Protamine mRNA ratio in stallion spermatozoa correlates with mare fecundity.

The research focused on understanding the correlation between protamines in stallion sperm and mare fertility. It was discovered that a specific ratio of protamine mRNA in sperm was associated with higher fertility rates.

Investigation of Protamines as Prognostic Factor

• The purpose of this study was to assess the potential of protamines as an indicator for stallion fertility.
• Protamines are proteins involved in the compaction of sperm DNA, crucial for the effective transfer of the paternal genome to the oocyte, or egg cell, during fertilization.
• The researchers performed in situ hybridization, an imaging technique, to observe the specific expression of Protamine 1 (P1) mRNA in the cytoplasm of developing sperm cells, also known as spermatids.
• Spermatids at different stages of development displayed different levels and timings of protamine protein expression. No staining, hence no spermatids, were found in the testes of cryptorchid horses, a condition where one or both of the testes fail to descend.

Protamine Variants and Stallion Fertility

• Through quantitative real-time polymerase chain reaction, an analytical technique to amplify and simultaneously quantify a targeted DNA molecule, mRNA transcripts of P1 and two variants of Protamine-2 (P2 and P3) were identified in ejaculated spermatozoa from 45 thoroughbred stallions.
• The stallions were categorized into three groups based on the mare fertility descriptor, also known as the “none-return-rate 28 percentage” or NRR28%, which calculates fertility based on whether a mare becomes pregnant after one breeding cycle.
• The researchers discovered that the ratio of P2/P1 mRNA was significantly lower in the group with reduced fertility, and this ratio also correlated slightly with sperm concentration.
• Additionally, a negative correlation was found between the P2/P1 mRNA ratio and the count of morphologically abnormal sperm, indicating a diminished protamine ratio in spermatozoa carrying head defects.
• On the other hand, a positive correlation was found between the P2/P1 ratio and mare fertility. However, the ratio between P3 and P1 remained constant across the groups, suggesting this variant plays a minor role in sperm chromatin compaction, the process of tightly packing DNA into sperm.

Conclusion

• The researchers concluded that, similar to humans, equine protamine expression could serve as a checkpoint of spermatogenesis, the process of sperm cell development.
• They suggested that the observed level of protamine mRNA could reflect the quality of spermatogenesis and the fertilizing ability of the sperm cells.
• Yet, they also clarified that abnormal protamine transcripts content in equine spermatozoa was not associated with DNA fragmentation rate, determined by flow cytometric acridine orange test, a technique to measure DNA damage in cells.