Odd Evolution

New research reveals the wild evolutionary origins of the platypus

The platypus may hold the key to understanding mammalian evolution.

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Most humans probably think very little of the platypus — if at all.

But this strange Australian mammal, with its webbed feet and beaver-like tail, is actually a treasure trove of genetic information.

Platypus (Ornithorhynchus anatinus) are part of a unique mammalian group known as monotremes. This group also includes echidnas (think Knuckles from Sonic the Hedgehog but real). They are the only group of mammals to lay eggs. As such, monotremes are a prime candidate for the study of mammalian evolution.

Research published Wednesday in the journal Nature provides a complete overview for the first time of the platypus' genome, revealing new information about the evolution of this odd group of mammals — and insight into the origins of human DNA.

An echidna walking in a coastal wilderness area in Victoria, Australia. Echidna, along with platypus, compose a unique group of mammals known as monotremes.Getty

Necessary background — The three main groups of mammals are: monotremes, eutherians, and marsupials.

The largest group by far are eutherians, which include animals that nurture their young in the mother's placenta (accordingly, the infraclass is sometimes referred to as "placentals"). Humans fit into this category.

Marsupials and eutherians are often lumped together into a subclass known as therian mammals, but monotremes are so distinct from other mammals that they deserve their own separate category.

Monotremes include only two animals that survive today: the platypus and the echidna.

Platypus are semi-aquatic creatures and echidna are land dwellers. Both share in common one habit unique to monotremes: egg-laying.

What was discovered — For the first time, the new research provides a complete chromosome map of the platypus genome, along with a less-complete map of the echidna genome.

The scientists used genetic data to analyze everything from the platypus' eating habits to its swimming routine. For example, the scientists found that the water-based platypus possesses far fewer "olfactory" or smelling genes compared to the terrestrial echidna.

This genetic data confirms the semi-aquatic lifestyle of the platypus, which closes its nasal cavity and eyes when in the water, relying instead on other senses — like electric stimuli — to detect prey.

But given the monotreme's unique reproductive strategy, the researchers focused specifically on its unique sex chromosomes. The monotreme has 10 sex chromosomes: five X chromosomes and five Y chromosomes. Humans, meanwhile, have two sex chromosomes.

Through their analysis, researchers found similarities between platypus and birds, but not so much between platypus and humans.

A platypus in Australia. Getty

Guojie Zhang, the corresponding author on the study and a professor in the biology department at the University of Copenhagen, tells Inverse, "From the genome sequencing, we found these 10 sex chromosomes do not have any homolog with the X/Y chromosomes in human, but they are more similar with the ZW chromosome of birds in the genomic sequencing structure."

The complex sex chromosomes in monotremes display unusual interactions during and after meiosis, which is a type of cell division.

"During the meiosis process, the homologous regions of the pair of chromosomes can match each other. Therefore, the sex chromosomes of platypus can form a ring structure during meiosis process," Zhang says.

That ring structure is perhaps the most intriguing finding of the study. This type of chromosome ring has been found in plants, but never before in animals, making it a groundbreaking finding.

The study also compared platypus genes to other animal genes — ranging from Tasmanian devils to humans — to construct a picture of ancient mammals' chromosome, also known as a karyotype.

An image from the study indicating genetic clusters of different types of animals.

Using these species, they could understand how platypus genes different from their ancient mammalian predecessors, and how modern platypus came to possess five different X chromosomes. They were also able to trace the last existing common ancestor of humans and platypus to 163 to 191 million years ago.

But what's even cooler: by examining the platypus' chromosomes, the scientists were able to better understand the origins of our own human DNA.

The study states that the scientists "confirmed that the X chromosome in humans was derived from the fusion of an original therian X chromosome with an autosomal region after the divergence from marsupials."

Why it matters — Without a complete map of an animal's chromosomes, it's hard to fully grasp its evolution. That's why a genomic analysis of this small, little-understood group of mammals is so valuable for the scientific community.

According to the study, the map produced for the platypus and echidna "enables us to infer the genomic changes that occurred in the ancestral monotremes and other mammals."

In other words: by looking at the DNA of platypus, we can learn more about the genetic changes that occurred in ancient mammals, helping us better understand the evolution of living animals today.

For example, the study found that "gene families associated with the immune response and hair growth were expanded considerably in the mammalian ancestor, perhaps contributing to the evolution of immune adaptation and fur, respectively, in mammals."

The research especially expands our understanding of the reproductive processes of mammals. Monotremes serve as an interesting transition point between oviparous (egg-laying) reptiles and viviparous animals — like humans — which grow an embryo inside the body.

The study states:

"Monotremes provide the key to understanding how viviparity evolved in mammals."

For example, the study found that the platypus contains fewer copies of an egg-producing protein, called vitellogenin, than reptiles, meaning that they are not as dependent on these proteins to lay eggs. But the existence of this gene may explain why platypus lay eggs in the first place.

"For example, birds have three copies of the vitellogenin gene. While in other therian mammals (like human), we do not have any vitellogenin gene," Zhang says. "But the monotreme still has one copy of the vitellogenin gene [that] maintains the same function in birds. This might explain why they can still produce egg."

"Our findings will be very interesting for those people who would like to investigate the reproductive systems in monotremes and also the organization of the sex chromosome during the meiosis process," Zhang adds.

An image of meiosis. Scientists found that platypus sex chromosomes have unusual interactions during meioisis.Getty

What's next — As the researchers acknowledge, there is still much that we don't yet know about these egg-laying mammals.

"Unfortunately, it remains a mystery why they have so many sex chromosomes," Zhang says.

But by providing such a clear genetic composition of monotremes, this study takes a new and exciting step forward in the world of mammal research.

"With the complete high-quality chromosome-scale genome, we can allow us to investigate the functions of the non-coding genomic sequencing that are important for the evolution of all mammals," Zhang says.

Abstract: Egg-laying mammals (monotremes) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution. Here we generate and analyse reference genomes of the platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus), which represent the only two extant monotreme lineages. The nearly complete platypus genome assembly has anchored almost the entire genome onto chromosomes, markedly improving the genome continuity and gene annotation. Together with our echidna sequence, the genomes of the two species allow us to detect the ancestral and lineage-specific genomic changes that shape both monotreme and mammalian evolution. We provide evidence that the monotreme sex chromosome complex originated from an ancestral chromosome ring configuration. The formation of such a unique chromosome complex may have been facilitated by the unusually extensive interactions between the multi-X and multi-Y chromosomes that are shared by the autosomal homologues in humans. Further comparative genomic analyses unravel marked differences between monotremes and therians in haptoglobin genes, lactation genes and chemosensory receptor genes for smell and taste that underlie the ecological adaptation of monotremes.
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