U.S. researchers have genetically altered the sperm of fruit flies to glow in the dark, giving new insight on how sperm from different males compete.

glowing-sperm

The internal sexual organs of a female fruit fly that was first mated with a male with sperm that glows green, and then mated with a male with red sperm. ((Science/AAAS))

Biologists at Syracuse University in New York say the genetically modified sperm, which glow in either fluorescent green or red, have allowed them to see what happens to live sperm inside the body of the female. 

"Our first goal with these flies was to tackle the mechanisms underlying sperm competition," said Scott Pitnick.

"Whenever a female mates with more than one male, and female promiscuity is more the rule than the exception in nature, there are conflicts between the sexes over paternity, as well as competition between rival ejaculates to fertilize eggs," Pitnick said in a statement.

'Our jaws hit the floor the first time we looked through a microscope and saw these glowing sperm.'—Scott Pitnick, Syracuse researcher

The researchers observed sperm movement inside female fruit flies after they mated with both males with sperm that glow red, and those with sperm that glow green.

The coloured genetic tags allowed the scientists to see how the sperm from the two males moved and how their distribution inside the female changed over time. They published their research this week in the journal Science.

"Our jaws hit the floor the first time we looked through a microscope and saw these glowing sperm. It turns out that they are constantly on the move within the female's specialized sperm-storage organs and exhibit surprisingly complex behaviour," said Pitnick.

The scientists found that the sperm from the most recent copulation ousted the sperm from previous males, but didn't damage them.

The Syracuse research team said it has created other animal strains with glowing sperm and plan to continue its research with other species, including mating different species that produce sterile hybrid offspring to see how sperm and females from mismatched species interact.

"I suspect we have just scratched the surface of using this material," said Pitnick.