Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2011
Publication Date: 7/1/2012
Publication URL: http://handle.nal.usda.gov/10113/59671
Citation: Echternkamp, S.E., Aad, P.Y., Eborn, D.R., Spicer, L.J. 2012. Increased abundance of aromatase and follicle stimulating hormone receptor mRNA and decreased insulin-like growth factor-2 receptor mRNA in small ovarian follicles of cattle selected for twin births. Journal of Animal Science. 90(7):2193-2200. Interpretive Summary: Reproductive rate of the beef cow is low relative to females from other meat-producing species. Most bovine females produce only one egg per reproductive cycle and length of pregnancy is 9 months long; so a beef cow typically produces only one calf per year. The research objective of the current study was to identify genes that result in the production of two eggs per reproductive cycle resulting in the birth of twin calves, essentially doubling beef cow production efficiency. Previous results suggest that the insulin like growth factor 2 receptor, aromatase (responsible for synthesis of the reproductive hormone estrogen) and FSH receptor (a primary control of the development of eggs in cattle) genes may play a role in the production of two versus a single egg per reproductive cycle. Comparison of gene expression between ovaries of cows normally producing single eggs with those selected to produce two or more eggs found that expression of both the aromatase and FSH receptor genes was greater, and insulin like growth factor 2 receptor expression was less in cows producing two or more eggs. These results are consistent with a role for these genes in the production of twins by beef cattle. Treatments that mimic these differences in gene expression could result in improved reproductive efficiency in beef cattle.
Technical Abstract: Cattle genetically selected for twin ovulations and births (Twinner) exhibit increased ovarian follicular development, increased ovulation rate, and greater blood and follicular fluid IGF 1 concentrations compared with contemporary cattle not selected for twins (Control). Experimental objectives were to 1) assess relationships among CYP19A1, IGF1, IGF2R, and FSHR mRNA expression in small (= 5 mm) antral follicles and 2) determine their association with increased numbers of developing follicles in ovaries of Twinner females. Ovaries were collected from mature, cyclic (d 3 to 6) Twinner (n = 11) and Control (n = 12) cows at slaughter and pieces of cortical tissue were excised, fixed, and embedded in paraffin. Expression of mRNA was evaluated by in situ hybridization using 35S-UTP-labeled antisense and sense probes for CYP19A1, FSHR, IGF1, and IGF2R mRNA. Silver grain density was quantified within the granulosa and theca cells of individual follicles (2 to 7 follicles/cow) by Bioquant image analysis. Follicles of Twinners tended to be smaller in diameter than Controls (1.9 + 0.1 vs. 2.3 + 0.1 mm; P = 0.08), but thickness of granulosa layer did not differ (P > 0.1) by genotype. Relative abundance of CYP19A1 (17.5 + 1.4 vs. 7.5 + 1.3% specific grains; P < 0.01) and FSHR (11.0 + 1.2 vs. 7.3 + 1.1% specific grains; P < 0.05) mRNA expression was greater in granulosa cells of Twinners vs. Controls, respectively; whereas, IGF2R mRNA expression was less in both granulosa (2.9 + 0.7 vs. 6.2 + 0.7% specific grains; P < 0.01) and theca (3.0 + 0.6 vs. 5.0 + 0.6%; P < 0.05) cells in follicles of Twinners vs. Controls, respectively. Abundance of CYP19A1 mRNA in granulosa cells was correlated negatively with IGF2R mRNA expression in both granulosa (r = 0.42; P < 0.01) and theca (r = 0.24; P = 0.05) cells. IGF1 mRNA expression was primarily in granulosa cells, including cumulus cells, and its expression did not differ between Twinners vs. Controls (16.5 + 2.5 vs. 13.2 + 2.4%; P > 0.10). Detected increases in CYP19A1 and FSHR, but not IGF1, mRNA expression along with decreases in IGF2R mRNA expression in individual small antral follicles of Twinners support the hypothesis that increased follicular development and steroidogenesis in Twinner females result from increased extra-ovarian IGF 1 production. Furthermore, a reduction in follicular IGF2R mRNA expression accompanied by a reduction in receptor numbers would increase availability of free IGF 2 and its stimulation of follicular development in Twinners.