Life Lines 43

THE PLATYPUS GENOME IS FULL OF SURPRISES: A MAMMAL WITH REPTILIAN AND BIRD GENES



About a year ago I wrote a Life Lines column about some fossil monotremes found in Australia. The monotremes are the oldest branch of mammals. They lay eggs rather than gestating an embryo in a uterus. They also nurse their newborn with milk but lack breasts with nipples. The marsupials (which have pouches into which their embryos crawl and attach to a nipple) and the placentals (like us) are the other two branches of mammals. The fossil monotremes had more bird-like and reptilian skeletal features than the platypus, the best-known living monotreme. For those unfamiliar with the platypus, it has a beak like a duck, hair instead of feathers covering its body, a tail like a beaver’s, and in platypus males, spurs on its lower legs that can inject venom.

An international group of scientists worked out the sequence of the platypus DNA. The platypus (Ornithorhynchus anatinus) has 1.84 billion base pairs in its DNA (we have about 3 billion). The platypus genome encodes 18,527 protein-coding genes (we encode about 23,000 protein forming genes). Some 82% of platypus genes are shared with other mammals. Its genes for producing milk are similar to those of humans and other placental mammals. It has some genes involved in making its eggs that are similar to those found in birds and reptiles. Those genes are absent in humans and other placental animals. Many genes found in birds, reptiles, and fish are found in the platypus but they are missing in more familiar mammals like cats, dogs, mice, cattle, or humans. They have special genes for smelling chemicals under water but they lack the genes for smelling in air when they emerge to breathe or make rare appearances on land. Most of the time the platypus lives in rivers, especially in or along its muddy banks.

The monotreme line split off from a mammal-like reptilian line to form an early branch of mammals some 166 million years ago. The remaining branch split again 148 million years ago to form the marsupials and the placental mammals. Note that all three branches of the mammals existed some 80 million years before the dinosaurs went extinct. The dinosaurs died out about 65 million years ago. Monotremes are found in Australia and neighboring South Pacific islands. Marsupials are largely confined to Australia, New Zealand, and Tasmania with the exception of the opossum in North America that we occasionally see flattened out as road kill. Placental mammals are now world wide and proved very versatile in hitching rides on debris and spreading from continent to continent.

Isolating the platypus genome and comparing it to fish, amphibians, reptiles, and other mammals will reveal what portion of the 18% unshared genes (those not found in mammals) are unique to the monotremes and what portion belong to earlier ancestors, especially among the reptilian denizens of our ancient past. These coming decades will be rich in phylogenetic trees constructed from comparative genomics. They will show far more step-by-step changes in the evolution of life than the scant surviving bones, teeth, shells, and scattered remains left behind tens of millions of years ago.