Science Literature

Ever since its discovery, the platypus has been known as "one of nature's oddest creatures, seemingly assembled from the spare parts of other animals". It was hoped that the genome sequence would cast light on its evolutionary history, but now that it has been published (in May 2008), the picture is no clearer. The mosaic of reptilian, avian and mammalian features apparent at the macro-level is replicated in its genetic code.

"There is nothing quite as enigmatic as a platypus," says Richard Gibbs, who directs the Human Genome Sequencing Center at Baylor College of Medicine in Houston, Texas. "You have got these reptilian repeat patterns and these more recently evolved milk genes and independent evolution of the venom. It all points to how idiosyncratic evolution is."

This graphic in National Geographic "illustrates the traits that the platypus shares with reptiles, birds, and mammals"

"Idiosyncratic" means 'peculiar to the individual', 'eccentric'. Whilst this is a fair summary of the quirky, unpredictable way evolutionary transformation is supposed to occur, the principles underlying the analysis of genomes is that the past course of evolution can be tracked. Cladistic techniques are supposed to offer the possibility of redrawing the Tree of Life. In this case, the real problem is not "idiosyncratic evolution" but the discovery of an idiosyncratic platypus genome!

There are several highlighted genetic features in the Nature News report.

• The platypus has "genes for the family of milk proteins called caseins, which map together in a cluster that matches that of humans."
• "The platypus shares with other mammals four genes associated with the zona pellucida, a gel-like coating that facilitates fertilization of the egg."
• It "has two matches for ZPAX genes that had previously been found only in birds, amphibians and fish. And it shares with the chicken a gene for a type of egg-yolk protein called a vitellogenin."
• "Male platypuses have spurs on their hind legs that are loaded with a venom so potent it can kill a dog. Like the venom of reptiles, the poison is a cocktail of variations on at least three kinds of peptide."
• "[N]one of the platypus X chromosomes resembles the human, dog or mouse X. [. . .] Instead, the platypus Xs better match the avian Z sex chromosome."
• "microRNAs, which regulate gene expression, [revealed] a mix of reptile and mammal examples. "We have microRNAs that are shared with chickens and not mammals as well as ones that are shared with mammals, but not chickens," Hannon says."

Despite the claims that the new data provides more "evidence for its place in the ancestral line of animal evolution", the evolutionary story is distinctly fuzzy. Identifying a reasonable lineage was deemed impossible in the case of the reptilian venom: this is regarded by the researchers as having an independent origin. "The similarity in venom is an example of convergent evolution between the two tetrapods."

These traits, and others, are considered in an article on the Truth in Science website here. The various evidences are weighed and it is concluded that evolutionary theory does not deliver a coherent story. Indeed, it is far worse than this: the platypus genome is truly idiosyncratic! The essay concludes:

"Thus the platypus will remain a significant misfit in any Darwinian scheme. Is it from a sauropsid lineage which includes reptiles and birds? Is it from a synapsid lineage which supposedly led to the emergence of the mammals? Or is it derived independently from some unknown ancestral amniote? Or could it be that the Darwinian hypothesis, cladistic analysis or any other classification system for that matter is just far too restrictive? Without doubt, there are mammal-like reptiles as there are reptile-like mammals. The platypus is a Darwinian cautionary tale. Is it a bird or is it a plain . . . old platypus?"

Genome analysis of the platypus reveals unique signatures of evolution
Wesley C. Warren, et al.
Nature 453, 175-183 (8 May 2008) | doi:10.1038/nature06936

Abstract: We present a draft genome sequence of the platypus, Ornithorhynchus anatinus. This monotreme exhibits a fascinating combination of reptilian and mammalian characters. For example, platypuses have a coat of fur adapted to an aquatic lifestyle; platypus females lactate, yet lay eggs; and males are equipped with venom similar to that of reptiles. Analysis of the first monotreme genome aligned these features with genetic innovations. We find that reptile and platypus venom proteins have been co-opted independently from the same gene families; milk protein genes are conserved despite platypuses laying eggs; and immune gene family expansions are directly related to platypus biology. Expansions of protein, non-protein-coding RNA and microRNA families, as well as repeat elements, are identified. Sequencing of this genome now provides a valuable resource for deep mammalian comparative analyses, as well as for monotreme biology and conservation.

See also:

Brown, S. Top billing for platypus at end of evolution tree, Nature, 453, 7 May 2008, 138-9 | doi:10.1038/453138a

A draft sequence of the platypus genome reveals reptilian and mammalian elements and provides more evidence for its place in the ancestral line of animal evolution. The platypus (Ornithorhynchus anatinus) is endemic to Australia and one of nature's oddest creatures, seemingly assembled from the spare parts of other animals. The semi-aquatic monotreme is a venomous, duck-billed mammal that lays eggs, nurses its young and occupies a lonely twig at the end of a sparse branch of the vertebrate evolutionary tree. Now, the structure of its genome has revealed new clues to how mammals evolved. [snip]

Platypus: a Darwinian Cautionary Tale (Truth in Science, 7 July 2008)