Wednesday, May 07, 2008

Weird animal, not-so-weird genome

(After reading PZ Myers's post on the matter, I've decided to stop using "weird" to describe the platypus's genome. However, I still think the animal itself can aptly be described as weird.)

Nature reports today that the platypus genome has been sequenced by a large international team (note: subscription needed for full article). Fittingly, it seems that the genome is just as delightfully weird interesting as the platypus itself - a mixture of mammal genes interspersed with ancestral reptilian and avian sequences, with a bit of novelty thrown in, all of which tells us a great deal about mammalian evolutionary history.

Some highlights:

  • The sequenced genome, with all of its peculiarities details, corroborates the idea that the platypus lineage is the earliest diverging mammalian lineage - diverging from the rest of the mammals over 160 million years ago. Side note: the marsupial-placental split occurred ~150 mya, and all modern placentals orders appeared by ~75 mya.

  • The genes for caseins, the proteins found in milk, align well with the casein gene families found in other mammals. This suggests, as expected, that milk production evolved before mammals evolved live birthing (or nipples - platypus secrete milk through pores in the skin).

  • Speaking of birthing, the platypus genome contains sequences that match mammalian genes for the zona pellucida, but also has copies of zona pellucida genes previously only found in amphibians, birds and fish. There is also a gene for the yolk-stored proteins, vitellogenins, that are also found in amphibians, reptiles, and birds, but that have been lost in non-monotreme mammals.

  • Genes for the proteins found in the platypus' reptilian-like toxins turn out to be novel sequences, independently evolved and are not homologous with reptile toxin genes. Toxin production in reptiles and platypus seems to be a case of convergent evolution. Interestingly though, the same defensin genes were co-opted in each case.

  • (Note: I was going to strike the word weird from this item, but I've decided that this is indeed weird.) Weirdest of all might be the sex chromosomes. Platypus sex chromosomes are known to be strange structures cytologically speaking, but it turns out that none of their X chromosomes match the X chromosomes of other mammals - they're more like the Z chromosome found in birds. A "normal" autosomal chromosome however matches the mouse X chromosome.

  • microRNA sequences are all over the place - platypus have both microRNA sequences that are found in birds, but not mammals, as well as microRNA sequences that are found in mammals, but not birds.

    Further study of the genome will certainly add to the uniqueness of the platypus and will be helpful in understanding early mammal evolution. I think it's rather poetic that such an odd cool creature has such an odd cool genome. Wouldn't it have been disappointing if it didn't?

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