Platypus Sex Chromosomes Could Be Missing Link Between Mammals and Birds

Typically, if it looks like a duck, it’s a duck. But what if it’s actually a mammal? Most people know by now that the duck-billed platypus is a pretty strange animal – one that doesn’t meet the conventional classification of a mammal because of the fact that it lays eggs, like birds and reptiles. However, scientists have discovered something even more perplexing: the platypus has five pairs of sex chromosomes – not one pair, as is the case with most mammals.

In birds and snakes, a ZW sex chromosome set determines the sex of females while a ZZ set determines the sex of males. In mammals, it is the XX sex chromosome set that determines female young and the XY set that determines male young.

The ten sex chromosome set of the male platypus follows a XYXYXYXYXY pattern. This is weird enough in its own right; but when studied, it was also realized that the platypus had chromosomal similarities to birds as well as mammals. The complex sex chromosomes of the platypus also resemble the ZZ/ZW sex chromosome system found in birds.

It was once believed that the sex determination processes of birds and mammals were autonomous, but this is no longer thought to be the case. “Mammal sex chromosomes may well have co-evolved with those of birds, and the platypus could be the key to finding out,” says Professor Marilyn Renfree of the University of Melbourne.

Because of the platypus’ combination of bird and mammal sex chromosome characteristics, the suggestion is that the two systems (those of birds and those of mammals) which were once thought to be independent are actually related.

Native to Australia, the duck-billed platypus not only has ten sex chromosomes; it has 26 total pairs of chromosomes compared with only 23 total pairs in humans.