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Tuesday, July 27, 2021
Fruit Fly

As a result of balancing selection, two different pigmentations of female fruit flies exist.

Female fruit flies adapt to male phallus

The fruit fly, as intolerable as they can seem, is integral to studying and understanding genetics.

John Pool, a UW-Madison assistant professor in genetics, studies population genetics primarily by using fruit flies. He looks at genetic variation to learn about how species evolve and adapt on a genetic and molecular level. A recently published paper of his lab dealt with the African fruit fly, Drosophila erecta.

Drosophila erecta is quite aptly named; the male flies have the unusual feature of very large and serrated phalluses. This unique feature of the males is not quite always beneficial to their female counterparts, to say the least, and so it has been found that the females have evolved several characteristics to counter the male sexual organ. 

Amir Yassin, a postdoctoral researcher in Pool’s lab, previously discovered that female flies had evolved armored plates to prevent damage. In addition, Pool and his lab focused on the coloring of the females and males.  

Typically, male fruit flies have a darkly pigmented abdomen, while females have a light coloring.  However, what they discovered in the African fruit fly was different from other fruit fly species.

“In Drosophila erecta, females come in two very distinct forms,” said Pool. “Either they’re a light-abdomen female that look like females of other species, or they’re a dark female morph that looks suspiciously like the males.”

Pool was curious as to what the genetic basis of this trait was at the molecular level.  Through a genetic mapping experiment involving crossing fly strains of light and dark females and studying the traits that their offspring displayed, they connected the pigmented traits with the “tan” gene.

Interestingly, the version of the “tan” gene that carried the light-pigment trait looked very different from the dark-pigment trait. But how long had these differences existed? It turns out there is a way to determine how long two traits have been diverging from each other. DNA mutations occur at an average rate through time, and by comparing sequences of DNA, scientists can use the number of differences to determine how long ago two traits had a common ancestor. Using this method, known as the “mutational clock,” Pool’s team was able to estimate that the two different versions of the “tan” gene have existed for several million years in the African fruit fly. 

The astonishing longevity of the two versions of the “tan” gene suggested that “natural selection had actually maintained these two genes in the population… it was keeping both around,” according to Pool.

This means that the different colors of the females were not random occurrences of mutation, but rather natural selection had determined the success of the females was dependent on having both dark- and light-pigmented females present in the population, a phenomenon called “balancing selection.” Without the light-pigmented females serving as a distraction to the males, the males would eventually learn that the dark flies masquerading as males were female and pursue them, regardless of coloring.

The lab’s study also raises provoking questions about the molecular basis behind female and male characteristics. In Drosophila erecta, a gene sequence called an enhancer effectively “turns on” or “turns off” the “tan” gene’s production of the male pigment.  

However, with this new study, it is possible that “there could be a special kind of DNA sequence that has the capability to turn on this gene differently in males versus females,” according to Pool.

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“That could be an interesting thing in terms of understanding how is it that males and females come to evolve different traits?” Pool said, “How is it that males and females can end up with different traits even though they basically have the same genome? This special type of DNA sequence [in Drosophila erecta] can be an example of how that happens on a molecular level.”

This illuminating study on the African fruit fly sheds light on various aspects of adaptive evolution, and is a step forward in further understanding how and why species evolve the way they do.

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