Remapping sex genes of the past

Researchers say humans shifted to new sex chromosomes at some point in history AFP

Swiss researchers have discovered that X and Y sex chromosomes in human beings were created more than 100 million years later than previously believed.

Lausanne University’s Centre for Integrated Genomics dates the creation of the sex determining chromosomes to 180 million years ago, in findings published in the Public Library of Science (PLoS) general biology journal on Tuesday.

Co-author Nicolas Vinckenbosch told swissinfo: “Before this, we thought that our sex chromosomes evolved through the common ancestry of all mammals between 280 and 310 million years ago.

“What we are saying now is that those sex chromosomes are not the same as ours.”

During the one-year study, the Swiss researchers looked at the movement of genes in humans, mice, dogs and opossums during spermatogenesis, the period when sperm cells are formed.

At this time, the X chromosome exports genes in an “out-of-X” process. The research team tried to understand the reason for the gene movement and wanted to date when it first started.

The researchers argue that this process existed in placental mammals and marsipuals but not in the early ancestors of humans and primitive mammals, such as the duck-billed platypus.

Using phylogenetic dating – the method of pinpointing when a mutation or a new gene occurs – the team dated the time of emergence of the X chromosome gene exportation and other gene movement.

“We saw that it started about 180 to 200 million years ago. That’s the time that we had a common ancestry with kangaroos,” Vinckenbosch said.

“We didn’t see an out-of-X movement in the ancestry branch that is common to all mammals. For example, in the ancestry we had with the platypus we don’t see this out-of-X movement.”

Historically important

The team say the findings show humans differentiate on the sexual plane from the duck-billed platypus.

Vinckenbosch, who co-authored the study with Lukasz Potrvebowski under a team headed by Professor Henrik Kaesmann, said the findings, although not immediately medically significant were important historically.

“Generally biologists have been very intrigued about sex and how it works, how it evolved,” he said.

“We had sex chromosomes before but in some part of our history we shifted to new sex chromosomes and that, for evolutionary biologists, is very fascinating data.”

swissinfo, Jessica Dacey

Sex in human make-up

Humans have 23 pairs of chromosomes which comprise around three billion base pairs of nucleotides arranged in a double helix. They include the X and Y chromosomes which determine the sex of many animal species.

Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Both men and women retain one of their mother’s X chromosomes, and females retain their second X chromosome from their father.

Genes are segments of DNA which carry information for making the amino acids and consequently proteins required by all organisms. The genes interact with each other and the environment to determine an organism’s genotype – its physical development and traits.

Humans are estimated to have 20,000-25,000 genes.