DNA

influence offspring traits," says Crean.

Healthy sperm within one particular ejaculate can vary greatly in their size, swimming speed and longevity.

Scientists don’t know what causes this variability, but they do know such phenotypic traits can affect how successful the sperm is at fertilising the egg.

To date, Crean says the phenotype of sperm wasn’t thought to affect offspring.

"We thought they were just little vessels to transport DNA," she says.

An egg, by contrast, is so much bigger than a sperm it was considered possible that it could transfer material other than DNA to influence the offspring.

But Crean still wondered whether it might be possible that sperm traits could also affect the next generation.

Sea squirt study -

To investigate this question she and colleagues carried out an experiment in sea squirts, which are easy to study because they simply squirt their eggs and sperm out into the ocean.

"It means that we can watch the sperm and manipulate fertilisation quite easily, whereas in other animals it all happens inside the female and it’s quite difficult to track," says Crean.

She and colleagues collected healthy sperm from a single ejaculate of a particular male.

They used half of them straight away to fertilise a batch of eggs and left the other half for an hour before fertilising a fresh pool of eggs.

Many of the sperm in this stored sample died off leaving a smaller skeleton of longer-living sperm for fertilisation purposes.

Eggs fertilised by the stored sperm were more likely to hatch into larvae, and these larvae were more likely to survive the first few weeks in the field.

"It looks like the offspring from those longer-lived sperm actually survived better," says Crean.

She says the "million dollar question" is why, but it’s possible that epigenetic effects on the DNA may be playing a part.

IVF caution -

Whether this applies in humans as well is an open question, says Crean, given the very different reproductive processes involved.

"A sea squirt is a few steps away from a human," she says.

But, Crean says, these findings suggest that it is no longer viable to assume the father only contributes DNA to their offspring.

She says IVF procedures can involve injecting low-motility sperm into eggs and the impact of this should be studied.

"By using these low-motile sperm we could be influencing the health of an offspring so that’s an important thing to test," says Crean.