The birds the bees and the platypuses

At first glance it’s simple, at least in placental mammals such as ourselves: males have a Y chromosome in place of a second X chromosome. More precisely, the presence of a single gene on the Y chromosome, known as SRY, is responsible for male characteristics. In its absence, development reverts to the default, which is female.

Things get more complicated when biologists start talking about the birds and the bees. In birds, it’s the other way round: males have a matched pair of sex chromosomes (ZZ), while females have an unmatched pair (ZW). In bees it gets even more complicated, and was only recently determined.¹

But the record holder for the most confusing sex determination system yet discovered 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 that seen in mammals, and one more reminiscent of that in birds.²

The 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 hybridisation (FISH), the group of Frank Gr?tzner at the Australian National University in Canberra has sorted out the platypus’ 10 sex chromosomes, which have the confusing habit of merging into one large chain during cell division. The researchers 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 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, which then evolved into what we have now. The platypus may have preserved the transition state of this important evolutionary change.

Michael Gross