Glow-in-the-Dark, Competing Sperm Reveal How New Species Form

It's a new take on evolution. It turns out that sperm competition can drive the formation of new species. The findings reveal a little bit more about how female promiscuity can actually aid the process of evolution.

Female promiscuity actually occurs in a majority of species, including humans. This can result in the ejaculates from two or more males overlapping within her reproductive tract. When this happens, sperm compete for fertilization of the female's eggs. In addition, the female has the opportunity to bias fertilization of her eggs in favor of one male's sperm over another's. In all, this process is called postcopulatory sexual selection.

Actually observing what sperm does in a female's body, though, is a bit difficult. This had made the process relatively unstudied until now.

"It's difficult to observe the competition between ejaculates and female discrimination among sperm, given that it takes place inside the female and may involve complex biochemical physiological and morphological interactions," said Scott Pitnick, one of the researchers, in a news release. "Although we have powerful tools for assigning paternity and for quantifying the outcome of sperm competition and cryptic female choice, it might as well be 1871 in terms of understanding the traits and processes of postcopulatory sexual selection."

In order to under to learn a bit more about this process, the researchers worked with closely related species of fruit flies that were genetically engineered with different colors of glow-in-the-dark sperm--red and green. This allowed the researchers to examine ejaculate-female interactions and sperm competition in hybrid matings.

In fact, the researchers were able to determine what happens when a female mates twice. They also were able to assess the predictability of the outcomes, based on knowledge of evolved ejaculate-female incompatibilities.

"The take-away from our study is that postcopulatory sexual selection can quickly generate critical incompatibilities between ejaculates and female reproductive tracts that limit gene flow between isolated populations or species," said Pitnick. "Because female promiscuity and, by extension, postcopulatory sexual selection is so ubiquitous, it is likely to be a widespread engine of speciation."

In other words, the researchers found that this type of selection could help drive the formation of new species. This particular finding is crucial for better understanding the process of evolution.

The findings are published in the journal Current Biology.