Duck-billed platypus boasts ten sex chromosomes
Roxanne Khamsi


Odd mammal hints at evolutionary origin of sex determination.
Everyone knows that the duck-billed platypus is pretty strange. But it seems
this mammal's eccentricities extend beyond its famous bill, and habit of
laying eggs, to the way its genes determine sex.

Not content with one pair of sex chromosomes, the platypus (Ornithorhynchus
anatinus) has five. This is the largest number found in mammals so far, and
also hints that the sex determination systems of birds and mammals may be
linked.

The platypus is native to Australia, and belongs to a primitive group of
mammals called the monotremes, along with only two other surviving species:
the long-beaked and short-beaked echidnas.

Monotremes were the first group to branch off after mammals evolved 210
million years ago. Their egg-laying shares a common origin with birds and
reptiles, although the bill is thought to have evolved independently.

Chromosome chain

The platypus has 26 pairs of chromosomes in total, compared with the 23
pairs present in humans. But researchers had long been confused about which
ones are autosomal (inherited equally by males and females), and which ones
determine sex.

Full Text at Nature
http://www.nature.com/news/2004/041025/full/041025-1.html

Comment:
Yes, we have some strange animals down here in Tasmania - the Tasmanian
Devil, the Platypus, the Echidna etc.  It would be interesting to know how
many sex chromosomes the other monotremes have - do they also have five
pairs?

Birds have a W and Z chromosome, and it is only the female that has a
mixture.  This makes the mammal and bird lines very different indeed - we
have to go all the way back to reptiles, that have no sex chromosomes, to
find any sort of common ancestor.  But the Platypus has sex chromosomes that
resemble those of both birds and mammals, making the mystery of the
evolution of sex chromosomes very interesting indeed.

Posted by
Robert Karl Stonjek
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