Platypus loaded with sex chromosomes
Monday, 25 October 2004
Dr Franz Grützner and others at the Australian National University (ANU) report their study of the duck-billed platypus online ahead of print publication today in the journal Nature.
Generally, most mammalian chromosomes have two copies, except for the sex chromosomes, X and Y.
But scientists have long been puzzled by the discovery that platypus have a whole bunch of chromosomes that don't match up with others.
"It's been a mystery for 30 years," said Grützner from ANU's comparative genomics group.
By labelling DNA in the 10 unpaired chromosomes with a fluorescent dye, the researchers tracked how the curious chromosomes behaved.
They found that the 10 chromosomes were made up of five X chromosomes and five Y chromosomes, making up 15% of the entire platypus genome.
"Having five X and five Y chromosomes, that's really unusual," Grützner told ABC Science Online.
A chain of chromosomes
During meiosis, the process by which chromosomes divide to produce germ cells like sperm, these chromosomes join up to form a chain-like structure, a structure thought only to occur in some termites, spiders and plants.
But how does the platypus deal with this large number of unpaired sex chromosomes during sperm production? After all, it would be a problem if sperm ended up with a mixture of X and Y chromosomes.
The researchers studied the way the chromosomes behaved during meiosis and found that the chain of sex chromosomes were in an alternating pattern of X-Y-X-Y etc.
During sperm production, all the odd chromosomes (X) went to one end of the dividing cell, and all the even (Y) chromosomes to the other.
Grützner and team analysed the genes in the "meiotic chain" of 10 sex chromosomes.
At one end of the chain was a chromosome similar to a mammalian X chromosome and at the other end was one similar to the sex-determining gene normally found in birds, DMRT1.
"The platypus actually links the bird sex chromosome system with the mammalian sex chromosome systems," said Grützner.
He added the team was "incredibly interested" to find out more about the other sex chromosomes in the chain.