Australia’s animals illustrate one of the key aspects of evolution – how reproductive barriers lead to new species – a leading geneticist will tell a conference celebrating 150 years since Charles Darwin and Alfred Wallace first outlined the theory of natural selection.
Scientists will gather today for the symposium ‘Wallace and Darwin’s Theory of Natural Selection - 150 years on’, hosted by the Research School of Biological Sciences (RSBS) at ANU.
Prof Jenny Graves’ team played a lead role in mapping the platypus genome, which includes 10 sex chromosomes. They found that platypus sex chromosomes are nothing like those of other mammals, but more like those of a chicken. “This means that an ancient hairy reptile-like mammal that lived 300-million-years ago probably determined the sex of their offspring like birds and many reptiles do today.
“Then something happened to the sex chromosomes so they worked in a different way,” Professor Graves said. “In birds and many reptiles, sex is determined by which sex chromosomes (Z or W) the female provides in the egg. But 200 or 300-million-years ago there was a flip-flop, so that it became the male that determined sex by providing either an X or Y chromosome in the sperm.
“But we know from gene mapping that the platypus X and the bird Z are the same chromosome. Now we are trying to figure out how the same genes on these chromosomes took on quite opposite roles.
“Though it was still closely related, the hairy reptile with a novel XY system could not mate successfully with a reptile with a ZW system. This reproductive barrier may have been what set mammals on the unique mammal evolutionary trajectory that eventually produced us humans!
“Then this ancient mammal system was taken over by a different X and Y in marsupials and placental mammals, with the evolution of the SRY gene from a gene called SOX3. We know this happened only recently because the human X and Y is still an ordinary chromosome in a platypus.”
Professor Graves said that Australian mammals illustrate how the deepest and most important evolutionary changes occurred accidently.
“The flip-flop from an ancient ZW system into the X and Y of our hairy reptile ancestor was a very trivial change, probably in a single gene,” she said. “The later takeover by a different X and Y in marsupials and placental mammals was the result of a single genetic accident to the SOX3 gene. This shows there was no design, and no celestial designer. In fact, any designer who put together the bizarre and inefficient 10 sex chromosome system of the platypus would have failed design school.”
