Female fruit flies choose a mate based on a 'hidden' feature of the males' wings – beautiful colours that only shimmer against a dark background thanks to a refraction phenomenon known as thin-film interference

Despite the fact that fruit flies have been intensively studied in genetics research for almost a century, the methods of choosing a partner have remained uncertain in Drosophila melanogaster – until now.

Fruit flies have very thin, (only nanometres thick) transparent wings that were once thought to be colourless. However, a few years ago researchers at Lund University in Sweden discovered that the wings shimmer with beautiful colours when viewed against a dark background.

A team from the same university have now demonstrated that females choose a mate based on these 'hidden' wing colours.

The results are published in the journal PNAS.

“Our experiment… is the first evidence that wing interference patterns have a biological signalling function between the sexes during sexual selection [in D. melanogaster],” said Dr Jessica Abbott, a biologist at Lund University.

Weird ways to woo

Male fruit flies are known to use a multitude of ways to woo a prospective partner. These include wing displays, emitting sounds and some flies even vomit a sugary liquid as a delightful dipteran "pre-nuptial gift".

Dr Abbott explained that, in some species of fruit fly, there is a lot of variation in males’ courtship sound, which females seem to use when choosing a mate, similar to crickets. However, research into the sounds male D. melanogaster create suggested they are likely only a signal as to which species they are.

“Males don't vary much in the sounds they produce, and as long as they produce the right frequency the females don't otherwise seem to care much about the sounds,” said Dr Abbott.

In species where males have very obvious marks or patterning on their wings, these visual cues provide important sexual signals to the females, so the team thought testing the interference colour patterns in D. melanogaster was “worth a shot”, according to Dr Abbott.

Females like flashy colours

“[I was surprised] that we managed to see such a clear effect of the wing interference pattern colours on female choice,” she said.

“The females like two things: an intermediate colour shade (hue), which in this case turned out to be magenta (pink), and a high colour saturation (how intense the colour is) – so they liked "flashier" colours.”

“Our results will hopefully stimulate more research on wing interference patterns in other species, and increase interest in the role that the light environment plays in mate choice,” said colleague, Professor Erik Svensson.

The team are hoping to expand on this recent study. For example, are there any costs of having colourful wings in terms of predation in wild D. melanogaster populations?

In particular, the team would like to find out if wing colours are influenced by waxy substances that the flies secrete and spread over their bodies when grooming themselves, known as cuticular hydrocarbons (CHCs).

“CHCs are used as contact pheromones, e.g. males and females use them to see if a potential partner "tastes good", but also for waterproofing and resistance to dehydration,” explained Dr Abbott. 

“We've been thinking that perhaps the CHCs could affect colour saturation, if a waxier wing reflects more light. If so, then this would be really interesting because then you can look at natural selection (e.g. dehydration resistance) versus sexual selection (e.g. wing colour intensity) for this trait. But at the moment this is just speculation.”