London, April 24 :
Ever thought even in your wildest dream that a very, very long time ago, the X and Y chromosomes were identical, until the Y started to differentiate from the X in males? Read on.
In a pioneering research, scientists have established that first sex genes appeared in mammals around 180 million years ago – and it took the researchers mind-boggling 29,500 hours to reach this conclusion!
To begin with, they took samples from several male tissues – particularly testicles – from different species.
The researchers recovered the Y chromosome genes from 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, we worked with samples from 15 different mammals, representing these three lineages, as well as the chicken which we included for comparison,” said Henrik Kaessmann, an associate professor at Center for Integrative Genomics (CIG) at University of Lausanne, Switzerland.
Instead of sequencing all Y chromosomes, which would have been a colossal task, the scientists opted for a shortcut.
“By comparing genetic sequences from male and female tissues, we eliminated all sequences common to both sexes in order to keep only those sequences corresponding to the Y chromosome,” explained Diego Cortez, a researcher at CIG and SIB Swiss Institute of Bioinformatics.
The findings 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, involved in testicular development, have thus emerged nearly at the same time but in a totally independent way,” Kaessmann noted.
So what triggered before 180 million years ago then that an individual was born male or female? Was this determination linked to other sex chromosomes, or even environmental factors such as the temperature?
“The latter is not an unreasonable scenario, given that temperature determines sex in present-day crocodiles. As far as mammals are concerned, the question remains open,” Cortez said.