Re-evaluating Percoll Density Gradient Centrifugation as a Bovine Sperm Sexing Method: DNA-Based Assessment and Field Fertility Outcomes in Holstein–Friesian Cattle in Lowland Indonesia

Technical Summary: Re-evaluating Percoll Density Gradient Centrifugation as a Bovine Sperm Sexing Method

Problem Statement
The application of sperm sexing technology is critical for accelerating genetic progress and enhancing profitability in livestock farming, particularly in dairy systems where increasing the proportion of female calves is advantageous. While flow cytometric cell sorting is the gold standard for sperm sexing, achieving high purity (up to 90%) requires expensive equipment and skilled operators, posing risks of sperm damage. In Indonesia, specifically in lowland areas where Holstein–Friesian (HF) cattle pregnancy rates remain suboptimal (24–28%), Percoll density gradient centrifugation (PDGC) has been widely adopted as a lower-cost alternative. Previous evaluations of PDGC in Indonesia relied heavily on sperm morphometry, assuming that X-chromosome-bearing spermatozoa are larger than Y-chromosome-bearing ones. However, the reliability of head size as a marker for chromosomal sex is controversial, with various studies reporting limited or no morphological differences. Consequently, there is a need to re-evaluate the efficacy of the 10-layer PDGC protocol using robust, DNA-based assessment methods and to verify its practical utility through field fertility outcomes.

Methodology
This study employed a split-ejaculate design using eight ejaculates from each of two sexually mature HF bulls. Each ejaculate was divided into a non-sexed (NS) control group and a PDGC-processed group. The PDGC group underwent a 10-layer Percoll gradient centrifugation (20% to 65%) to separate spermatozoa into putative X-enriched (SX) and Y-enriched (SY) fractions based on density. All samples were cryopreserved using standard protocols.

The study utilized a multi-faceted evaluation approach:

1. Post-Thaw Sperm Quality: Assessed via Computer-Assisted Semen Analysis (CASA) for motility and kinematics, Chlortetracycline (CTC) staining for capacitation and acrosome reaction, and the Halomax assay for sperm DNA fragmentation (SDF).
2. Chromosomal Enrichment Assessment:
   • Imaging Flow Cytometry: Used Hoechst 33342 DNA staining to quantify the proportion of X- and Y-chromosome-bearing spermatozoa based on DNA content.
   • RT-qPCR: Analyzed relative abundance of X-linked (PLP) and Y-linked (SRY) molecular markers using cDNA derived from total RNA. Commercially sorted semen served as positive controls.
3. Field Fertility Trial: A large-scale field trial involved 1,649 HF cattle inseminated with either NS or PDGC-derived SX semen. Outcomes measured included pregnancy rate (PR), calving rate (CvR) among pregnant cows, and the proportion of female calves born.

Key Results

• Sperm Quality: PDGC processing significantly reduced progressive motility and increased the proportion of immotile spermatozoa compared to NS semen. While capacitation status and SDF were not significantly affected, the acrosome reaction showed a significant treatment effect. The authors interpret these changes as consequences of the overall PDGC workflow (centrifugation, washing, handling) rather than specific chromosomal enrichment effects.
• Chromosomal Enrichment:
  • Flow Cytometry: Detected statistically significant but biologically negligible shifts in X:Y ratios. All groups remained close to the physiological 50:50 ratio (e.g., NS: 49.02% X; SX: 47.10% X).
  • RT-qPCR: Showed only modest, non-significant differences in PLP and SRY marker signals between SX and SY fractions. In contrast, positive controls (sorted semen) demonstrated clear marker predominance, validating the assay's sensitivity.
• Field Outcomes: There was no significant difference in pregnancy rates (21.59% for NS vs. 24.61% for SX) or the proportion of female calves (42.17% for NS vs. 42.41% for SX) between the groups. However, the calving rate (CvR) was significantly lower in the SX group (75.42%) compared to the NS group (87.37%). The authors caution that this difference may be influenced by uncontrolled field confounders (farm, season, inseminator) and should not be solely attributed to the semen processing.

Significance and Claims
The paper concludes that the evaluated 10-layer PDGC protocol did not achieve biologically meaningful chromosomal enrichment of X- or Y-chromosome-bearing spermatozoa in HF bulls. The study challenges previous morphometry-based claims of high sexing efficiency, suggesting that the small DNA content difference (~3.8%) between bovine X and Y sperm is insufficient for effective separation via Percoll density gradients under the tested conditions.

The authors assert that while PDGC may select for sperm with favorable functional characteristics, it fails as a reliable method for sperm sexing. Consequently, the practical effectiveness of this specific PDGC protocol for increasing female calf production in lowland Indonesian dairy systems should be critically reconsidered. The study emphasizes the necessity of using DNA-based validation over morphometry and calls for future research involving larger bull populations, sham-processed controls, and more rigorously controlled field designs to definitively assess the viability of density gradient centrifugation for sperm sexing.