New genes found on Y chromosomes

An international study involving researchers from the University of Adelaide and UNSW has revealed new genes on mammalian Y chromosomes.

The Y chromosome, which plays a key role in determining male sex, is notorious for being difficult to study. Despite male genomes being included in early whole-genome sequencing projects, the Y chromosome was largely ignored because of the challenge in obtaining useful data from the chromosome, which is rich in repetitive and palindromic sequences.

This has meant the gene content and evolution of the Y chromosome has long been a mystery in most mammals, particularly the monotremes (the platypus and echidna).

The researchers developed a new sequencing technique that helped them analyse billions of genetic sequences from the Y chromosome of 15 representative mammal species.

The work revealed, for the first time, new gene repertoires for all major mammal groups, tracing the evolution of the Y chromosome in unprecedented detail.

“Finding these new genes is a major breakthrough for us. It finally reveals the gene content of the two different Y chromosome systems that evolved in mammals,” said Dr Frank Grützner, from the University of Adelaide, who has led research into monotreme sex chromosomes for more than 10 years.

The research included the chicken, in which males have two Z chromosomes and females have one Z and one W chromosome (the ovum determines the sex of the offspring in the ZW system), and the platypus, which has five X and five Y chromosomes.

The results enabled the researchers to generate a broader picture of sex-chromosome evolution. In particular, they found that the sex chromosomes of placental mammals, birds and monotremes had essentially independent origins, which means that patterns of gene loss and of specific retention of classes of genes on their Y (or W) chromosomes can be compared.

Dr Paul Waters, at UNSW, said that the X and Y chromosomes started out as normal autosomes (non-sex chromosomes) harbouring the same genes.

“As the Y chromosome evolved, it withered away, losing most of the 1000 genes that are found on today’s X chromosomes. The preserved genes were then recruited into male-specific functions,” he said.

“The most important aspect of this work for us was to identify more genes on platypus Y chromosomes to reveal new leads about potential sex determining genes in these animals,” said University of Adelaide PhD student Deborah Toledo-Flores.

“The next step will be to discover which gene on the Y chromosomes determines sex in the platypus and echidna.

“Y chromosomes are vital for male development and fertility and it will be fascinating to gain a better insight into the function that these newly discovered genes have in different species,” she said