The mystery of the non-fertilising sperm
In many species, males have two types of sperm – one that can fertilise and one that cannot. Scientists have known this for a long time, but the function of the non-fertilising sperm remains largely a mystery.
Luisa Kumpitsch, a marine biologist at the Department of Marine Sciences, has come a step closer to the answer in her research.
”I remember the first time I saw them. I was completely fascinated,” says Luisa Kumpitsch, who has spent the last few years researching so-called parasperm, sperm that cannot fertilise the female’s egg.
In the species of snails that Luisa Kumpitsch has studied, the male produces parasperm alongside the significantly more numerous fertilising sperm – known as eusperm. Under the microscope, it is clearly visible how the long, slender eusperm attach themselves around the parasperm.
”They look like little suns under the microscope.”
A common phenomenon in the animal kingdom
Non-fertilising sperm are found in such diverse animal groups as insects, annelids, echinoderms and molluscs. In periwinkles, the two types of sperm have been known for over a hundred years. But the function of the non-fertilising sperm remains largely unknown.
One theory likens them to a kind of public transport system.
“The idea has been that they function much like a bus, transporting the fertilising sperm to the female. Once that’s done, their job is finished,” says Luisa Kumpitsch.
Her own research suggests that they probably have a more complex function.
When Luisa Kumpitsch sliced the sperm cells into ultra-thin sections and examined them under an electron microscope, she found large, membrane-bound structures. There were also what appear to be lipid droplets – small accumulations of fat – and mitochondria, the cell´s power stations.
Do they help the sperm cope with stress?
– The biggest surprise was these really large, membrane-enclosed structures. At first I thought they might be filled with lipids but they weren’t. So I actually don’t know what they contain.
Luisa Kumpitsch does not dismiss the ”bus” theory entirely. But her findings suggest that transport is hardly the sole function of parasperm.
– If that were the case, I don’t think they would need so many energy-intensive components. They have mitochondria, lipids and these large structures, she says.
Instead, her research, including studies of the protein in parasperm, suggests that parasperm cells may help to neutralise reactive oxygen acids formed in the fertilising sperm. These acids can cause stress and damage the spermcell. In this way, the parasperm could help the fertilising sperm remain functional for longer.
Another theory, previously discussed, is that the parasperm acts as a sort of gift of nutrients to the female snail, which could increase a certain male’s chances of paternity. Female snails can mate with several males, and can also store the sperm inside them.
Luisa’s research provides some support for this theory. But it is still unclear whether the parasperm are actually transformed to the female together with the fertilising sperm.
Fulfils an important function
“What I can say is that the parasperm appear to have a supportive function. But I cannot yet scientifically substantiate exactly what that consists of,” says Luisa Kumpitsch, who has recently completed her doctoral thesis about sperm evolution in marin organisms.
The fact that different types of non-fertilising sperm have nevertheless evolved in different parts of the animal kingdom suggests that they fulfil important functions.
“They aren’t closely related to one another, so having two different types of sperm must be a successful strategy, given that it has evolved in so many different groups.”
If you were to continue your research into parasperm, what would your next experiment be?
– The next step would definitely be to look inside the female. There are many scientific articles stating that parasperm are transferred to the female, but none of them actually show them there. The first step would therefore be to confirm that they are present in the female, says Luisa Kumpitsch.
Read more: Sperm Evolution in Marine Organisms
Text: Mikael Andersson