A member of the class of mammals known as monotremes, the platypus lays eggs yet produces milk; is furry but has a malleable duck bill; produces venom, but not quite like snakes’ and has electroreceptors similar to some types of fish. It’s these genetic contradictions of the platypus, possessing both reptilian and mammalian features, that has fascinated scientists for over a century. In fact, the initial discovery of the platypus was thought to be a hoax. Today, we clearly know the platypus is classified as a mammal, but the mystery of its evolution remains.
Since the platypus genome represents the basal (primitive) mammalian lineage, it is expected to be a major contributor to studies of evolutionary divergence, chromosome evolution and human genome annotation. For these reasons, an international consortium was formed to analyze the platypus genome. This consortium consisted of over 100 researchers from the United States, United Kingdom, Germany, Israel, Japan, Spain, New Zealand and Australia. The platypus genome, with its unique chromosome structure, 10 sex chromosomes (unique to monotremes), was sequenced and assembled at The Genome Institute at Washington University. The findings were published in May 2008 and were funded by the National Human Genome Research Institute (NHGRI), one of the National Institutes of Health (NIH).
The paper, which appeared in Nature, analyzed the genome sequence of a female platypus named Glennie from New South Wales, Australia. Geneticist Dr. Frank Grützner of the University of Adelaide in Australia is one of the collaborators on the Platypus project.
Q: What’s unique about the platypus (besides the obvious)?
Dr. Frank Grützner: Almost everything! I think people often focus on the reptilian and mammalian characters and regard the platypus as an ancient creature. It is worth highlighting that they are a semiaquatic mammal that has evolved a number of highly specialized features that distinguish them from both reptiles and other mammals. One feature in particular is the electroreceptors, which were discovered in the 1980s. These neurons are not related to electric organs in fish, but they allow the platypus to detect its pray in the muddy water (with its eyes closed). Another feature is a massive toe web at their feet that makes the platypus very fast underwater, and in the burrow they can fold the toe web back, so it does not get in the way when they dig. Their body design, fur and tail are highly adapted to the aquatic lifestyle. Still, they can move quite quickly on the ground. In Tasmania, they are often spotted in small ponds and even on roads, which shows that they can wander around to find new food sources.
(Another interesting fact that) most people would not know is that monotremes have a placenta despite the fact that they lay eggs!
Q: Why was it important to geneticists to analyze the platypus genome?
To me, two things are important. First, as our most distant relative amongst living mammals, they are our best guess at what happened in early mammalian evolution. The work we have done with Hitoshi Niwa (researcher with RIKEN CDB) on the evolutionary and functional analysis of the Oct4 (the important stem cell renewal gene) can illustrate that. By showing that the platypus Oct4 gene can encourage stem cell renewal in mouse cells lacking Oct4, we found that birth and functional diversification dates back to early mammalian evolution. Second, we can learn about the platypus’ own biology, which is also important for conservation biology. For example, the massive loss of pepsin genes in the genome nicely mirrors the fact that they have a small stomach.
Q: Were you surprised by the results of your findings?
As monotreme researchers, we are used to expecting the unexpected. The total lack of sex chromosome homology to other mammals was a big surprise. Now, after the genome project, we know that the homology to bird sex chromosomes is quite substantial and not only restricted to one of the sex chromosomes. The platypus sex chromosomes have no homology to the sex chromosomes of other mammals at all! These findings truly change how we think about sex chromosomes in birds and mammals. Birds and mammals shared sex chromosomes; marsupials and placental mammals then evolved a new sex chromosome system, after the divergence of monotremes more than 160 Million years ago.
Q: What sort of practical applications resulted from the platypus study?
In my area, we currently focus on understanding how monotremes determine sex without the primary sex determination locus SRY (the sex-determining gene on the Y chromosome in placental mammals and marsupials). This will give us important insights into how sex determination mechanisms evolved in all mammals. In a more general sense, the genome project provides us with a fantastic resource for future research. We went from 30 genes identified in 30 years to over 18,500 genes identified over the four years of the platypus genome project. There is also a surge in research activity on monotremes and we have captured this momentum at a recent conference on platypus genomics and its future.
Q: What does the future hold for the platypus genome?
I think we are only beginning to explore the potential of these genomes for understanding these most amazing creatures and mammal genome evolution. Hopefully, we will soon unlock the genome of the close relative of the platypus, the short-beaked echidna. These two close relatives are amazingly different. The echidna genome will provide insights into how they can be so different with a divergence time being similar to that of mouse and rat and echidnas feature fascinating biology in particular in terms of reproduction, thermoregulation and longevity.