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Scientists at Indiana University have uncovered one key to the differences between the two sexes.

The researchers have found the “master gene”—called doublesex or dsx— that regulates the physical differences between males and females and plays a complex role in matching the right physical trait to the right sex.

“We want to know more about this gene because it helps us answer a major question about development and evolution: How do animals with similar genomes—such as males and females of the same species—produce different versions of the same trait?” Cris Ledón-Rettig, a postdoctoral researcher in the IU Bloomington College of Arts and Sciences’ Department of Biology, who led the study, said in a statement.

The study represents the first look at the effect of dsx across the whole genome. The researchers found that the gene isn’t simply a “switch” that turns off certain male traits as females, which was originally thought but rather it plays a highly complex role in controlling the expression of physical differences at different points in the genome based on sex.

The fine-grained control that dsx exerts over male and female traits is possible because the gene acts in a surprising variety of ways, according to Ledón-Rettig.

By activating different genes in males and females it can promote male or female versions of the same trait or by activating the same genes in males while simultaneously inhibiting them in females, it can promote opposite traits.

"The power to prevent the expression of male traits in females, and vice versa, is a critical feature," Armin Moczek, a professor in the Department of Biology and senior author of the paper, said in a statement. "It buffers traits that benefit only members of one sex from causing harm in members of the other."

The researchers compared the genes expressed in normal beetles—Onthophagus taurus—to the genes expressed in beetles in which dsx was suppressed. The experiments showed over 1,000 points on the genome in normal beetles where dsx affected gene expression in males and over 250 points where it affected gene expression in females.

Ledón-Rettig said the majority of these points did not overlap, which indicated that dsx didn’t simply turn certain genes on and off for most the traits studied but rather affected gene expression at different locations in the genome based on sex.

"Essentially, dsx instructs the development of male and female versions of the same trait by influencing different genes in each sex," she said.

This was the case when the researchers looked at the effect on the brains, which regulates sex-specific behavior and genitalia. However, for one trait—large head horns prevalent in males—the study showed that dsx sometimes targets the exact same genes in both sexes. When the scientists disabled dsx, both sexes developed similarly sized horns.

Ledón-Rettig said the group will continue to study the impact of the gene on the beetles.

"We're eager to extend our work on role of dsx -- and other genes -- to sexual differences across other, closely related species of beetles," Ledón-Rettig said. "These beetles are really a powerful platform for unraveling the fundamental mechanisms that underlie evolutionary diversification of sexual traits across species."

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