Sex chromosome-dependent differential viability of human spermatozoa during prolonged incubation.

STUDY QUESTION: Are there significant differences in the ability of X chromosome-bearing (X) spermatozoa and Y chromosome-bearing (Y) spermatozoa to survive incubation under stressful conditions?

SUMMARY ANSWER: Y spermatozoa are more vulnerable to stress than their X counterparts depending on culture period and temperature, and show higher expression of apoptotic proteins.

WHAT IS KNOWN ALREADY: The primary sex ratio is determined by there being an equal number of spermatozoa carrying X and Y chromosomes. This balance can be skewed by exposure to stressful environmental conditions such as changes in pH, pollutants or endocrine disruptors. However, less is known about the ability of sperm carrying either sex chromosome to withstand environmental stress.

STUDY DESIGN, SIZE, DURATION: The difference in survival between X and Y spermatozoa was evaluated by measuring motility, viability and Y:X chromosome ratio during incubation for 5 days, at three temperatures (4, 22 and 37°C), and three pH conditions (6.5, 7.5 and 8.5). To identify the critical factors that determine the survival of X and Y bearing spermatozoa, we analysed the expression levels of apoptosis-related proteins (Bcl, Bax and Caspase-3), as well as the extent of DNA damage under a subset of conditions.

PARTICIPANTS/MATERIALS, SETTING, METHODS: Semen samples were obtained by masturbation from normozoospermic donors after 3 days of sexual abstinence. Four samples with >60% motility from different donors were mixed to obtain sufficient semen and eliminate sampling-related bias. Data are presented as mean ± SD of three independent experiments. Mean age of donors was 28.7 ± 3.2 years.

MAIN RESULTS AND THE ROLE OF CHANCE: In total, 58 489 spermatozoa were scored. The viability of Y spermatozoa was lower after exposure to different temperatures and culture periods than that of X spermatozoa (P < 0.05). Increased expression of apoptotic proteins in live Y spermatozoa was observed, despite the addition of tocopherol to the culture medium (P < 0.05).

LIMITATIONS, REASONS FOR CAUTION: Spermatozoa were cultured in vitro during the treatment period. It is difficult to extrapolate the observed lifespan differences to spermatozoa survival in vivo. The experiments were replicated only three times.

WIDER IMPLICATIONS OF THE FINDINGS: The prolonged survival of X spermatozoa under stressful conditions might lead to shifts in the ratio of male-to-female births.

STUDY FUNDING/COMPETING INTEREST(S): This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. NRF-2014R1A2A2A01002706). The authors declare no competing financial interests.