Sperm Telomere Dynamics: Natural Variation and Sensitivity to Environmental Influences in House Sparrows (Passer domesticus)
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Understanding the mechanisms that contribute to variation in lifespan is of central importance to diverse fields including life history theory. Although the causes of aging are not fully understood, telomere dynamics (length and loss rate) is a potentially critical mechanism underlying longevity. Telomeres are highly conserved, non-coding regions of DNA at the ends of eukaryotic chromosomes. Telomere loss occurs throughout life due to accumulating oxidative damage and normal cell replication. When telomeres reach a critically short length, they stop dividing and functioning normally.</DISS_para><DISS_para>While early life telomere length is predictive of lifespan in birds, the mechanism of inheritance of telomere length is unknown. One hypothesized mechanism is by direct transfer from gamete telomeres. However, very little is known about telomere dynamics in gametes. Stress exposure has been shown to accelerate telomere loss and reduce longevity, particularly when stress is experienced early in life. Exposure to elevated glucocorticoid hormones during activation of the stress response is thought to lead to increased oxidative damage, and thereby accelerate telomere loss. Sperm are particularly sensitive to oxidative damage. Therefore, exposure to stress may accelerate aging within individuals, but also may accelerate sperm telomere loss and thereby impact the telomere dynamics of their offspring. To test this hypothesis, I measured natural variation in sperm telomere length and offspring early life telomeres, sperm telomere length in response to acute and chronic stress exposure, and the relationship between stress sensitivity and telomere length. In free-living sparrows, I found no relationship between paternal sperm telomere length and offspring early life telomere dynamics. Across studies, there was a consistent positive correlation between blood and sperm telomere length, suggesting that sperm telomeres may decline with age in birds. I also found variation in sperm telomere length across the breeding season, and no relationship between stress sensitivity and sperm telomere length. Finally, I found that while sperm telomere length in free-living birds exceeded blood telomere length, in captivity sperm telomeres were equal length or shorter than blood telomeres, potentially related to the duration of captivity. These findings suggest that sperm telomeres are sensitive to environmental factors including stress exposure.