Identification and characterization of 2 major mutations affecting sire fertility in Jersey cattle: Effects on...

Male fertility is a key determinant of reproductive efficiency in dairy cattle; however, some sires with normal semen parameters exhibit reduced fertility due to intrinsic sperm defects, including those arising from genetic mutations. Previous research has identified 2 genomic regions, located on BTA11 (2.6 Mb) and BTA25 (19.1 Mb), that severely affect service sire fertility in US Jersey cattle. However, the causal mutations and their impact on male fertility remain unknown. The first goal of this study was to identify the causal mutations using targeted sequencing. For each genomic region, 2 DNA pools were constructed with high- and low-fertility Jersey sires. The DNA-sequencing analysis included genome alignment, variant calling, and variant annotation. We identified a deleterious missense mutation on FER1L5 (BTA11) and a deleterious frameshift variant on ABCA16 (BTA25) carried by low-fertility sires. The second goal of this study was to determine whether these mutations affect sperm function, early embryo, and post-hatching development using frozen semen from Jersey sires homozygous for the FER1L5 and ABCA16 variants or wild-type sperm (WT). Both variants affected sperm acrosomal reaction, with lower proportions of reacted sperm in the FER1L5 and ABCA16 variants compared with WT. As a consequence, oocytes fertilized with sires carrying these variants had reduced fertilization rates (P = 0.0001), with 60.2 ± 8.8%, 76.2 ± 7.1%, and 94.2 ± 2.5% for the FER1L5, ABCA16, and WT, respectively. Cleavage rate differed among groups (P < 0.0001), with 42.0 ± 6.4%, 54.0 ± 6.5%, and 78.6 ± 4.5% for the FER1L5, ABCA16, and WT, respectively. Blastocyst formation was reduced in FER1L5 variant (11.6 ± 1.6%) and ABCA16 variant (15.4 ± 1.9%) versus WT (26.7 ± 3.3%). The proportion of cleaved embryos developing to blastocysts was lower in FER1L5 variant (26.6 ± 2.4%) than WT (35.7 ± 2.5%), while the ABCA16 variant (30.1 ± 2.2%) was intermediate. Blastocysts had similar total cell numbers, although mutations affected lineage proportions. During extended culture, mutations affected embryo survival, with reduced survival in the ABCA16 variant compared with the FER1L5 variant and WT. Embryo attachment was not different among groups, but trophectoderm outgrowth area was affected by the genotype. Overall, the candidate causal mutations identified on FER1L5 (BTA11) and ABCA16 (BTA25) impair sperm fertilization competence and early embryo development and may contribute to reduced sire fertility, highlighting the importance of identifying and managing deleterious variants in dairy cattle breeding programs.