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The clock gene Period 1 (Per1) may be a prolificacy gene, because it localized to the mouse oocyte and Per1-null drosophila shed fewer eggs. Because Per1 mapped to a region of mouse chromosome 11 syntenic to bovine chromosome 19 where a quantitative trait loci (QTL) for ovulation rate existed, we hypothesized that Per1 influenced folliculogenesis and ovulation rate in ruminants. Ovarian cortex was collected at slaughter on days 5, 12, 15, 17, and 20 after estrus for real-time RT-PCR evaluation of Per1mRNAexpression in Dorset (n = 18), Romanov (n = 10), Romanov/Dorset (n = 21), and Composite (n = 22) ewes. Ovarian cortexwas also collected from cows selected for increased ovulation rate (n = 37) or unselected controls (n = 28) on days 4, 5, and 6 of the estrous cycle for in situ hybridization and real-time RT-PCR. To examine the role of Per1 in early follicular development, ovarian cortex from neonatal calves (n = 5) was cultured for 10 days and Per1 mRNA levels were measured on day 0 and on day 10 of culture. The primers generated a 483 bp amplicon with 100% sequence homology to bovine RIGUI-like protein (Per1). In silico mapping of this sequence placed Per1 on bovine chromosome 19; however, it was 20 cM from the QTL. Per1 mRNA expression was unaffected by prolificacy, day of the cycle, or pregnancy status in ewes or cows. The riboprobe hybridized to oocytes of bovine preantral and antral follicles. In bovine ovarian cortical cultures on day 0, the tissue contained mostly primordial follicles (5.6±0.6 follicles/section); however, after 10 days in culture, the number of primordial follicles per section decreased (0.5 follicles/section) and the number of primary follicles increased as follicles activated (day 0 = 0.5±0.6 versus day 10 = 10.4±0.6 primary follicles/section; P < 0.001). Per1 mRNA did not change over time in culture.We conclude that Per1 mRNA is expressed by ruminant oocytes in preantral and antral follicles; however, its physiological role in mammalian ovarian function remains to be elucidated.