Summer induces DNA damage in boar sperm: implications for the management of seasonal infertility

At 40% share, pork is the most widely eaten meat globally. As such, research efforts must improve production and efficiency in the pig industry to meet growing demand. However, summer heat stress has a significant negative impact on pig fertility; causing embryonic death and decreased litter size that cost the industry millions in productivity losses. This problem is particularly prevalent in the tropics where ambient temperatures rise beyond the animal’s zone of thermal comfort. Boars are particularly vulnerable to the effects of heat stress due to their inefficient capacity to sweat; non-pendulous scrotum; and the high susceptibility of boar sperm to temperature shock. Moreover, due to limited endogenous antioxidant systems inherent in mammalian spermatozoa and the loss of cytosolic repair mechanisms during spermatogenesis, the DNA in these cells are particularly susceptible to oxidative damage. While a seemingly healthy looking sperm may swim and fertilize an oocyte normally, studies in mice demonstrate that heat stress-induced DNA damage can disrupt expression of key developmental genes in early embryos after fertilization and distort the formation of the blastocyst; resulting in implantation failure and pregnancy loss. The aim or our study is to determine whether heat stress induces DNA damage to boar sperm that could significantly contribute to the high rates of embryo loss and pregnancy failure observed in sows during summer infertility. The quality of sperm obtained from n=6 Large White boars housed in the dry tropics of Townsville, North Queensland, Australia was evaluated across different seasons (summer, winter and spring) during 2014 - 2015. Sperm motility was characterised by Computer-Assisted Sperm Analysis (CASA; IVOS version 10: Hamilton Thorne, USA), and sperm DNA integrity evaluated by Terminal deoxynucleotidyl transferase dUTP Nick-End Labelling (TUNEL; In situ cell death detection kit, fluorescein: Roche, Germany). Twenty-thousand spermatozoa per boar per treatment were analysed using flow cytometry (CyAn ADP analyser: Beckman Coulter, USA). Sperm had equal motility across all seasons (total motility: 70.8 ± 5.5% vs. 71.3 ± 8.1% vs. 90.2 ± 4.2%, P ≥ 0.05; progressive motility: 41.7 ± 2.8% vs. 35.4 ± 7.0% vs. 46.6 ± 4.0%, P ≥ 0.05 for spring, summer and winter respectively). However, sperm in summer exhibited ~9-fold higher DNA damage than that in winter and spring (16.1 ± 4.8% vs. 1.1 ± 0.2% and 1.8 ± 0.4% respectively; P ≤ 0.05). These results demonstrate that summer negatively affects sperm DNA integrity in boars without depressing sperm motility. This means traditional methods of evaluating semen quality may not detect inherently compromised spermatozoa. We are currently evaluating the effect of this DNA-damaged sperm on rates of fertilization, development and survival in pig embryos. Our study emphasizes the need for improved management practices and development of strategies to mitigate heat stress in boars during summer