Platypus Genome Published...and Analyzed

Animal's reptilian-mammalian mix reflected in its DNA

The first analysis of the genome sequence of the duck-billed platypus was published today by an international team of scientists, revealing clues about how genomes were organized during the early evolution of mammals. The research was supported in part by the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health (NIH).

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n a paper published in today’s issue of the journal Nature, researchers analyzed a high-quality draft genome sequence of Glennie, a female platypus from Australia. The consortium included scientists from the United States, Australia, England, Germany, Israel, Japan, New Zealand and Spain. Sequencing of the platypus genome was led by the Genome Sequencing Center at Washington University School of Medicine in St. Louis, a part of NHGRI’s Large-Scale Sequencing Research Network.

Once the sequence was produced, researchers began comparing the genome of the platypus, whose ancestors split from the rest of mammalian lineage some 166 million years ago, with the well-characterized genomes of the human, mouse, dog, opossum and chicken, as well as the draft genome sequence of the green anole lizard. The chicken genome was chosen because it is descended from the ancestral group of egg-laying animals, including extinct reptiles, which passed on much of their DNA to animals like the platypus. Scientists were particularly interested in finding features within the platypus genome that could explain the odd mix of characteristics seen in the platypus, those that were more like reptile and birds and which were like mammals.

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.”

Alright, first argh: mix of reptilian, mammalian and unique...that hurt to read. As we all now know, mammals did not evolve from reptiles. They both came from basal amniotes. That said there are some interesting bits of information that have come out.

First, platypus venom is genetically encoded in the same way as in diapsids: through gene duplication of the same genes. This seems to be a case of parallel evolution - so say the paper authors - rather than conserved basal genes.

Second, its RNA evolution is...interesting. There seems to be I am wholly unqualified to comment though.

Third, the sex detemination genes for the platypi have de nada to do with XY chromosomes. That's a therian...thing. Instead, their sex chromosomes share extensive homology to those of birds.” Waters notes that this finding suggests our ancient mammal-like reptile ancestor may have had bird-like sex chromosomes and sex determination system.

Then...well...nuts. Go read it yourself.

Here's a link with all of the above.

This has a lot of exciting potential for uncovering some of the basal traits for the therapsids and, indeed, the ancestral amniotes. Based on this and previous studies it seems highly likely that the therapsids were egg layers and had a sex determination system that was inherited from the amniote ancestors. I am stoked that they have looked at least a bit at the echidna genome and I hope that the same sort of analysis will be conducted there too relatively soon.