The weird and wonderful platypus genome

Modern comparative biology has truly entered a new age. The list of species for which researchers have completely sequenced their genomes continues to rapidly grow. Fruit flies (Drosophila melanogaster), chickens (Gallus gallus), sea urchin (Strongylocentrotus purpuratus), pufferfish (Fugu rubripes), Short-tailed Opossum (Monodelphis domestica), mosquitos (Anopheles gambiae), Rhesus Macaque (Macaca mulatta), several plants such as rice (Oryza sativa) and cottonwood (Populus trichocarpa), numerous microbes, and even Humans (Homo sapiens) all have complete genome sequences completed.

Add to that list the Duck-billed Platypus (Ornithorhynchus anatinus). A research team has just completed the first sequencing of the platypus genome. The platypus is truly among the strangest of mammals. Found exclusively in Australia and Tasmania, they have hair and produce milk as do the rest of their mammalian kin but they also lay eggs and have a brain much like a reptile. Male platypus also sport a spur on their hind feet that can deliver a venomous sting. Because of this odd mix of reptilian and mammalian characters the first platypus specimens brought back by the early explorers of the Australian continent were thought to be a hoax, patched together from bits and pieces of other animals. Cincinnati Museum Center's Zoology Collection has an old platypus specimen in it's holdings (see photo left).

Like it's reproductive behavior, physiology and morphology the genome of the platypus reveals it's key evolutionary position at the base of the mammal family tree. For example, mammalian ova have an outer membrane called the zona pellucida which aids in fertilization. Of the proteins make up the zona pellucida in mammals four found in the platypus match those found in the human genome, however, the platypus genome has two additional ova membrane proteins previously found only in birds. Additionally, the platypus genome contains genes for the yolk protein vitellogenin, a protein found in the eggs of birds but neither marsupials or placental mammals.

The genes underlying the venom found in the spurs of male platypus also tell an interesting evolutionary story. Platypus venom, like many venoms found in reptiles, is a complex mix of different proteins. Platypus venom contains 19 different compounds. The venom proteins in platypus venoms appear to have arisen through duplications of genes. Gene duplication is a common evolutionary process that can give rise to new characteristics. When a gene is duplicated the new duplicate is free to accumulate new mutations and take on new functions while the original gene retains it's original function. Not only has gene duplication played a role in the evolution of platypus venom but the same process likely led to the evolution of venoms in reptiles. Also, the venom proteins in the platypus arose from the same gene families as in venomous reptiles providing an interesting case of convergent evolution (evolution of similar traits arising independently in different lineages).

The complete sequence of the platypus genome follows previous work on the sex-determination chromosomes in the platypus (Grutzner et al. 2004. Nature 432: 913-917). For mammals, sex is determined by two sex chromosomes, X and Y. Females have two X chromosomes and males have one X chromosome and one Y. But, platypus have ten sex chromosomes! These ten chromosomes are arranged in a chain such that females are have five pairs of X chromosomes and males have five XY pairs. In birds the sex determination system is different. The sex chromosomes in birds are called W and Z and rather than males being the sex with two different sex chromosomes (called the heterogametic sex) the females are the ones with different sex chromosomes (female birds are WZ and male birds are ZZ). Interestingly, like much of the rest of the platypus genome the sex chromosomes belie their position in the mammalian tree. At one end of the chain of X-chromosomes in the platypus genome is an X chromosome with sequence similarity to the avian Z chromosome. This suggests evolutionary links between the sex chromosomes of birds and mammals and thus a common evolutionary history for these two different groups of animals.

Surely further investigation of the platypus genome will reveal more insights not only into platypus evolution but the evolution of the whole mammalian family tree, including us. As more and more organisms are sequenced we will gain more insight into evolutionary history and processes.

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