The Y chromosome appeared more than 180 million years ago and plays a more important role than just determining the sex of a person.
Y chromosomes have been shrugged off as mere "sex switches," popularly known as sex-determining genes. However, a new discovery revealed that these chromosomes appeared more than 180 million years ago and play a bigger role in human biology than just determining the sex of a person.
Y chromosomes are present only in males while females have two X chromosomes but researchers from the Swiss Institute of Bioinformatics found that this wasn't always the case. A long time again, Y and X chromosomes were the same until suddenly the Y chromosome started differentiating itself from X chromosomes. During this transition period, it progressively shrank to such an extent that it contains only about 20 genes while the X chromosome contains more than a thousand genes. The current study looked into when this transition started taking place and what genes the Y chromosome managed to retain.
Researchers used samples of male tissues of different species. Most of the samples were derived from the testicles. The species included 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). They used tissues from the chicken for comparison.
The researchers then compared genetic sequences from male and female tissues which helped them eliminate all sequences common to both sexes in order to keep only those corresponding to the Y chromosome. This led to the establishment of the largest gene atlas of this "male" chromosome to date.
After 29,500 computing hours, researchers found that the sex-determining gene (SRY) in Y chromosomes of placentals and marsupials first formed in the common ancestor of both lineages, some 180 million years ago. Researchers also noted that another gene named AMHY was responsible for the emergence of Y chromosomes in monotremes and appeared about 175 million years ago.
"Both genes involved in testicular development have thus emerged nearly at the same time but in a totally independent way," study author Henrik Kaessmann said in a statement.
The sex-determining system of common mammal ancestors continue to remain a mystery as Y chromosomes had not yet formed at that time. Researchers of the current study stated that the factors which trigger whether an individual is born male or female remain an "open question."
