|
Australia: Study Unravels Secrets of Platypus Gender Selection |
| October 26, 2004 |
 | The world's only egg-laying aquatic mammal, the platypus is one of the curiosities of science.
Photo: © The State of Queensland (Environmental Protection Agency) 2004. |
The mystery surrounding how gender is determined in the unique Australian platypus has yielded a surprising secret: it has 10 sex chromosomes.
The research has implications for understanding the evolution of so-called “sex determination,” or the way chromosomes interact to determine gender.
“Platypus are remarkable animals, having the defining mammal characteristics of fur and milk, but also retaining some ancestral characteristics like laying eggs,” said Professor Jenny Graves, from the Research School of Biological Sciences at the Australian National University. “Many aspects of their anatomy, physiology and lifestyle have puzzled scientists since they were discovered — it turns out their chromosomes are just as weird.”
Professor Graves and Dr Frank Grützner were part of a research team that tagged genetic material (DNA) from platypus and compared it to chromosomes of human males and females. Human males have one X and one Y chromosome, while females have two X chromosomes.
The scientists were surprised to find that platypus have 10 sex chromosomes – females have 10 X chromosomes and males five X and five Y. Further analysis showed that Y chromosomes are specific to male platypus, just as they are in humans.
During cell division in male platypus, 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 produce female offspring, while 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 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.”
The research was published in the science journal Nature.
Source: Vancouver Aquarium Marine Science Centre
|
