Battle of the Sexes

In the developing embryo, the gonads have a very important decision to make: whether to become testes or ovaries. That makes them rather special.

“The gonad is the only organ that forms and then differentiates as two completely different things,” says Blanche Capel, a professor in the Department of Cell Biology. “The fact that it determines our sex also captures the imagination.”

Different organisms determine sex in different ways. In mammals, including us, the key factor is genetics. Those who inherit a Y chromosome from their father will become males and those who inherit an X will become females. In turtles, temperature reigns. In some fish, one dominant male can prevent its fellow fish from following the male path. Some species even experience partial sex reversals over the course of a year as the seasons change.

“We’re beginning to get a picture of the gonad at a very different level than people have ever had before.”
—Blanche Capel

Despite those differences in the master switches that send an individual down one path or another, Capel suspects that a similar underlying network of genes might be at play. And even in the seemingly simple XY system, ultimately controlled by a single gene on the Y chromosome, that network involves what Capel has referred to as “complex antagonistic signaling pathways that stage the battle of the sexes in the bipotential gonad.”

Capel’s lab got its start in sex determination with more classic developmental biology experiments, work that is still ongoing today. She says it has really only been in the last couple of years that they’ve begun to branch out, incorporating genomic technologies and a systems biology approach in mice. To the extent possible, they aim to do similar work in red-eared slider turtles as well, with plans to sequence the transcriptomes of embryonic turtles in the IGSP’s Genome Sequencing and Analysis Facility both before and after sex is determined.

Already, the mouse work has led to new insight. For instance, before the gonad has made its decision one way or another, it appears that there are two, partially overlapping networks running simultaneously, one male and the other female.

“It’s exciting to see the science evolve,” Capel said. “We’re beginning to get a picture of the gonad at a very different level than people have ever had before while looking at it one gene at a time. We’re attempting to understand it as a system.”

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