When evolution hits on a solution that works, that solution tends to get reused. Researchers have found that genes that play key roles in development typically get deployed over and over again in different tissues. Once this happens, however, it can create a problem: you can't make major changes to the gene without messing up a whole lot of essential processes.

This week, however, a team of researchers from the Janelia Research Campus describe a case in which an essential gene that's critical for neural activity was tweaked in an incredibly subtle and specific way. The new version of the gene changed only a single feature of the species it evolved in: the details of the male courtship song.

You might not think fruit flies would do much in the way of singing, and they don't in the traditional sense. But their courtship behavior involves a song created by rapid vibrations of their wings. The song is a mixture of repeated chirps interspersed with longer, buzzing vibrations. The details of this song—the frequency of the buzzing, the space between the chirps, etc.—often varies among the dozens of species of Drosophila we've identified.

To understand how different species have evolved distinctive songs, the researchers focused on two species that have been separated by a quarter-million years of evolution: Drosophila simulans and Drosophila mauritiana. Despite this difference and their different mating songs, members of the two species will still mate if they're not given any other options (options being a member of the same species but opposite sex). These matings allowed the researchers to map the genes responsible for the different mating songs.

Turns out, a remarkably small number of genes are involved. Two were involved in controlling the interval between chirps, and two others influenced the frequency of different parts of the song. The work the researchers describe in a recent paper focuses on one of the genes that control the frequency, which differs by about 10 Hz between the two different species.

After a tremendous amount of work, including creating their own mutations through genome editing, the team was able to narrow the difference down to a single gene. The gene is called slowpoke in flies, and it encodes a protein that controls the flow of ions during nerve cell activity (specifically, it's a calcium-activated potassium channel). Its function is essential for proper neural activity, as evidenced by the fact that embryos lacking the gene entirely end up dead. It's also a complex gene, with a variety of different protein forms generated through differences in where the gene is first transcribed into RNA or the processing that RNA undergoes when transcription is done.

The two species have a number of differences in the slowpoke gene, but the researchers were ultimately able to narrow things down to just one of them. A transposable element—a parasite that operates at the DNA level by hopping around the genome—had hopped into a part of the gene that's normally processed out of the RNA. Its presence subtly changed the dynamics of that processing, which in turn subtly changed the ratio of different forms of the slowpoke protein.

As far as the researchers could detect, the only thing this changed was part of the mating song, an astonishingly specific effect for changes in an essential gene. Still, the overall impact can be dramatic, since a mating song helps species separate from their evolutionary relatives, even if they happen to occupy the same territories.

Nature, 2016. DOI: 10.1038/nature19093 (About DOIs).