Australian platypus research has implications for understanding the evolution of human sex determination

The mystery surrounding sex determination of the unique Australian platypus has yielded a surprising secret: it has 10 sex chromosomes.

The research has implications for understanding the evolution of sex determination — it seems that human sex chromosomes may have evolved from an ancient bird-like system, according to Professor Jenny Graves and Dr Frank Grützner from the Research School of Biological Sciences at ANU.

“Platypus are remarkable animals, having the defining mammal characteristics of fur and milk, but also retaining some ancestral characteristics like laying eggs,” Professor Graves said. “Many aspects of their anatomy, physiology and lifestyle have puzzled scientists since they were discovered — it turns out their chromosomes are just as weird.”

In collaboration with colleagues from University of Cambridge, Professor Graves and Dr Grützner used new molecular methods to tag DNA from isolated chromosomes with fluorescent dye and ‘painted’ it on to the chromosomes of males and females. The tagged DNA homes in on sequences that are the same (such as the X and Y sex chromosomes).

“We found that the platypus is not content with the usual way mammals determine sex – with females having two X chromosomes and males a single X and a Y,” Dr Grützner said.

Surprisingly, platypus have 10 sex chromosomes – females have 10 X chromosomes and males five X and five Y. DNA isolated from one of the Y chromosomes by fellow researcher, Dr Enkhjargal Tsend-Ayush, showed that it was specific to males.

In the male platypus, the ten sex chromosomes line up as a chain of XYXYXYXYXY at meiosis, the special cell division that distributes chromosomes to eggs and sperm. When the cell divides, the five X chromosomes divide into one cell and the five Y chromosomes into another.

This makes two kinds of sperm; half of the platypus sperm have XXXXX and determine female young, and the other half have YYYYY and are male-determining.

“It’s amazing that these complicated sex chromosomes work so efficiently to produce platypus babies. So although there are more than two sex chromosomes in males, they separate perfectly into five X and five Y each time,” Professor Graves said.

“The most interesting and surprising aspect about the platypus chain of sex chromosomes is that it links mammal sex chromosomes with bird sex chromosomes,” Dr Grützner said.

According to the researchers, the X at one end of the platypus chromosome chain shares genes with the human X and Y chromosomes. The X at the other end shares genes with bird sex chromosomes.

This was shown by mapping a gene of particular interest, called DMRT1 (thought to be the sex determining gene in birds) to a platypus X chromosome by ANU PhD student Nisrine El-Moghabel.

“This completely changes our picture of how human sex chromosomes came to be,” Professor Graves said.

“Birds have a different sex chromosome system, in which ZZ is male and ZW female. It has been thought that the XY mammal system was unrelated to the bird ZW system, and the two systems evolved independently.

“Now it may be that our sex chromosomes have evolved from an ancient bird-like system and platypus chromosomes may be the key to finding out just how.”

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