Published online 25 October 2004 | Nature | doi:10.1038/news041025-1

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Odd mammal hints at evolutionary origin of sex determination.

Even the duck-billed platypus's chromosomes are weird. © The State of Queensland (Environmental Protection Agency)

Everyone knows that the duck-billed platypus is pretty strange. But it seems this mammal's eccentricities extend beyond its famous bill, and habit of laying eggs, to the way its genes determine sex.

Not content with one pair of sex chromosomes, the platypus (Ornithorhynchus anatinus) has five. This is the largest number found in mammals so far, and also hints that the sex determination systems of birds and mammals may be linked.

The platypus is native to Australia, and belongs to a primitive group of mammals called the monotremes, along with only two other surviving species: the long-beaked and short-beaked echidnas.

Monotremes were the first group to branch off after mammals evolved 210 million years ago. Their egg-laying shares a common origin with birds and reptiles, although the bill is thought to have evolved independently.

Chromosome chain

“Mammals are pretty boring when it comes to sex chromosomes. The platypus is a huge exception.” Steve Rozen

Whitehead Institute for Biomedical Research

The platypus has 26 pairs of chromosomes in total, compared with the 23 pairs present in humans. But researchers had long been confused about which ones are autosomal (inherited equally by males and females), and which ones determine sex.

Frank Grützner of the Australian National University in Canberra and his colleagues used fluorescent tags to study the animal's chromosomes. They were amazed to find that five separate pairs, which join together in a chain during cell division, determine an individual's sex.

"Mammals are pretty boring when it comes to sex chromosomes," says geneticist Steve Rozen of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts. "The platypus is a huge exception."

Like a bird

In humans, sex is determined by one pair of chromosomes: a woman has two X chromosomes, and a male has an X and a Y. One of the platypus's sex chromosome pairs contains units similar to these, but another resembles the ZZ/ZW sex chromosome system found in birds.

Bird and mammal sex chromosomes both evolved from autosomal chromosomes. But they evolved from different pairs in the two classes, so scientists had believed the systems arose separately.

But because the platypus system contains elements of both, it is possible that the two systems are related. The result "challenges the accepted view that mammal and bird sex chromosomes evolved independently", the researchers report online in Nature1. They now plan to study the two echidna species to see if they use the same system.

So what is the advantage of having so many sex chromosomes? "It's hard to speculate on how that could have evolved," says Rozen. Grützner's colleague Willem Rens from the University of Cambridge, UK, says he has an idea, but isn't ready to discuss it yet. The team's next findings are scheduled to appear in the Proceedings of the National Academy of Sciences.

Whitehead Institute for Biomedical Research