134 Consequences of two major mutations for Jersey bull fertility on fertilization and early embryo development

In US Jerseys, two genomic regions, located on BTA11 (2.6 Mb) and BTA25 (19.1 Mb), show significant effects on dairy bull fertility. Each of these regions explains ~4%–6% of the observed differences in conception rate among Jersey bulls and harbor strong candidate genes, including FER1L5 and ABCA16, that have known roles in sperm biology. The objective of this study was to evaluate how these two major genetic mutations affect fertilization and early embryonic development. Abbatoir-derived ovaries were used to retrieve cumulus–oocyte complexes (COCs) by slashing all the visible follicles between 4 and 8 mm in diameter. COCs were matured in vitro in groups of 50 oocytes in a humidified atmosphere of 5% CO₂ in air at 38.5° for 20–22 h. After maturation, COCs were distributed equally to be fertilized with frozen-thawed semen from one of four Jersey bulls: two bulls homozygous for the mutation on BTA11, one bull homozygous for mutation on BTA25, and one non-carrier (wild-type) bull that was used as control (WT). Following an 18-h co-incubation of gametes, the co-incubated oocytes (n = 2545) were cultured in groups of 50 in 500 µL of Synthetic Oviduct Fluid-Bovine Embryo 2 for 7 days to evaluate early embryo development. In addition, a subset of 369 co-incubated oocytes were subjected to DNA fluorescence staining (Hoechst) to evaluate fertilization. An oocyte was considered fertilized based on the presence of two pronuclei or sperm inside the oocyte. All statistical analyses were carried out using SAS V 9.4. Given that the objective was to evaluate the consequences of the mutations rather than individual bull performance, data on fertilization and embryo development from the two bulls homozygous for the BTA11 mutation were combined because no significant differences were observed between them. Mutation effects on fertilization rate and development were analyzed using logistic regression. The bulls carrying the mutations showed a low fertilization rate (P < 0.01) (15.4 ± 0.02% for BTA11, 63.5 ± 0.06% for BTA25, and 82.7 ± 0.03% for WT). Cleavage rates were as follows: 28.8 ± 0.01% for BTA11, 56.1 ± 0.01% for BTA25, and 74.7 ± 0.02% for WT, with all groups being different from each other (P < 0.01). Additionally, bulls carrying the mutations had significantly reduced proportions of cleaved embryos reaching the blastocyst stage (P < 0.01), with 22.8 ± 0.02% for BTA11, 29.06 ± 0.02% for BTA25, and 35.5 ± 0.02% for WT. Overall development of co-incubated oocytes to blastocysts was 6.6 ± 0.00% for BTA11, 16.3 ± 0.01% for BTA25, and 26.5 ± 0.02% for WT (P < 0.01). In conclusion, semen from Jersey bulls carrying these two major fertility variants exhibited a diminished ability to fertilize, and resulting cleaved embryos showed decreased competence to develop to the blastocyst stage. Moreover, the low fertilization and embryo development rates associated with the BTA11 mutation suggest that it affects fertility differently compared with the BTA25 mutation. A limitation of this study is the reduced sample size used for testing. However, efforts are ongoing to increase the number of bulls carrying these mutations available for study. Research is ongoing to assess the impact of these mutations on placental and embryonic development.

This research was supported by the Wisconsin Dairy Innovation Hub.