Paternal influence on S-phase in the first cell cycle of the bovine embryo

Abstract

The objective of this study was to determine whether the mutation and length of the zygotic S-phase differs for embryos sired by bulls demonstrating high versus low fertility in vivo. Bovine oocytes were matured in vitro for 24-27 h and fertilized with frozen-thawed bovine semen. Six bulls that differed in fertility level were used in the study. The bulls were classified into two groups: those demonstrating high fertility in vivo (in vivo high-fertility bulls; n = 3), with a group mean ± SEM lifetime nonreturn rate of 78 ± 2%, and those demonstrating low fertility in vivo (low fertility bulls; n = 3), with a group mean ± SEM nonreturn rate of 69 ± 1%. The S-phase in zygotes was identified by means of an immunocytochemical technique after pronuclear-stage zygotes were labeled with 5'bromo-2-deoxyuridine (BrdU). To visualize all pronuclei, presumptive zygotes were also stained with propidium iodide. In the first experiment, zygotes were labeled with BrdU at 2-h intervals from 8 to 20 h after sperm addition. There were no differences between bull fertility groups in the time course of pronuclear formation (p > 0.05). The beginning of S-phase was earlier in zygotes sired by high- compared to low-fertility bulls (p < 0.05). The end of S-phase was not affected by sire fertility group (p > 0.05). In the second experiment, zygotes were labeled with BrdU continuously from 8 to 20 h after sperm addition. There was no effect of bull fertility group on the percentage of zygotes in S-phase (p > 0.05), and overall, 98 ± 1% of the zygotes were in S-phase at some point between 8 and 20 h after sperm addition. From these results, we conclude that there are paternal effects on the initiation and length of the S-phase during the first cell cycle of the bovine embryo. A longer S-phase was correlated with higher bull fertility in vivo.