Discover Magazine : Sex, Ys, and Platypuses

Sex, Ys, and Platypuses
By Jocelyn Selim
April 25, 2005 Biology & Medicine


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...

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