Duck-billed platypus genome sequence published

The team found that the platypus genome contains about the same number of protein-coding genes as other mammals -- approximately 18,500. The platypus also shares more than 80 percent of its genes with other mammals whose genomes have been sequenced. Next, researchers combed the platypus genome looking for genetic evidence of sequences unique to platypuses, which have been lost from mammalian genomes. Scientists were also eager to find out what characteristics of the platypus were linked at the DNA level to reptiles or mammals.

“The mix of reptilian, mammalian and unique characteristics of the platypus genome provides many clues to the function and evolution of all mammalian genomes,” said Richard K. Wilson, Ph.D., director of Washington University School of Medicine’s Genome Sequencing Center and the paper’s senior author. “Now, we’ll be able to pinpoint genes that have been conserved throughout evolution, as well as those that have been lost or gained.”

The female platypus lays eggs, a reptilian characteristic, yet also produces milk to nourish its young, which is a mammalian characteristic. Interestingly, the platypus genome harbors both reptilian and mammalian genes associated with fertilization of eggs. However, researchers discovered that, like other mammals, the platypus genome contains a tightly clustered set of genes that produce the casein proteins that make up milk.

A good sense of smell is something mammals have in common. Compared to most mammals, chicken and lizard have a relatively poor complement of odorant receptor genes responsible for their ability to detect smells. Interestingly, platypus has about half as many odor receptors as do the mouse and other mammals. However, the researchers also found that the platypus genome has experienced an expansion in the genes that code for a particular type of odor receptor, called a vomeronasal receptor. Researchers think the expansion of this particular set of genes may be involved in the ability of the platypus to detect odors during underwater foraging.

Researchers also found that genes related to the immune system of the platypus are very similar to those of other mammals, with a few key differences. For example, the platypus genome contains expansions in the gene family that codes for a microbe-fighting peptide called cathelicidin. Primates and rodents have only a single copy of the cathelicidin gene. Scientists think that the expansion of this gene family in platypus may be involved in boosting the immune system of their very immature offspring, which hatch very developmentally immature. Scientists postulate that the microbe-fighting peptide may have been less critical for other mammals, whose offspring spend relatively long periods of time developing within the womb and are born at a more advanced stage of development.