Let’s step into our time travel machines and rewind all the way back to biology class in high school. Remember those chromosome charts? They were square in shape, had “XX” on one axis and “XY” on the other. XX was Mom (female), and XY was Dad (male). The interactions of these chromosomes at the embryonic level, we were taught, determined our sex, our anatomy, and likely a large part of how we see ourselves as individuals.
The “chromosome narrative,” as Molly Webster reports in a recent Radiolab podcast, is truthful to a certain extent. But it doesn’t tell the whole story of how we became male or female. Chromosomes are large structures (“kidney beans” at the cellular level) containing up to 200 genes in the human body. Which genes, specifically, are responsible for choosing sex?
As babies, and later as children, we are told that we have our “father’s nose” or our “mother’s eyes.” Physical appearance is a “blending” of genetic material from our parents. With sex, there is no such blending. We are born with either female or male reproductive organs – but not a mixture of both.
The chromosome narrative began to receive scrutiny at the onset of the DNA revolution in 1979, according to David Page, a professor of Biology at MIT. In his work with the “Human Genome Project,” Page came across genomes of anatomical females that had XY chromosomes, and anatomical males that had XX chromosomes. Page looked closer at the XY chromosomes present in females and noticed that a chunk was missing. The missing chunk, oddly enough, was present in males that had XX chromosomes.
Page thought, “this must be the bit that matters” – the so-called sex-determining grand master switch. Instead of an entire XY chromosome being responsible for sex selection, it was a small part of that chromosome: a single gene, called “SRY.”
The SRY gene sets off a cascade of reactions in the embryo, creating chemicals that send signals to create male reproductive organs (testes). If this trigger does not show up in the early days of the embryo, female reproductive organs form.
One of the signals created by the SRY gene is “DMRT-1,” a gene discovered by University of Minnesota Genetic Cell Biologist David Zarkower in the early 1990s. The gene, Zarkower says, is essential to “finishing the job” of forming the testes.
Zarkower wanted to know: what happened if he removed DMRT-1? So, like any good geneticist, he went to work on mice. He snipped the gene from the testes of male mice, then taking a slice of the testes to examine under a microscope. He then recalled having a “That’s weird” moment. The male cells, usually long and slender, became smaller and rounder. They then started making estrogen. Absent the DMRT-1 gene, they eventually turned into ovary cells.
It gets weirder. Whereas most genes in the developmental stage go away, their work being done, DMRT-1 in humans stays active throughout our lives –as embryos, babies, children, and adults. The genetic code for our other male or female selves never went away, says geneticist
Blanche Capel –it is just being “shushed” by the ever-watchful DMRT-1. Or in Zarkower’s words: “SRY casts the deciding vote, but every day there’s a recall.”
In his work as a Marine Biologist at UC Santa Barbara, Bob Warner studies blue-headed wrasse, a reef fish in the Florida Keys that lives in groups made up of one male and many females. Every day, each female shoots up in the water column to mate with the male, to (we can only assume) have their own moment of privacy.
But the real magic happens when the male dies or is picked off by a predator. Soon after, in a process that is rapid and not entirely understood, the ladies pick who will be the new male. The chosen one, who up to that point was anatomically female, begins to change behavior. She will “wave a pectoral fin” and get friendlier with the other females. Her body starts to change, producing bright blue that starts at the nose and spreads to the chin and face. She develops a black and white crown consistent with the male. She produces testosterone. Her ovaries disintegrate, and she begins to make testes.
“She” becomes “he” in a process that Warner considers unremarkable, seeing as hundreds of other fish species –parrot fish, groupers, damsel fishes, clown fish—already possess the capability. And it’s not only fish. Sex change also occurs in shrimp, worms, alligators, lizards, birds… and don’t even get Warner started on turtles.
“Humans tend to separate things into lumps,” says Capel—“male” and “female” being one of countless examples. Yet as the discoveries of SRY and DMRT-1 show, there is perhaps little that separate the two, and a tremendous amount of middle ground.
Check out the Radiolab series called “Gonads,” wherever you get your podcasts!