Paternal effects on pre-implantation embryo development in cattle
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Currently, sire fertility is measured using sire conception rate (SCR), which is not always indicative of embryo development. Since the majority of pregnancy loss in dairy cattle occurs during the early embryonic period, it is important to determine the effect of sire during this time period. Therefore, the goal of this research is to identify sires with high and low capacities to produce embryos and elucidate the effect of sire on early embryo development. To investigate this, 65 Holstein sires with SCRs ranging from -14.2 to 5.3 were run through an in vitro embryo production system and embryo development was monitored. Based on their in vitro development performance, eight high performing (HP) and 9 low performing (LP) sires were identified. The average blastocyst rate (BL) was 48 percent for HP and 14 percent for LP sires, respectively. In this dataset, there was no correlation between SCR and BL. However, there was an increase in embryos arrested at the 5-6 cell stage in LP sires compared to HP sires. Next, embryos were produced from HP and LP to determine autophagy levels, and blastocyst cell number. LP sires had a higher rate of autophagy than high performing sires, with no effect of SCR. However, the ratio of trophectoderm to inner cell mass cells in blastocysts did not differ between sire performance groups. RNA-Seq on 4-cell embryos identified 687, and 1411 genes with increased expression in HP and LP sires, respectively. Genes with increased expression in HP sires were involved in mRNA and cell cycle regulation, chromosome segregation, and sperm mitochondria clearance. Genes with increased expression in embryos from LP sires were indicative of sperm mitochondria retention, and an increase in DNA damage and apoptosis. Lastly, embryos were produced in vivo from HP and LP sires. Interestingly, LP sires generated twice the number of degenerated embryos as HP sires. In conclusion, this research demonstrates a clear effect of sire on preimplantation embryonic development, where LP sires produced a higher proportion of embryos with developmental delays, increased autophagy and expression of DNA damage and pro-apoptotic genes, resulting in embryonic arrest at the 5-6 cell stage. Interestingly, SCR was not indicative of preimplantation development in this study. The in vitro model used in this study to identify sires with negative effects in embryo development represents a useful tool to build a robust predictor of sire fertility that accounts for the sire's influence on the early stages of pregnancy.