In humans and other mammals, the difference between sexes depends on one single element of the genome: the Y chromosome.
It is present only in males, where the two sexual chromosomes are X and Y, whereas women have two X chromosomes. Thus, the Y is ultimately responsible for all the morphological and physiological differences between males and females.
However, a very long time ago, the X and Y were identical, until the Y started to differentiate from the X in males.
It then progressively shrank to such an extent that, nowadays, it only contains about 20 genes (the X carries more than one thousand genes).
The question of when did the Y originate and which genes have been kept has now been answered by the team of Henrik Kaessmann, Associate Professor at the Center for Integrative Genomics - CIG UNIL and group leader at the Swiss Institute of Bioinformatics, and their collaborators in Australia.
Researchers have established that the first "sex genes" appeared concomitantly in mammals around 180 million years ago.
By studying samples from several male tissues - in particular testicles - from different species, the researchers recovered the Y chromosome genes from the three major mammalian lineages: placentals (which include humans, apes, rodents and elephants), marsupials (such as opossums and kangaroos) and monotremes (egg-laying mammals, such as the platypus and the echidna, a kind of Australian porcupine).
In total, the researchers worked with samples from 15 different mammals, representing these three lineages, as well as the chicken, which they included for comparison.
Instead of sequencing all Y chromosomes, which would have been a "colossal task", researchers compared genetic sequences from male and female tissues to eliminate all sequences common to both sexes in order to keep only those sequences corresponding to the Y chromosome.
By doing so, they established the largest gene atlas of this "male" chromosome to date.
The study showed that the same sex-determining gene, named SRY, in placentals and marsupials had formed in the common ancestor of both lineages around 180 million years ago.
Another gene, AMHY, is responsible for the emergence of Y chromosomes in monotremes and appeared some 175 million years ago.
Both genes, which according to Kaessmann are "involved in testicular development," have thus emerged "nearly at the same time but in a totally independent way".