With their little beady black eyes and dappled gray fur, the mice born in a recent lab experiment in Hong Kong are unlike any others of their species, or indeed any other animal.
And yet, in spite of their fundamental differences, they couldn’t appear more ordinary, a fact that reveals an astonishing truth about our evolutionary history.
The mice were spliced with genes from a single-celled microbe called a choanoflagellate. Though not an animal itself, the microorganism is closely related to them, having changed little since a time before complex, multicellular life even existed.
Remarkably, the success of a selection of the choanoflagellate’s genes in something as complex and multicellular as a mouse gives us new insights into the evolutionary origins of animal traits.
Animals feature what is known as pluripotency: an ability for embryonic stem cells to differentiate and develop into the variety of tissues that make up a fully developed organism. In spite of not having this talent, choanoflagellates have their own versions of the genes responsible for pluripotency in animals.
By swapping mouse genes for the version found in choanoflagellates, researchers could determine just how similar the two are in their functionality.
“By successfully creating a mouse using molecular tools derived from our single-celled relatives, we’re witnessing an extraordinary continuity of function across nearly a billion years of evolution,” says geneticist Alex de Mendoza of Queen Mary University in the UK.
“The study implies that key genes involved in stem cell formation might have originated far earlier than the stem cells themselves, perhaps helping pave the way for the multicellular life we see today.”
Pluripotency is thought to have emerged with the appearance of multicellular animals some 700 million years ago, so it stands to reason that transcription factors associated with stem cell pluripotency, such as those in the Sox and POU families, are thought to be restricted to multicellular animals.
But prior research conducted on animal-adjacent microbes suggests that the origins of pluripotency predate multicellularity. If this is the case, it could be one of the drivers of animal evolution, rather than a consequence of it.
Choanoflagellate Sox genes have traits similar to those found in mammalian Sox2 genes. In mice, Sox2 interacts with a POU member called Oct4, but choanoflagellate POU genes are incapable of generating pluripotent stem cells.
A team of researchers led by Ya Gao and Daisylyn Senna Tan of the University of Hong Kong and Mathias Girbig of the Max Planck Institute for Terrestrial Microbiology in Germany wanted to know what might happen if they replaced the mammalian Sox2 gene with a choanoflagellate Sox gene.
They grew cloned mouse stem cells and reprogrammed their genomes, replacing Sox2 with choanoflagellate Sox. These cells were injected into embryonic mouse blastocysts that were then implanted into pseudopregnant mouse surrogates to be gestated, birthed, and raised in a nurturing environment.
The chimeric pups were born with a mix of traits based on their spliced heritage. Obviously they were mice, but they had dark eyes and dark fur patches that indicated their mixed genetics. Otherwise, they were pretty normal, which suggests that choanoflagellate Sox genes were able to create stem cells compatible with the mouse’s development.
This suggests that the tools for creating pluripotency developed in choanoflagellates before multicellularity emerged.
“Choanoflagellates don’t have stem cells; they’re single-celled organisms, but they have these genes, likely to control basic cellular processes that multicellular animals probably later repurposed for building complex bodies,” de Mendoza says.
The findings suggest that the Sox transcription factors in choanoflagellates hundreds of millions of years ago were biochemically similar to the Sox genes that serve important functions in multicellular organisms today. The inability of choanoflagellate POU to produce pluripotent stem cells, on the other hand, suggests that POU members had to undergo modification to take up the role they play in pluripotency now.
These results could have implications for stem cell research and stem cell therapies, the researchers say. And they add an interesting layer of complexity to the story of how life diversified on Earth.
“Our data clearly shows that two of the main gene families involved in vertebrate pluripotency and key developmental genes across animals were already present before the origins of multicellularity,” the team writes in its paper.
“Eventually, their biochemical capabilities were enhanced to build one of the defining cell types of a complex multicellular entity.”
