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WU research advances knowledge of the platypus' peculiarities
(by Tim Woodcock - May 20, 2008)
Researchers at Washington University School of Medicine’s Genome Center are generating headlines around the world — and not because of medical research per se, but because of a DNA analysis that has shed light on the origins of one of the animal world’s most peculiar creatures, the platypus.
The platypus, a mammal that lays eggs and can produce venom, has long confounded zoologists because so many of its characteristics are outside the norm for mammals. Its improbable bill caused early European visitors to the platypus’ native land of Australia to speculate that the first examples they saw were frauds — something like a beaver with a duck’s bill sewn on.
Because of its peculiar appearance and abilities, the platypus fascinates both the scientific community and the general public, and is a “generally loved creature,” said Wes Warren, assistant director of the Genome Sequencing Center and one of the lead authors of the study. Warren said he was recently at an academic conference presenting the results of the study, but his time at the conference was limited because he spent so much time on the phone with reporters discussing the research, which was featured in the May 8 issue of Nature, an international journal of science.
As part of an international consortium, the WU team analyzed platypus DNA to determine where in its ancestral history the platypus diverged from related creatures.
The implications of the research go beyond understanding the oddities of this one particular animal because it helps refine scientists’ ideas about how genes are passed on and mutate. “While we’ve always been able to compare and consider all of these creatures on the basis of their physical characteristics, internal anatomy and behavior,” said Richard Wilson, director of the Genome Sequencing Center, “it’s truly amazing to be able to compare their genetic blueprints and begin to get a close-up look at how evolution brings about change.”
DNA is considered the “code of life” — the genetic signature that differentiates one organism from another — and in a platypus there are 2.2 billion genomes that needed to be examined. The analysis was done several times, and the information was then compared to genome sequences from other animals to spot where the most interesting variations are, Warren said.
The platypus, in its current evolutionary form, can be traced back 166 million years; it retains some of the features of reptiles, from which all mammals evolved.
“What is unique about the platypus is that it has retained a large overlap between two very different classifications, while later mammals lost the features of reptiles,” Warren said.The DNA markers that correspond to egg laying were identified by the researchers.
They also found genes that support lactation, which is significant because a female platypus does not feed her young through nipples, instead milk is secreted through the abdominal skin.
In these two cases, the genetic research supported already known facts about the creature’s oddities, but the scientists involved also came across one especially remarkable way in which the platypus breaks all the rules.
The chromosome system for determining sex in humans and most other mammals — XX for females, XY for males — is not at work in the platypus, Warren said. The study revealed there are 10 chromosomes affecting sex, although the outcome is the same, with only males and females being produced.
The genetic markers look more like the markers found in birds than mammals, he said. “It’s a complete mystery as [to] why it has evolved. There’s no other thing like it in biology,” he said.
After this study Warren will be moving on to a follow-up study looking at the evolutionary roots of the creature’s venom gland, which is not found near the platypus’ mouth but in a spur on its hind leg. While he’s working on that one, other biologists will have plenty of remaining questions to consider about the curious life of the platypus.
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