Neither fish nor fowl: Platypus genome decoded
PARIS (AFP) — Arguably the oddest beast in Nature's menagerie, the platypus
looks as it if were assembled from spare parts left over after the animal
kingdom was otherwise complete.
Now scientists know why. According to a study released Wednesday, the
egg-laying critter is a genetic potpourri -- part bird, part reptile and part
The task of laying bare the platypus genome of 2.2 billion base pairs spread
across 18,500 genes has taken several years, but will do far more than satisfy
the curiosity of just biologists, say the researchers.
"The platypus genome is extremely important, because it is the missing link
in our understanding of how we and other mammals first evolved," explained
Oxford University's Chris Ponting, one of the study's architects.
"This is our ticket back in time to when all mammals laid eggs while suckling
their young on milk."
Native to eastern Australia and Tasmania, the semi-aquatic platypus is
thought to have split off from a common ancestor shared with humans
approximately 170 million years ago.
The creature is so strange that when the first stuffed specimens arrived in
Europe at the end of the 18th century, biologists believed they were looking at
a taxidermist's hoax, a composite stitched together from the body of a beaver
and the snout of a giant duck.
But the peculiar mix of body features are clearly reflected in the animal's
DNA, the study found.
The platypus is classified as a mammal because it produces milk and is
covered in coat of thick fur, once prized by hunters.
Lacking teats, the female nurses pups through the skin covering its
But there are reptile-like attributes too: females lay eggs, and males can
stab aggressors with a snake-like venom that flows from a spur tucked under its
The bird-like qualities implied by its Latin name, Ornithorhynchus anatinus,
include webbed feet, a flat bill similar to a duck's, and the gene sequences
that determine sex. Whereas humans have two sex chromosomes, platypuses have 10,
the study showed.
"It is much more of a melange than anyone expected," commented Ewan Birney,
who led the genome analysis at the European Bioinformatics Institute in
The animal also possesses a feature unique to monotremes -- an order
including a handful of egg-laying mammals -- called electroreception.
With their eyes, ears and nostrils closed, platypuses rely on sensitive
electrosensory receptors tucked inside their bills to track prey underwater,
detecting electrical fields generated by muscular contraction.
"By comparing the platypus genome to other mammalian genomes, we'll be able
to study genes that have been conserved throughout evolution," said senior
author Richard Wilson, a researcher at Washington University.
In captivity, platypuses have lived up to 17 years of age.
In the wild, they feed on worms, insect larvae, shrimps and crayfish, eating
up to 20 percent of their body weight every day.
Males grow to a length of 50 centimetres (20 inches) and weigh about two
kilos (4.5 pounds), with females about 20 percent shorter and lighter.
The genome sequenced for the study belongs to a female specimen from New
South Wales nicknamed Glennie and can be accessed at www.ncbi.nih.gov/Genbank.