Platypus Genome Found Fittingly Strange

One of the more surprising elements was the animal's system for sex determination. Most mammals have two sex chromosomes, either two "X" chromosomes (to make a female) or an "X" and a "Y" (to make a male). Not only do platypuses have 10 instead of two, but they seem closer to the "Z" and "W" chromosomes of birds.

Moreover, the key gene on the Y chromosome that confers maleness in most mammals is not on any of the platypus's sex chromosomes. It is on another chromosome, where it seems to have nothing to do with sex. In its place, another gene seems to be central to sex determination in platypuses -- evidence of a shakeout of various evolutionary efforts to settle on a system of sex determination in early mammals.

Other genes show how platypuses were transitional creatures on the road from egg laying to internal gestation. There is just one gene for one kind of yolk protein, for example, while chickens have three. That is consistent with the idea that the platypus represents a shift in strategy toward providing more nutrition after hatching, rather than during incubation, and lends credence to the poet Ogden Nash's famous appreciation of the platypus's approach to child-rearing: "I like the way it raises its family/Partly birdly, partly mammaly."

Platypus milk appears to be a modified version of a moisturizing fluid that ancestral platypuses once used to keep their leathery, lizardlike eggs from drying out during incubation. It is secreted from "milk patches" on the mother's abdomen.

As with kangaroos, platypus milk becomes more nutritionally complex over a period of months while the young suckle and grow, the result of at least five different genes turning on in sequence.

"The dairy industry is actually very interested in this and want to get their hands on the controlling gene elements that turn these milk genes on and off," Graves said.

Another surprise was that platypuses have a huge array of genes that help them detect chemical signals released underwater by other animals. That makes sense, scientists said, since platypuses close their eyes and nostrils while diving for the small aquatic crustaceans that make up the bulk of their diet.

These "vomeronasal" genes, active in the back of the mouth, give the platypus a highly sensitive system for detecting waterborne pheromones -- hormonelike signals released by potential mates or prey.

Yet another surprise came from an examination of the genes that make platypus venom, which males can deliver from a sharp spur on each of their rear legs.

The platypus is the only mammal to make venom, and the chemicals in it are almost identical to those in some snake venoms. Yet the new analysis shows plainly that the two classes of animals came up with the innovation independently and by different evolutionary routes, though both built their poisons from the same starter molecule, an immune system chemical.

Disappointingly, scientists said, they have been unable to find any genes involved in the platypus's elaborate system for detecting electrical fields, which it does through its bill, perhaps to help navigate through narrow waterways. But that is just one of many avenues, they said, that promise to keep them busy with duck-billed DNA.

"We're going to be using the platypus genome for the next 50 years," said Ewan Birney of the European Bioinformatics Institute in Cambridge, England, which was involved in the analysis.

"The platypus gives us a perspective that is deep in time, that tells us what was going on 170 million years ago, when all these traits were being developed," Birney said. "Every time there's a difference in the DNA between human and dog, or human and some other mammal, and you want to know which one changed more recently, you need these outgroup species to be able to answer that."