sexy platypuses

Following on the crazy camels, here's a chapter from the "sexy" section of the book. It's the chapter which gave the book its weird and wonderful title:

The birds, the bees and the platypuses

I suggest a title with every piece I write, but editors tend to have a mind of their own, so less than half of my suggested titles survive in the published versions. This one got through, and it must be my favorite title ever. But the story is sexy, too.

What is the molecular difference that makes us male or female? At first glance it’s simple: the male has a (fairly degenerate, see page XXX) Y chromosome instead of the second X chromosome. More precisely, the presence of a single gene in the Y chromosome, known as SRY, makes you develop male characteristics. In its absence, the development reverts to the default option which is female. Things get more complicated when biologists start talking about the birds and the bees. In birds, you know, it’s the other way round: females have a pair of different sex chromosomes, while males have a matched pair -- and don’t even think about the bees. But the record holder for the most confusing sex determination system must be the duck-billed platypus. After decades of uncertainty, Australian researchers have established that this animal has no less than five pairs of sex chromosomes, including one that resembles ours, and one that is more reminiscent of birds.

The ever-popular platypus is one of only three surviving species from the deepest branch of mammalian evolution, the monotremes. Thus, its sex determination is of interest not just as a curiosity but also for any light it might throw on the early evolution of our mammalian ancestors. Using fluorescence in situ hybridization (FISH), the group of Frank Grützner at the Australian National University in Canberra has sorted out the platypus’s ten sex chromosomes, which have the confusing habit to merge into one large chain during cell division. They found that there are five male-specific (Y) chromosomes, which can pair up with five different X chromosomes. In the chain, they are always found in the same order. At one end of the chain there is a pair that resembles our own XY pair (although it confusingly lacks the SRY gene), but the pair at the other end shows some similarity with the ZW chromosomes of birds. The authors even suspect that the latter pair was the first to develop a sex-specific difference, while the others were recruited later, and the one that resembles ours came in last.

This surprisingly bird-like feature in the mammal that lays eggs and sports a duck-like bill might overthrow the old dogma that sex chromosomes evolved independently in birds and mammals. Maybe we originally inherited the same system still found in birds and morphed it into ours. Platypus may have preserved the transition state of this important evolutionary change.

(2004)


Further reading

F. Grützner et al., Nature, 2004, 432, 913.


What happened next

I’m ashamed to admit that I haven’t followed platypus’s sex life as closely as I should have, so I don’t really know. I’ll make a New Year’s resolution to catch up with this.

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watch this space, there is more platypus news coming up very soon !!!