The gene that most likely determines the sex of the platypus and echidna has been identified by Australian and Swiss researchers.
The study also shows that the Y chromosome, contrary to previous assumptions, carries genes that are important to the basic viability of male mammals, says geneticist Dr Paul Waters from the University of New South Wales.
Although the Y chromosome is known to be important in sex determination, little is known about the function and evolution of its genes, says Waters.
He says this is because it has so many repetitive and palindromic sequences, which make it hard to reconstruct the true sequences of its genes from fragments of sequenced DNA.
Monotremes (the platypus and the echidna), whose males have 5 X chromosomes and 5 Y chromosomes, are especially challenging.
"No one had really characterised any Y chromosomes in platypus before because they've got quite a complex sex chromosome system," says Waters.
Waters and colleagues from the University of Adelaide and the University of Lausanne now report on their new analysis of male and female DNA from 15 representative mammals, including human, elephants, marsupials and monotremes.
The study, reported recently in the journal Nature, is the largest of its kind, and relied on a rapid new sequencing technique.
For each species, the researchers identified Y chromosome genes by looking for those DNA sequences that were specific to males.
By using a molecular clock, which combines fossil evidence and rates of change in DNA sequences, the researchers were also able to work out when specific genes evolved.
Waters says the process uncovered for the first time a gene, called AMH [for Anti-Müllerian hormone], on the oldest of the platypus Y chromosomes that appears to determine if an animal becomes male.
"If an animal has that gene it will act as a master switch to turn on testis development," he says.