"That was quite a big finding because we had no idea what might be causing male platypus to develop as males."
Waters says short of knocking out the gene in platypus and finding out how it affects their sex, this is the "best evidence to date" on what determines if an animal becomes male.
Unlike monotremes, male marsupials, like kangaroos, and placental mammals, like humans, have only one X and one Y chromosome, and a different sex-determining gene, called SRY.
Waters and colleagues worked out that both this Y chromosome system and the monotreme system evolved around 180 million years ago, but arose independently of other.
"It's a case of convergent evolution," he says.
The researchers are not sure what determined sex prior to the evolution of these genes.
"It could have been an environmental cue such as temperature or could have been a different genetic sex determining system that doesn't exist today," says Waters.
Waters says Y chromosomes started their existence as non-sex chromosomes but over time lost a number of genes.
"As the Y chromosome evolved, it withered away, losing most of the 1000 genes that are found on today's X chromosome."
He says because females don't have a Y chromosome it was believed that over all genes on the Y chromosome could not be too important.
"It was hypothesised that the Y chromosome can't hold anything that's critical to life."
But it turns out this is not the case.
The new study has found that while some of the genes preserved on the Y chromosome evolved for male-specific functions, such as testis development or sperm production, in most species, most Y-specific genes are actually important for the male's basic viability.
They include genes involved in regulation of protein production, which require two copies to be functional. In males, this means having one copy on the X chromosome and one on the Y.