Genes Down Under
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By any standard, platypuses are weird, with a seemingly random collection of features that look like they pressed the evolutionary shuffle button a little too enthusiastically. Among their bizarre characteristics are a bill like a duck, a tail like a beaver, and no stomach or teeth. They have webbed feet with electroreceptors that help them to track prey, decorated with venomous spurs and attached to limbs that stick out from their sides like reptiles. And although they’re warm-blooded and technically classed as mammals, they lay eggs rather than giving birth to live young, and feed their babies by sweating milk out through their skin (yuck).
Platypuses and their closest relatives - four species of echidna - are all found in Australia and are the only living examples of monotremes - mammals that lay eggs. Platypuses are often considered a mix of mammalian and reptile because of their features but, in truth, they are the least understood mammal. Learning more about the platypus and echidna could help uncover evolutionary clues about things like how life evolved from egg laying reptiles to birthing mammals.
In search of some genetic explanations for the platypus’ unique characteristics, back in January 2021 scientists published the most complete genome sequence of the platypus to date. Although the platypus genome was first sequenced in 2008, scientists had only properly mapped about 25% of the genes in there, with the rest remaining a mystery. Researchers from Universities in Australia, China, Japan, USA and Denmark have now mapped 96% of the platypus genome and have already started uncovering clues that could help us understand more about the evolution of mammals.
For example, most birds and insects have multiple copies of a gene called vitellogenin, which is involved in producing egg yolk. Most mammals don’t have this gene, but platypuses and their cousins do have one copy of the gene, allowing them to continue to lay eggs. So, presumably, some ancient ancestral mammals lost their vitellogenin genes and with it, our egg-laying talents, while the ancient ancestor of the monotremes held on to it. However, Platypuses have mammal-like genes for making milk, suggesting that the trait is derived from a common ancestor that was around before monotremes and live-birthing mammals diverged.
Still, there are plenty of mysteries about the platypus left to unravel, like the fact that their sex chromosomes seem to have more in common with birds than regular mammals. Platypuses have five pairs of sex chromosomes compared to our one pair. Based on the latest genome mapping, the researchers think they these sex chromosomes were once in a ring formation and then broke up into pieces. It seems pretty bizarre, but then that’s only to be expected with such a fascinating and unique creature.
References:
Genomes reveal insights into much-loved Aussie animals, Eurekalert
Platypus and echidna genomes reveal mammalian biology and evolution, Nature
A Question Hidden in the Platypus Genome: Are We the Weird Ones?, The New York Times
Researchers Sequence Platypus and Echidna Genomes, Sci News
Mapping the platypus genome: How Earth's oddest mammal got to be so bizarre, ScienceDaily
