Register for an account


Enter your name and email address below.

Your email address is used to log in and will not be shared or sold. Read our privacy policy.


Website access code

Enter your access code into the form field below.

If you are a Zinio, Nook, Kindle, Apple, or Google Play subscriber, you can enter your website access code to gain subscriber access. Your website access code is located in the upper right corner of the Table of Contents page of your digital edition.


Sex, Ys, and Platypuses

By Jocelyn SelimApril 25, 2005 5:00 AM


Sign up for our email newsletter for the latest science news

For mammals, gender is usually a simple affair. From mice to elephants, a single pair of chromosomes control sex—inheriting double Xs makes females, while inheriting a Y makes males. But for the platypus, the story is a bit more complicated.

Unable to locate a single Y chromosome in the platypus, researchers had long considered its gender-determining genes a mystery. By using fluorescent labeling to track platypus chromosomes during cell division, Frank Gruetzner, a molecular biologist at the Australian National University, solved the puzzle. Instead of a single pair of sex chromosomes, the platypus has five—a record for vertebrates. “It’s not as confusing as it might be,” Gruetzner says. “The sex chromosomes link up in a chain, so a male platypus is always XYXYXYXYXY.”

Even more intriguing, one of the platypus’s Y chromosomes shares genes with the ZZ/ZW sex chromosomes found in birds, which are thought to have evolved separately. “We’re not certain exactly how the two are related, because in birds the system is inverted,” says Gruetzner. “Males are the ones with two identical sex chromosomes.” The presence of both bird and mammal chromosomes in the platypus implies that sex determination evolved just once among a common ancestor of both groups, and then diverged into distinct systems.

The platypus has always looked a little ambiguous. Its signature duckbill and egg-laying are birdlike, but its fur and milk are mammalian. “Now,” says Gruetzner, “we know this ambiguity goes all the way down to its genome.”

    3 Free Articles Left

    Want it all? Get unlimited access when you subscribe.


    Already a subscriber? Register or Log In

    Want unlimited access?

    Subscribe today and save 50%


    Already a subscriber? Register or Log In