The platypus laid bare

It's a little over two centuries since NSW Governor Captain John Hunter sent a platypus pelt and a sketch of the strange mammal to London, only to have the learned natural philosophers of the Royal Society dismiss it as an elaborate hoax.

The platypus' improbable combination of reptilian, avian and mammalian features provoked great skepticism, even before any European scientist learned that the bizarre creature laid eggs, and suckled its young, without benefit of nipples.

At a genetic conference held earlier this month at Lorne in Victoria, Professor Wes Warren, assistant director of Washington University Medical School's Genome Sequencing Centre in St Louis, Missouri, presented preliminary results from the Platypus Genome Project.

Already, it is clear that the platypus' bizarre combination of physical features runs more than skin-deep: the platypus' 52 chromosomes are bird-like, but the genome of Ornitorhynchus anatinus is unmistakably mammalian, despite diverging from eutherian and marsupial mammals in the Triassic period, some 200 million years ago.

Warren says the platypus's draft DNA sequence has been completed, at 6X coverage, and a preliminary gene set has been catalogued, ready for ordering on a chromosomal map.

One of Washington University's collaborative efforts with another US research group involves the evolution of testicular descent. Male platypuses retain their testes inside the abdominal cavity, unlike male eutherians and monotremes, whose testes descend into a scrotum, in effect becoming external organs.

Testicular descent in eutherians and monotremes is thought to be an adaptation for cooling the testes. If the testes fail to descend, the higher temperature of the abdominal cavity, where they testes develop, inactivates sperm.

Where eutherians and marsupials typically have a core temperature around 38 degrees, the platypus has a core temperature of only 31-32 degrees.

Genetic amalgam

Warren says the unique biology of the platypus promises to yield numerous interesting genetics tales - for example, about the multiple, daisy-chained sex chromosomes, which give the platypus a very different mode of sex determination to the standard X-Y chromosome system in eutherians and placentals - and different again from the W-Z system in birds, where males are the heterogametic sex.

As in birds, most of the platypus' 52 chromosomes are acrocentric - the centromere is located near one end of the chromosome, instead of centrally. It combines very large chromosomes with microchromosomes - again, a bird-like characteristic.

The platypus karyotype is an amalgam of macro- and micro-chromosomes. Warren says it may provide insights into why huge chromosomes evolve: the same little-and-large pattern is a consistent feature in all the world's 9600-odd bird species. One hypothesis suggests it is an evolutionary adaptation for flight, that keeps the genome compact.

There is also great interest in the genes that endow the platypus with its unique ability to sense the faint electrical pulses generated by muscle activity in its prey, via electroreceptors in its bill - a system shared with the echidna, he says.

But the large amounts of repetitive DNA scattered throughout the platypus genome are distinctively mammalian, not avian.

"Given that the platypus sits at the basal lineage of mammals, it's a fascinating question as to why these repeat structures expanded in mammals," Warren says.

"Why did the platypus keep bird-like chromosomes, yet evolve mammalian-like repeat structures?"

In evolutionary terms, the platypus and its monotreme cousins, the echidnas, descended from primitive egg-laying mammals called multituberculates, whose fossils have been found in Mongolia.

The basal divergence that gave rise to monotremes and marsupials occurred around 200-210 million years ago, with eutherians branching much later from the marsupial line.

Warren says these relationships look "pretty solid" and doubts that the platypus genome will require the phylogeny of mammals to be revised after it is compared with those of marsupials like South America's grey opossum and Australia's tammar wallaby, and eutherian mammals.

The platypus genome is Warren's focus, but his Washington University research team is working on other mammalian and avian genome projects, as well as on nematode and fly species to complement the historic genome projects on C. elegans and Drosophila melanogaster.

The Washington University genome centre will begin a genome project on Australia's zebra finch later this year and is also working on reference genomes for non-human primates including the orang-utan, gibbons and highly divergent clades like prosimians, including South America's marmosets.

Warren says while one aim is to establish the evolutionary history of these primate clades, the more important objective is to garner non-human primate data that should provide insights into human disease.