A mammal that lays eggs: the genome of the platypus provides scientists with new information about these intriguing animals

The platypus feeds its young with milk like a mammal, lays eggs like a bird, and produces venom like a reptile. It shocked zoologist George Shaw when he came across the mammal in 1799. Over 200 years later the near complete genome of the Platypus has been mapped by researchers and published in Nature. This research gives new insight into the evolutionary mechanisms involved in creating one interesting creature.

The platypus species is one of only five examples of monotremes left on earth

This newest platypus genome sequencing attempt has contextualised 96% of the platypus’ genome. Up until now only 25% of the genome had been mapped in a 2008 study. Mammals are split into three different types: monotremes, eutherians, and marsupials. The platypus species is one of only five examples of monotremes left on earth including the echidna species (also known as spiny anteaters) which has also been sequenced in this research. This type of mammal both lays eggs and feeds its young with milk. The platypus does this through sweat. Humans belong to the eutherian subclass which give birth to live young. Koalas, for example, belong to marsupials because they have permanent pouches. The new research offers insight answering questions about when these three types of mammals diverged from each other. It suggests monotremes diverged from the other two mammal types 187 million years ago. 

The platypus however has 10 sex chromosomes. This  suggests that these chromosomes may have been arranged in a ring shape

The new genome helps explain the peculiar overlap of the platypus with bird and reptile characteristics. Most birds have the vitellogenin gene, this gene encodes egg yolk proteins which provide nutrients to developing embryos. Unsurprisingly this type of gene is not usually found in mammals such as humans. However, platypuses and echidnas have one copy, providing a good reason as to why these mammals lay eggs compared to giving birth to live young. Another surprising difference between monotremes and the other two types of mammals is their sex determining system. Eutherian and marsupials have an XY system, meaning that the sex of an offspring is determined by two chromosomes, one chromosome received from each parent. The platypus however has 10 sex chromosomes. This  suggests that these chromosomes may have been arranged in a ring shape and as mammals have evolved it has slowly broken down into smaller pieces of X and Y chromosomes. This broken-down sex determination system may be more comparable to the ZW sex determination of an egg-laying bird rather than a mammal’s XY system. Evolutionary biologist and research leader Guojie Zhang explains this new research: “holds the key as to why we and other eutherian mammals evolved to become animals that give birth to live young instead of egg-laying animals.” ​ The monotreme’s egg-laying ability may have been something other mammals lost as they evolved compared to an ability that monotremes gained after mammals diverged.

In a 2010 study Wesley Warren, genomics professor at University of Missouri, found that 83 toxins in platypus venom contained genes of animals such as snakes and spiders

Further, like some whales and anteaters, platypuses are toothless. They use gravel to help them chew their food into smaller pieces. Platypuses have teeth while in the embryo and after hatching but lose them before the animal even leaves the breeding burrow. The recent research has shown that platypuses are missing genes which support dental development.

Platypuses also have venom producing glands in its hind feet. This venom is strong enough to kill a dog and protects the platypus against predators and other males during mating season. In a 2010 study, Wesley Warren, genomics professor at University of Missouri, found that 83 toxins in platypus venom contained genes similar to those found in animals such as snakes and spiders. This 2021 research reconfirms the idea: the platypus has similar venom producing genes to that of a reptile. 

It would be misleading to say that platypuses are more bird and more reptile than we are. It is argued that these similar genes are an example of convergent evolution. Organisms can produce similar genes because they are in similar environments and not because they are closely related. What is interesting about this research is its significance to support the mapping of mammal evolution; understanding where monotremes lost traits we used to share millions of years ago and which ones they gained throughout their evolution. Like Warren says, the platypus helps to show “the ancestral state of what terrestrial mammal genomes could have been before adapting to various environments.” The platypus is not ‘part’ bird or ‘part’ reptile, it is a mammal like us. This research is exciting because it can help scientists to understand why some mammals can evolve so differently.