Heat stress-induced DNA damage and its mitigation in boar spermatozoa
DR Damien Paris gave quite a memorable presentation at this year’s Australian Pig Veterinarians Annual Conference. ‘Pork is the most consumed meat in the world and summer infertility equals reduced reproductive performance, costing the industry $300 million annually in the US.’
The thermal comfort zone of pigs is 16-26C and tropical countries exceed these temperatures in summer easily, with spermatogenesis in boars impaired above 29C. Boars are vulnerable to heat stress due to inefficient sweating and non-pendulous scrotum. Libido, semen volume, sperm concentration and mobility decreases, while abnormal sperm and disturbed testosterone production increases.
Sperm DNA may be susceptible to damage due to loss of cytosolic repair mechanisms during maturation. In pigs, 1.5 fewer embryos survive conceived from heat stressed boars. However, important links between heat stress and DNA damage in boar sperm is still missing.
Damien and his team are working on figuring out what those missing links are.
The aims of their project are to:
- Determine effect of summer heat stress on boar sperm quality and DNA integrity; and
- Evaluate therapeutic effect of antioxidant supplements.
The goal is to prove boars can significantly contribute to summer infertility in sows and develop a mitigation strategy to solve the problem.
Methods – sperm DNA fragmentation
- Sperm purified by 40/80 Percoll gradient centrifugation;
- Breaks in 3’-OH end of sperm DNA labelled by TdT-mediated dUTP Nick End Labelling and nucleus counterstained with DAPI; and
- 20,000 sperm/boar/treatment evaluated by flow cytometry.
- Boar feed supplemented with 100g/boar/day custom-made antioxidant during summer; and
- Semen collection.
Temperature and sperm concentration
- Summer lower than winter but did not differ to spring;
- Consistent with previous studies; and
- Did not improve after 42 or 84 days antioxidant treatment.
Results – sperm DNA fragmentation
- Summer increase 16-fold higher than winter and nine-fold higher than spring;
- Significant decrease after 42 and 84 days antioxidant treatment;
- More than 6 percent DFI cause decrease in piglets born per litter;
- First definitive link between heat stress and DNA damage in boar sperm; and
- Developed first putative mitigation strategy to improve DNA integrity of boar sperm during summer.
- However, summer did not differ to winter or spring;
- Contrasts with previous studies;
- Did not affect motility after 42 and 84 days antioxidant treatment; and
- Traditional measures of sperm motility do not detect inherently compromised or improved sperm.
- Summer induces DNA damage and reduces sperm concentration without depressing sperm motility;
- Traditional evaluation of sperm motility may not detect inherently compromised or improved sperm;
- Heat stress-induced, DNA damaged sperm from boars may contribute significantly to early embryo loss in sows; and
- Antioxidant therapy during summer appears to significantly alleviate heat stress-induced DNA damage but not concentration nor motility in boar sperm.