiol within the ovaries was significantly greater in B-crossbred ewes than in C ewes; F ewes were intermediate and did not differ from C ewes. The increased production of IGF-I in the ovaries of ewes from the Booroola Merino breed appears to stimulate development of an increased number of small follicles into large ovulatory follicles and thus the increased ovulation rate in Booroola Merino ewes. Contrary to previous reports, an increase in FSH secretion was not found in these Booroola Merino populations. No reason for the increased ovulation rate in Finn ewes was identified.
Technical Abstract: The relationship between ovulation rate and follicular fluid (FFL) concentrations of estradiol, progesterone and IGF-I or ovarian content of IGF-I mRNA was assessed in 10 ewes from each of five genotypes: Finn (F), Composite III (C, Columbia x Suffolk x Hampshire), 1/2 Booroola Merino x 1/2 F (BxF), 1/2F x 1/2C (FxC), 1/2B x 1/2C (BxC). Ovulation rates for F, C, BxF, FxC and BxC ewes were 3.1, 1.6, 3.8, 2.9 and 2.9 +/- .5, respectively. Estrous cycles of ewes were synchronized with prostaglandin F2alpha (PGF) and serum was collected every 6 h from 6 h before to 42 h after PGF (ovariectomy=48 h post-PGF). FFL samples were collected for RIA, and whole ovaries were processed for RNA isolation and Northern blot analysis. Concentrations of IGF-I in FFL were 53% greater (P<0.05) in large than in small follicles. Conversely, total IGF-I binding activity and binding activity of three species (34, 27-29 and 24 kDa) of IGF binding proteins were lower (P<0.05) in FFL of large versus small follicles. FFL IGF-I and IGFBPs did not differ (P>0.10) among genotypes. Ovarian IGF-I mRNA levels in BxF (.13+/-.02 densitometric units) and BxC (.16+/-.02) ewes were greater (P>0.05) than in C ewes (.05+/-.02); F (.08+/-.02) and FxC (.09+/-.03) ewes did not differ (P>0.10) from C ewes. Small follicles from BxC and BxF ewes had 10- to 16-fold greater (P<0.05) estradiol levels than F or C ewes; progesterone did not differ (P>0.10) among genotypes. Serum FSH, inhibin, IGF-I and progesterone did not differ (P>.10) among genotypes. In conclusion, some genotypic differences in ovulation rate (i.e., BxF and BxC vs C) were related to differences in FFL estradiol and ovarian IGF-I mRNA levels but were not associated with differences in FFL progesterone, IGF-I or IGFBPs.