Poisonous platypuses confirm convergent evolution
Genome analysis shows that the monotremes and snakes have similar venoms.
Don't be fooled by the playful-looking duck's bill — platypuses deliver a venom containing more than 80 different toxins.
The finding, from an analysis of the genes encoding the dangerous mixture, also reveals the striking similarities between the poisons of different animals. The genes resemble those of other venomous animals, such as snakes, lizards, starfish and sea anemones.
Like eyes, fins and wings, which have evolved independently in a number of different lineages, platypus venom looks to be an example of convergent evolution, says Wesley Warren, a genomicist at Washington University in St Louis, Missouri, who led the study, published in the journal Genome Biology.1
The platypus — a semi-aquatic egg-laying mammal found in Australia — is one of few mammals to make venom, which males produce in abdominal venom glands and deliver through spurs on their hind legs. They only make the poison during breeding season, and Warren thinks that males probably deploy it to defend their turf against other males.
By some accounts, being poisoned by a platypus could qualify as punishment in one of Dante's circles of hell. In one case report2, Australian doctors described their treatment of a 57-year-old man a few hours after he grabbed one of the small marsupials while fishing. The pain was "so bad I started to become incoherent" the man said, and far worse than the shrapnel wounds he took as a soldier. Ibuprofen and morphine provided no relief, and one finger was swollen and ached more than 4 months after the run-in.
Efforts to find the molecules capable of inflicting such anguish have focused on separating and characterizing proteins in venom extracts. This approach identified three of the most abundant ingredients of platypus venom, but Warren's team suspected that more molecules were present at lower levels.
What's your poison?
His team sequenced messenger RNA transcripts from the venom gland of a male platypus, killed by a dog in breeding season. To identify venom ingredients they looked for genes that were not produced in other tissues and which resembled venom genes from other animals. This scan turned up 83 genes in 13 different families of toxins, linked to effects including inflammation, nerve damage, muscle contraction and blood coagulation. For instance, platypuses make 26 different kinds of serine protease enzymes, which are also found in the venom of most snakes, and seven of their venom genes resemble a neurotoxin produced by spiders called a-latrotoxin.
Additional tests will be needed to determine what each venom ingredient does, says Warren. He also thinks that his team's study undercounted the number of toxin-encoding genes in the platypus 'venome', because the method used overlooks genes that bear little resemblance to other animal toxins. To find these, his team plans to look for genes switched on during the seasonal development of the platypus venom gland.
Nonetheless, the platypus venome supports work in other animals showing widespread convergence in venom gene evolution. Warren says that this probably happens when genes that perform normal chores, such as blood coagulation, become duplicated independently in different lineages, where they evolve the capacity to carry out other jobs.
Animals end up using the same genes as building blocks for venom because only a subset of the proteins the genes encode have the structural and functional properties to become venoms, he adds.
Despite such convergence, closely related animals tend to produce similar venoms, says Bryan Fry, head of the venomics laboratory at the University of Melbourne, Australia.
An evolutionary outlier such as the platypus is likely to produce a venom with new components, says Fry. "If you want to find something potentially useful in drug design and development from a venom, you're much likelier to find it in a novel venom such as platypus venom than if you are looking at, say, rattlesnakes."