Want a new type of man? Maybe less brawn and more brains?

May 17 2004

Want a new type of man? Maybe less brawn and more brains?

A new version of the human male is a distinct possibility, given the genetic erosion of the “junk” sex-determining chromosome that makes men male.

The Y-chromosome, which carries an essential gene known as SRY that induces embryonic testis development (and thus the release of male hormones), is self-destructing, shedding 97 per cent of its other genes in the 300 million years since it evolved, ANU geneticist Professor Jenny Graves said in a Jubilee Lecture delivered at University House this week. 

“The human Y chromosome is running out of time. It has lost 1393 of 1438 genes it began with 300 million years ago. At this rate it would lose the last 45 in just 10 million years.”

In humans, as in other mammals, females have two X chromosomes, and males a single X and a Y. The X and Y evolved from an ordinary pair of chromosomes, as the Y chromosome was progressively degraded. This evolution must have happened in the last 300 million years, as birds and reptiles have completely unrelated sex-determining systems.

Professor Graves, the Head of the Comparative Genomics Group in the Research School of Biological Sciences at ANU, has compared the Y chromosome of the three mammalian groups, allowing her and her team to subdivide the human Y into a tiny ancient region shared by all.

“We saw that most of the original human Y has been lost - it was saved from extinction only by adding bits from another chromosome,” Professor Graves said. “Most genes on the human Y have partners on the X from which they evolved.”

“Even the sex-determining gene SRY has a partner on the X, the brain-expressed SOX3, raising questions about how a gene involved in brain development became the gene for testis formation.”

Although it may seem the decline of the Y chromosome and the essential SRY gene would lead to the end of the human race, Professor Graves said nature has proved this is not necessarily the case.

“SRY has been lost in at least two groups of rodents. The mole voles of eastern Europe, and the country rats of Japan have no Y chromosome, and no SRY,” Professor Graves said. “Somewhere else in their genome, a new sex determining gene must have taken over the function of SRY. Which gene or genes took over this task, and how they work, are questions we will be investigating in future.”

Professor Graves predicts that as the human Y-chromosome deteriorates, one or more sex-determining genes will develop, possibly within different human populations.

“What would happen if different new sex determining genes arose in different human populations? Could mole vole man breed with country rat woman? Probably not, so the two populations would ultimately become different hominid species.”

Amanda Morgan
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