Nature reports today that the platypus has five chromosomes determing sex

"What we've discovered is that these five Xs and five Ys line up in a great big long chain, that go XY XY XY XY XY XY, and then all the X chromosomes move to one pole, and all the Y chromosomes move to the other," she said.

Professor Graves says there is another unexpected finding.

"One end of the chain looks like human sex chromosomes but the other end of the chain looks like bird sex chromosomes, so the chain is actually linking a very ancient system of sex determination in birds and probably reptiles too," she said.