A Query Hidden within the Platypus Genome: Are We the Bizarre Ones?

When British zoologist George Shaw first came across a platypus in 1799, he was so confused that he looked for stings and thought someone might try to outsmart him with a Frankish creature. It’s hard to blame him: what other animal has a rubbery bill, ankle tips full of venom, luxurious fur that glows under black light, and a tendency to lay eggs?

Centuries later, we are still trying to pull the platypus apart, now with more subtle tools. What we find can make us ask: is the platypus normal and are we what has been found to be strange?

On Wednesday in Nature, the researchers presented the most complete platypus genome to date, along with a closely related genome, the short-beaked echidna. By delving into their DNA, researchers can uncover the genes and proteins that underlie some of the distinctive traits of these creatures and better understand how mammals like us evolved to be so dissimilar to them.

The platypus and four species of Echidna, all native to Australia, are the only living monotremes in the world – a group perhaps best known for their unique reproductive strategy, which involves laying eggs and caring for their young after hatching.

“They’re very bizarre in so many ways,” said Guojie Zhang, genomicist at the University of Copenhagen and head of sequencing efforts.

But because the Monotreme differed from other mammals so early – about 187 million years ago – they are also “very important in understanding mammalian evolution,” he said. Indeed, some monotreme traits that seem so strange to us might actually be present in the ancestor we all share.

The platypus genome was first sequenced in 2008. Since then, technological advances have made it much easier to map certain genes onto chromosomes. In the earlier experiment, only about 25 percent of the platypus genome was so contextualized, said Dr. Zhang, while the new version is 96 percent mapped.

“It’s very complete,” he said. “We find a lot of genes that were overlooked in previous congregations.”

The new genomes confirm much of earlier findings about the platypus and, when combined with the new Echidna genome, provide greater clarity on the evolutionary mechanisms involved, said Wesley Warren, a professor of genomics at the University of Missouri who led the sequencing study in 2008, but was not involved in this.

“In my opinion, the mammalian platypus is the most fascinating species of all,” he added. “They represent the ancestral state of what mammalian terrestrial genomes might have been before they adapted to different environments.”

Such a comprehensive map enables comparisons between the genomes of different species and helps fill in gaps in the step-by-step history of how mammals appeared and then diverged. For example, many birds and insects have multiple copies of a gene called vitellogenin, which is involved in the production of egg yolks.

Most mammals don’t have a vitellogenin gene, said Dr. Zhang. However, the new genomes show that platypus and echidnas have a copy of what helps explain their abnormal oviposition – and suggests that this gene (and possibly the reproductive strategy itself) was something the rest of us lost, and not an innovation of the Monotreme. They now also have milk-producing genes similar to ours and other mammals, so they can feed their young.

Other properties went other ways. The new genome shows that toothless monotremes have lost several genes associated with tooth development that are present in other mammals. Platypuses also have venom-producing genes that other mammals lack, but that are similar to those of some reptiles, possibly explaining their toxic toe tips.

Less visible, but equally confusing, is the fact that other mammals generally have one pair of sex chromosomes, while Monotreme have five pairs. The structure of the newly uncovered genomes suggests that these sex chromosomes were once in a ring formation and then broke into pieces – although more research is needed to find out how this happened.

Dr. Zhang and his colleagues plan to further investigate the many remaining monotonous secrets. “You are a very important line to understand,” he said.

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