Platypus sex is XXXXX-rated
18:00 24 October 04
The weird and wonderful duck-billed platypus just got even more weird and more wonderful.
Platypuses are famous for laying eggs yet producing milk, having a bird-like bill and a skeleton with reptilian features. Now it turns out that the mammal has an equally eye-catching way of deciding its sex, according to a study by Frank Grützner and Jenny Graves at the Australian National University in Canberra, and colleagues.
In most mammals, including humans, sex is decided by the X and Y chromosomes: two Xs create a female, while XY creates a male. In birds, the system is similar: ZW makes for a female, while ZZ makes for a male.
But in platypuses, XXXXXXXXXX creates a female, while XYXYXYXYXY creates a male. In other words, rather than a single chromosome pair, platypuses have ten-chromosome chains that determine their sex.
The researchers worked out the make-up of platypus sex chromosomes by using fluorescent markers to stain chromosomes in platypus cells before examining them under a microscope.
The researchers also found that when sperm is produced by male platypuses, the chromosomes in the chain are precisely distributed to form XXXXX-bearing sperm and YYYYY-bearing sperm. When an XXXXX-bearing sperm fertilises an egg it produces a female platypus. YYYYY-bearing sperm would produce a male.
It is certainly unusual. You only need one sex-determining gene on one chromosome on a chromosome pair. Its intriguing why it evolved such a complex system, says cytogeneticist Jon Martin at Melbourne University.
Very few other mammals are known to have more than two sex chromosomes. The black howler monkey, for example, has four sex chromosomes - the most known until the platypus finding. But the platypus is outstanding, as usual, says Grützner.
The extra chromosomes in the monkey are probably due to a hiccup in evolution whereby non-sex chromosomes mutated to become similar to the sex chromosomes.
This similarity would then cause both pairs to be passed on during reproduction. A similar process might have led to the ten-chromosome system in the platypus, though researchers admit to being stumped.
Mammals and birds
Another intriguing discovery from the study is that one end of the platypuses chromosome chain shares similarities with mammalian sex chromosomes, while the other end shares characteristics with the sex chromosomes of birds.
The platypus X1 chromosome has 11 genes that are found on all mammalian X chromosomes. The X5 carries a gene called DMRT1, which is also found on the Z chromosome in birds.
But although many people believe that DMRT1 will turn out to be the key sex-determining gene in birds, Grützner cautions that there is no evidence that it plays a similar role in the platypus.
The way in which chromosomes determine sex in mammals and birds was thought to have evolved independently after the two classes diverged 300 million years ago.
"But this links the two systems together and raises the issue of whether the ancestral mammal had a sex chromosome system similar to birds, says Grützner.
Journal reference: Nature (DOI 10.1038/nature03021)