The platypus (Ornithorhynchus anatinus) is an odd-looking creature whose features combine the furry torso and wide, flat tail of a beaver with the rubbery bill and webbed feet of a duck. But its looks are not all that is strange about it. A new study indicates that the distinctive mammal's genetic code is an eclectic brew of bird, reptile and mammal.
This mix-and-match animal is more than just an oddity, though. Researchers report in Nature that its genome provides important clues into how mammals, birds and reptiles evolved from a common ancestor some 315 million years ago. And researchers at Stanford University School of Medicine report in Genome Research that they linked the evolution of a gene in the old platypus to a mutated version in humans responsible for moving the testes outside of the body and into an external pouch, or scrotum.
"As you look at the [platypus] genome, effectively what you've got is a patchwork: places that look a bit more avian, places that look a bit reptilian, and places that look a bit mammalian," says Mark Batzer, a geneticist at Louisiana State University in Baton Rouge and co-author of the Nature study. "Looking at different genomes and seeing where various processes were created gives you some insight into how things work and why they were generated."
Batzer was part of an international team of researchers that sequenced and analyzed the genome of a wild female platypus named Glennie, which lives in southeastern Australia. Among its findings: The platypus' genome is two thirds the size of the human genome and contains 18,500 genes. (The human genome comprises 20,000 to 25,000 genes.) Eighty-two percent of the animal's genes are found in other mammals. The genome is organized into 52 chromosomes (tightly packed structures of DNA stored in a cell's nucleus), 10 of which determine the animal's sex. (In humans, there are 46 chromosomes—23 pairs—and only two (X and Y) are sex-determining.)
The Stanford group compared the platypus genome with those of fish, frogs, birds, marsupials and placental mammals. Their goal: to track the evolution of a family of genes known as relaxins. Relaxins are behind the development of features unique to most mammals—placentas (organs that allow nutrients to pass from pregnant females to their developing fetuses), mammary glands for milk production, nipples and external testes.
The only type of mammal that does not have these traits belongs to the classification monotreme, an order that includes the platypus and the echidna (or spiny anteater) and which split off from other mammals 170 million years ago. Monotremes have a single orifice (called a cloaca) for urinating, defecating and laying eggs. Like other mammals, the platypus secretes milk through its skin to feed offspring and is warm-blooded—though its body temperature is nine degrees Fahrenheit (five degrees Celsius) cooler than that of a human. Like reptiles, it lays eggs, has no nipples and houses its testicles inside its body near its kidneys.
"Testicular descent is a very specialized process that required the evolution of specific genes," says Sheau Yu Teddy Hsu, a Stanford assistant professor of obstetrics and gynecology and co-author of the study in Genome Research. "The platypus serves as a 'bridge' animal between nonmammals like birds and reptiles, which maintain their testicles in a body cavity, and placental and marsupial mammals, which hold their testes in an external scrotum."
External testes allow for animals to have a higher body temperature without jeopardizing heat-sensitive sperm cells, Hsu says. (An increased body temperature allows for faster energy production, leading to greater agility and faster reflexes—all of which are reproductively advantageous.)
Scientists already know that a gene called Insl3 is important for testicular descent in placental mammals and marsupials. During their study, Hsu and his colleagues determined that Insl3 and a second relaxin, Rln3, whose function is unknown, evolved from an ancestral gene found in fish.
Hsu says that fish have one copy of the ancestral gene; the frog evolved to have has two copies of the same gene. "In order for something to evolve, duplication of a gene is the most important event," Hsu says. "We found that in all [four-legged animals], there is a duplication of the ancestral gene." One copy of this gene went on to become Rln3; the other developed into Insl3 sometime after the monotremes split off from the rest of the mammals.
Hsu says that learning how Insl3 adapted may eventually offer a way to reverse the fortunes of the 30 percent of boys born prematurely whose testes fail to properly descend. The Stanford team is now looking into the evolution of Rln2, the gene believed to be linked to mammary gland and nipple formation.