Scientists have for the first time unveiled the unusual genetic make-up of the Australian platypus.

According to the study released this morning in the journal Nature, the semi-aquatic animal is a genetic potpourri - part bird, part reptile and part lactating mammal.

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 mainland 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 abdomen.

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 hind feet.

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," said Ewan Birney, who led the genome analysis at the European Bioinformatics Institute in Cambridge.

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 per cent of their body weight everyday.

Males grow to a length of 50 centimetres (20 inches) and weigh about two kilos, with females about 20 per cent 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 http://www.ncbi.nih.gov/Genbank.

Jenny Graves from the Australian National University says by mapping the genome, the scientists also found the platypus has an unusual genetic sexual make up.

"In fact the platypus does sex like a bird, we know that other mammals have an X and a Y chromosome and there's a gene on the Y chromosome that makes you male, that's SRY and we found there is no SRY in a platypus," she said.

"In fact, the platypus sex chromosome is derived not from other mammal sex chromosomes but from bird sex chromosomes."

The genome mapping also revealed the platypus venom produces some useful chemical compounds which may eventually help develop human medicines such as painkillers and potent antibiotics.

- ABC/AFP