By Stephen Beech via SWNS
Ugly flies make up for being less attractive by fighting more fiercely, according to a new study.
Male stalk-eyed flies with longer eyestalks are more attractive to females and more intimidating to males, say scientists.
However, males with a genetic variant that causes shorter eyestalks are fiercer fighters, according to, new research.
They have a copy of the X chromosome which always causes short eyestalks, according to the findings published by the journal Frontiers in Ethology.
American scientists investigating why the mutation hasn’t died out, despite sexual selection, have discovered that the flies could be compensating for their shorter eyestalks with increased aggression.
Corresponding author Dr. Josephine Reinhardt, of the State University of New York, said: “It’s the first time I’m aware of that there’s evidence of a link between a selfish gene and aggressive behavior.
“These driving X chromosomes are pretty interesting because they are an example of how parts of our genetic code aren’t necessarily working together, but have their own selfish interests.
“This is an extreme example, but simply carrying one of these selfish chromosomes impacts so many parts of these animals’ biology, even their behavior.”
Dr. Reinhardt explained there are two different types of X chromosomes in stalk-eyed flies.
She said the X chromosome carrying the mutation for short eyestalks is a meiotic driver: it carries alleles that are over-represented in a male’s sperm, meaning it’s much more likely to be passed on.
Dr. Reinhardt said: “The driving X chromosome has a huge natural advantage because it passes itself on more than the fair 50-50 ‘coin flip’ rule of genetics that most of us learned in high school biology.
“Up to 100% of a male’s offspring end up inheriting the X and therefore are female.
“Because of this, we might assume the X will keep increasing in the population and even cause extinction.
“Since that hasn’t happened, we’re interested in understanding what other traits could counteract that advantage.”
She says male stalk-eyed flies defend access to mates by intimidating displays and fighting.
To test whether flies carrying the driving X are more aggressive, the research team used populations of flies carrying either type of X.
Flies display more aggression against flies with similarly-sized eyestalks, so the researchers matched up competitors with similar eyestalks, then filmed their contests and analyzed their behavior.
The researchers found not only that fighting was more common when the two flies were closely matched in eyestalk size, but it was more commonly seen in males with the driving X.
Males that used more of the fighting behaviors were more likely to win contests.
The research team also observed that males with the driving X chromosome were more likely to win if they engaged in more fighting than displaying.
Dr. Reinhardt said: “When fighters are mismatched, fights tend to end quickly, with the smaller male retreating.
“When a male with the driving X chromosome is fighting a male with similar-sized eyestalks, he is more aggressive.
“But because driving X males are on average smaller, it is likely still a disadvantage.”
She said that could explain why the flies with short eyestalks were able to mate.
Dr. Reinhardt says longer eyestalks signal a larger body size and a more dangerous opponent, which is why flies with shorter eyestalks usually retreat from contests.
She said if males with the driving X chromosome are more aggressive or don’t accurately assess the threat from other males, these males could choose to compete with rivals with longer eyestalks, bringing themselves into contact with the females initially attracted to their opponent.
Although the extra aggression could be dangerous, Dr. Reinhardt says it could also help the flies get mating opportunities they otherwise wouldn’t.
However, she said it can’t fully counterbalance sexual selection.
Modeling of the spread of the driving X suggests that it could explain why it hasn’t taken over: females still prefer males with longer eyestalks, keeping the variant’s frequency low.
Dr. Reinhardt added: “I would say that this study is an initial finding.
“A larger study might be done in which we specifically test for the increase in high-intensity behavior that we saw here in a larger sample.
“In addition, this is a laboratory study, so it is not totally clear how well it would apply to field behavior.
“Finally, females were not tested. If the driving X chromosome directly increases aggression that might impact females – whereas if it’s an indirect effect to do with the eyestalk size, it might not.”
