In the world of stalk-eyed flies, it’s the males with longer eye stalks that get the females. Not only do the females prefer males with longer antennae and stippled eye stalks, but other males are also less likely to fight with these better-endowed flies.
However, some males will always have short eye stalks due to a copy of the X chromosome that makes the eye stalks smaller. While investigating why this genetic mutation has not gone away – despite generations of sexual selection – a team of scientists discovered that the flies may be compensating for their shorter eye stalks with more aggression. The findings are detailed in a study published Oct. 21 in the journal Frontiers in ethology.
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“These floating said in a statement. “This is an extreme example, but simply carrying one of these selfish chromosomes affects so many parts of these animals’ biology, and even their behavior.”
Gene drives and dueling flies
Stem eye flies are a collective name for the insects in the fly family Diopsidae and order Diptera. They are typically found between low-lying vegetation in humid areasusually near rivers and streams. They are only about an inch long and feed on decaying animals and plants.
There are two types of X-chromones present in stalk-eyed flies. The one carrying the mutation for short eye stalks is a driving X chromosomeor more specifically, a meiotic driver. This means it carries alleles that are overrepresented in a man’s sperm, making it much more likely to be passed on to the next generation.
“The driving “Up to 100% of a male’s offspring eventually inherit the X and are therefore female. Therefore, we can assume that the X in the population will continue to increase and even become extinct. Since that did not happen, we would like to understand what other characteristics could counteract that advantage.”
To defend their access to mates, male flies generally use stalk-eyed flies intimidating physical displays and fights. They will also show more aggression towards other flies with eye stalks of similar size
To test whether the flies carrying the driving X are more aggressive, the team will investigate this study used populations of flies that carried both types of X chromosome: the one that always results in flies with shorter eye stalks and the one that does not. They also paired competitors with similar eye stalks, recorded their matches and analyzed their behavior.
The fighting behavior was was more common when the two flies had closer eye cell sizes. These aggressive actions were also seen more often in male flies with the driving X. The males that exhibited more of these fighting behaviors were also more likely to win in these competitions. Men with the floating X chromosome were also more likely to emerge victorious if they spent more time fighting than showing.
“If the combatants are not well matched, fights often end quickly, with the smaller men retreating,” Reinhardt said. “If a male with the floating X chromosome fights a male with eye stalks of similar size, he is more aggressive. But because riding X-males are smaller on average, it is probably still a disadvantage.”
Mating opportunities
According to the teamthis spirited behavior could explain why the flies with short eye stalks were able to mate. Because longer eye stalks indicate larger body size and a potentially more dangerous enemy, the flies with shorter eye stalks will typically withdraw from competitions with these larger flies.
However, if males with the floating X chromosome are more aggressive or cannot accurately assess the threat of other males, they may choose to compete with males with longer eye stalks. This brings them into contact with the females who are initially attracted to their opponent.
While this extra aggression is potentially dangerous, it can also give the flies access to mating opportunities they wouldn’t otherwise have. However, this cannot fully compensate for sexual selection. The team’s modeling of the spread of the driving X suggests this could explain why the shorter eye stalks have not taken over. The females still prefer males with longer eye stalks, keeping the frequency of the variant low.
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“I would say this study is an initial finding,” says Reinhardt. “A larger study could be done where we specifically test for the increase in high-intensity behavior that we saw here in a larger sample. Furthermore, this is a laboratory study, so it is not entirely clear how well this would apply to field behavior.”
The study also didn’t test female flies. If the floating X chromosome directly increases aggression, this could have consequences for females. However, if it is an indirect effect related to the size of the eye stalk, the floating X chromosome may not affect the females.