Sperm that swim for longer before fertilising an egg produce offspring with a higher survival rate, a new study has found.
The finding suggests that it’s not just the genes they carry but also the quality of individual sperm that can influence reproductive success, says the study led by UNSW researcher Dr Angela Crean and published today in the journal PLoS One.
The unusual reproductive strategy of sea squirts – Styela plicata – allowed Dr Crean and colleagues to manipulate which sperm fertilised eggs and track the resulting offspring. Dr Crean, of the UNSW Evolution & Ecology Research Centre, worked with researchers from the University of Western Australia and Monash University.
Sea squirts are hermaphrodites that reproduce by releasing both sperm and eggs into the surrounding water where fertilisation occurs, they note. Each sperm lives from as little as a few seconds to a few hours.
Dr Crean isolated some sperm from the one ejaculate of a sea squirt and kept them separate for an hour before they fertilised an egg. She then compared offspring of the hour-old sperm with that of fresh sperm from the same ejaculate.
The longer-lived sperm produced offspring that were more likely to hatch into larvae and survive the first few weeks of life.
“This is surprising because sperm are thought to be little more than transport vessels for the male genome,” Dr Crean said. “While sperm traits such as size and speed are known to influence the fertilization success of sperm, that influence was expected to end at fertilization.”
By linking sperm traits to offspring fitness, Dr Crean’s findings suggest that the sperm type of a male, and not simply the genes that sperm carries, can have important effects on offspring survival and success.
"The findings add to the growing evidence that sperm characteristics, such as longevity and motility, and not just the genes the sperm carries, can have important effects on the survival and success of offspring,” she said.
"This has the potential to change the way we view and study inheritance. It will be exciting to find out if sperm traits are linked to offspring fitness in other species."
In previous studies, Dr Crean found that the sea squirt can change its sperm according to local conditions. Sea squirts living in high-density populations produce larger and stronger sperm than those in low-density groups, giving them a better chance of beating competitors to fertilise an egg.
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