Stallion semen quality depends on major histocompatibility complex matching to teaser mare.

The research article focuses on how the genetic compatibility of horses, specifically the major histocompatibility complex (MHC), affects the quality of stallion semen. It was discovered that being exposed to a genetically dissimilar mare improves the quality of a stallion’s semen in terms of sperm count and viability, although not necessarily in velocity. The overall genetic similarity between mare and stallion outside the MHC had no significant impact.

Major Histocompatibility Complex and Mate Selection

• It’s observed that in many vertebrates, the Major Histocompatibility Complex (MHC) plays an important role in mate selection. Typically, animals have a preference for mating with individuals whose MHC is dissimilar to their own to maximize genetic diversity for offspring.
• The study applies this understanding of MHC’s role in mate selection to horses, investigating how MHC compatibility affects a male horse’s reproductive investment following short-term exposure to a female horse.

Effects on Semen Quality

• The researchers found that exposure to an MHC-dissimilar female horse led to enhanced male plasma testosterone levels and resulted in ejaculates with higher sperm count compared to exposure to an MHC-similar female horse.
• Unexpectedly, sperm velocity was not significantly affected by MHC-matching.
• Interestingly, overall genetic similarity, beyond the MHC, between male and female horses did not play a meaningful role in these outcomes. Only differences in the MHC had measurable impact on semen characteristics.

Implications for Cold-Stored Semen

• Not only did MHC differences affect fresh semen, but the effects were also seen in semen stored at low temperatures for 24 and 48 hours.
• Sperm viability (the proportion of living sperm in a sample) improved when a male was exposed to an MHC-dissimilar female horse, which fits with the principle of ejaculate economics, where the quality and quantity of sperm is adapted based on the likelihood of fertilization.
• However, the study also found that oxidative stress and the percentage of sperm with high DNA fragmentation were mostly increased after exposure to an MHC-dissimilar female horse, dependent partly on whether the female was in oestrus or not.

Conclusion

• The study concludes that stallions can rapidly adjust the quality of their ejaculates in response to a female’s MHC. Therefore, this male reaction can affect not only the immediate characteristics of the ejaculate but also the characteristics of semen when stored at low temperatures.