‘Are old males still good males and can females tell the difference?’ is the subject of research at the University of Otago which is showing some unexpected pros and cons in the issue of older dads.
Zebrafish are commonly used as animal models for research as they develop quickly, and have transparent embryos Photo: Wikimedia Commons
Using quick-growing and short-lived zebrafish as her study animals, Sheri Johnson has been looking at sperm quality, fertility success and mating attractiveness in a cohort of fish, starting when they were young and following them as they aged.
“Although males can continue to reproduce through most of their lives that doesn’t necessarily mean that they are reproducing well,” says Sheri. “They may not be producing high quality sperm. So we set out in 2012 to do this longitudinal aging study where we sample the fish every three months through to 28 months of age … when we think that they are on the downhill side to senescence.”
Females were less interested in mating with older males than they were with young males, and older males had poorer sperm quality and lower fertilisation success, but the study has revealed an intriguing and unexpected positive benefit.
“The most interesting thing is that we’re seeing that older males are potentially delivering fitness benefits, despite the significant decline in sperm performance that we see,” says Sheri. ‘The survival of the progeny that was generated from the older males was significantly higher.”
Survival is measured by how many of the offspring survive beyond the first critical 15 days. Sheri wondered whether this fitness benefit might be in turn passed onto the next generation of offspring (from the F1 to the F2 generation), but when Alison Ballance visited Sheri at the university’s zebrafish facility, she found that work done to date has not shown this.
It’s well known in both humans and other animals that the quality and quantity of sperm declines as males age, and as part of her Marsden-funded research Sheri uses computer assisted sperm analysis. She is also looking rates of mitochondrial mutation and at degrees of DNA fragmentation in sperm from different males. Sheri says that in human fertility clinics DNA fragmentation is increasingly being used as an important predictor for fertilisation success.
Sheri Johnson and student Thomas Gardiner in front of the zebrafish used in their study (left), and motion analysis (right) of sperm collected from a young zebrafish: the sperm with tails are motile or moving, while stationary dots indicate inactive sperm Photo: RNZ / Alison Ballance
Sperm cryopreservation and the use of computer assisted sperm analysis has featured in previous Our Changing World stories looking at attempts to improve kakapo breeding through artificial insemination.
Older Dads and Reproductive Success
