Disadvantaged Stalked-eyed Flies Fight to Beat the Odds

Article title

"Stalk-eyed Flies Carrying a Driving X Chromosome Compensate by Increasing Fight Intensity"

Summary

Males with a selfish X chromosome engage in more aggressive behaviors when battling opponents. While the extra fervor may put them in more physical danger, it also gives them a fighting chance, actually increasing their odds of winning.

Abstract

Male stalk-eyed flies fight to protect their access to food and females. The longer their eyestalks, the more attractive they are to females and the more intimidating they are to males. However, some male stalk-eyed flies have an X chromosome variant which means their eyestalks are shorter, and this disadvantageous variant is passed on to offspring nearly 100% of the time. Scientists trying to understand how this selfish gene is perpetuated, despite sexual selection against it, have now discovered that these males fight more aggressively. Specifically males engaged in more behaviors that involved contact or likely contact with the opponent, such as jumping on or lunging at an opponent, but not more behaviors that did not involve contact, such as lining up of eyestalks or waving of the forelegs. Our data also suggested that male driving X carriers that engaged in more highly intense behaviors were more likely to win their contests against a standardized competitor, whereas this wasn't the case for males without a driving X. We are suggesting that the males carrying driving X are "compensating for" the fact that they have smaller eyestalks on average by increasing aggressive behavior in a way that may allow them to win more contests than would be expected based on their eyespan alone. We also confirmed previous findings about aggressive contests in this species - specifically that a smaller difference in ornament size predicts more aggressive behavior, and that contests tend to escalate from low to high intensity behavior, then end with one animal retreating before a new bout of fighting starts.

Primary research question

Are there differences in aggressive behavior between males carrying a "selfish" meiotic drive X chromosome and those that do not?

What the research builds on

We knew that driving X chromosomes impacted sexual interactions between males and females - intersexual selection - in this species but also others. But no one had looked at whether it impacted sexual contests between males. Because these male-male contests are known to be so important to male fitness in this species, we thought it was worth looking into.

What the research adds to the discussion

It's the first time that an association between a selfish X chromosome like this and aggressive behavior has been demonstrated. It can help understand how these selfish genes are maintained.