Sequenced Platypus genome reveils unexpected evolution…

ResearchBlogging Medium White Sequenced Platypus genome reveils unexpected evolution...
What makes mammals so special and so different from other animals? To understand a question like this from the evolutionary point of view, it is a must nowadays to sequence the complete genome of a corresponding species. This allows for full comparative studies, which can reveal unexpected insights. In this case, an intermediate species has been selected to represent the beginning of mammal traits. Although the Platypus (Ornithorhynchus anatinus) is a true mammal, wich means, that it posseses fur, feeds milk and secretes sweat, this duck-billed animal also exhibits reptilian features, like venomous spines at the hind limbs and the laying of fertilized egg, while the hatched offspring later gets nursed. The bill only superficially looks like that of a duck, while on its inside it harbours a complex electrosensory system that enables the animal to forage underwater. Sounds like an unbelievable list of attributes? Well, it is. In my opinion, this is one of the most amazing creatures on our planet. The sum of genes, or genome, which were published in Nature reflects those oddyties. [PubMed]

 Sequenced Platypus genome reveils unexpected evolution...
image: DenDenis

It is the first fully sequenced genome of a “venomous” mammal, maybe it will be the only one? This study revealed, that the venom-genes are similar to the genes used for venom production in reptiles, while another number of genes was lost during evolution.
Another highlight is the fact, that irrespective of “laying” eggs, they are relatively poor in yolk, and this fact is reflected by the loss of components of the yolk production machinery, in comparison to egg-laying ancestors. The examined genome offers an explanation on the development of milk production. Usually, new genes are established by duplication of DNA-sections, allowing the second new gene to mediate a new function. Genes for milk production for example seem to have evolved from tooth enamel genes, due to similarities and location. This is also the fact in Platypus, which suggests that arround the origin-time of the Platypus, more than 160 million years ago, these genes were duplicated and led to the production of milk.
In a second article, Wes Warren desribes the extraordinary sex chromosome complex of these animals. [PubMed]
There are five X and five Y chromosomes which pair in a chain of alternating X and Y chromosomes. In comparison, other mamals usually bear one XY- or XX combination. Additionally, a comparative mapping showed that, in contrast to earlier reports, there is no homology between the Platypus and therian (Marsupials and Placentals, to which humans belong) X chromosomes. Evenmore, platypus sex chromosomes have much more homology with bird sex chromosomes. This implies, that the therian sex-chromosomes evolved from an independent source. This results in the hypothesis, that the therian sex-chromosomes are more than 145 million years younger than previously thought.

Warren, W.C., et al.(2008). Genome analysis of the platypus reveals unique signatures of evolution. Nature, 453(7192), 175-183. DOI: 10.1038/nature06936

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