The platypus has always been considered odd. Not only does it have webbed feet, a tail like a beaver’s, a coat of fur, and a large bill, but it also lays eggs and nurses its offspring through a set of glands on its abdomen. Biologists classify the platypus as a monotreme, an egg-laying mammal with a single opening for reproduction and excretion. But is it truly a mammal?
A draft of the genetic sequence of Glennie, a female platypus in Australia, answers the question. “We found that the platypus has reptilian, avian, and mammalian genome features in one organism,” says Mark Batzer, a biologist at Louisiana State University. “It looks like a car that was built on a Friday. They used the parts they had left to put it together.” This mix of genes, he says, supports the classification of the platypus as a unique and very early mammal. Among its oddities: 52 chromosomes, including 10 sex chromosomes, as well as the highest number of repeated segments in the genome of any mammal sequenced so far.
The repeats are mobile elements called transposons, also known as jumping genes, which can trigger mutations in the genes around them and lead to genetic disorders. “Learning when transposons first appeared may give us some insight into how they spread through the mammalian lineage and how they are expressed in humans,” Batzer says.