Oct. 25, 2004 — An animal said to be the "world's weirdest" — the platypus — has just become a bit more unusual with a report that claims the mammal retains a distinctly birdlike sex chromosome.

The finding, published in the current journal Nature, not only helps to explain why the platypus retains characteristics common to both mammals and birds, but it also suggests that the sex chromosomes for these two diverse groups might have an evolutionary link.

Previously it was believed that sex systems for mammals and birds evolved completely independently from each other. If the current research holds true, all mammals, including humans, may hold a bit of unexpected bird history from the distant past.

The platypus, with its furry body and duck-like bill, has puzzled scientists for centuries.

"The first specimens were thought to be a scientific hoax, parts of different animals skillfully stitched together," said Frank Grützner, lead author of the study and a geneticist at the Research School of Biological Sciences at the Australian National University.

Grützner, biologist Jennifer Graves, and colleagues studied platypus sex-determining chromosomes by dying them with fluorescent paints.

The different fluorescent colors enabled the researchers to both identify and track the chromosomes and their involvement in meiosis, which is the process of cellular division that produces eggs in females and sperm in males.

Humans and most mammals have just one set of sex chromosomes for meiosis. Females possess a set of X chromosomes, and males possess one X and one Y chromosome. The overall system is described as XX/XY, with the resulting sex of the mating determined by a gene on the male Y chromosome.

In birds, the male and female chromosomal roles somewhat are reversed, as female birds carry two different chromosomes, called Z and W. The overall bird system is called ZZ/ZW.

The platypus, in contrast, has an unprecedented ten sex chromosomes. The set for females consists of X1X1X2X2X3X3X4X4X5X5, with the numbers indicating slight variations. The male platypus set is X1Y1X2Y2X3Y3X4Y4X5Y5.

Notably, the researchers discovered that X5 resembles the bird Z chromosome in terms of appearance and the signaling of associated genes.

"The platypus sex chromosome system is the first example that shows links to both the bird ZZ/ZW and the mammalian XX/XY system," Grützner told Discovery News.

The platypus, along with two species of echidna, or spiny anteater, emerged 210 million years ago and today represent some of the world's first mammals.

They embody a unified mixture of mammal, reptile, and bird physiology, with their sex chromosomes hearkening back to very early days of life when perhaps all sex chromosomes and sex determining genes evolved from a common ancestor.

Even so, Grützner is quick to point out that the platypus is still a mammal.

"Monotremes are clearly mammals! They have fur and suckle their young," he said. "Nevertheless, no other mammal shows so striking links to reptiles and birds. The skull and the shoulder girdle of the platypus and echidna share similarities with birds and reptiles and, of course, they lay eggs. Our results show a new stunning similarity between the sex chromosome systems of mammals and birds."

He added, "This won't affect the evolutionary position of the platypus as a mammal, but it will revolutionize how we think about the origin and evolution of mammalian sex chromosomes and sex determining genes."

David Haig, head tutor in biology at Harvard University, is "somewhat skeptical" about the proposed evolutionary link, but he said Grützner and his colleagues utilized a "very clever approach" in their study.

Dave Rowell, reader in evolutionary genetics at the School of Botany & Zoology at the Australian National University, is less skeptical.

Rowell told Discovery News, "The fact that platypus chromosomes contain a gene associated with bird sex chromosomes is a surprise ... we need to rethink (current theories) ... (but) how the system as a whole functions is still a mystery."

Despite the current findings, the platypus remains a living puzzle. Among the mysteries yet to be resolved is what exact gene determines the sex for these unique mammals.