Chinese scientists create first mammal with fully reprogrammed genes

A team of scientists in Beijing has announced they have achieved a complete recombination of the chromosomes of a mouse, a genetic engineering breakthrough that could pave the way for the design and creation of mammal species that do not exist in nature.

The mouse – called Xiao Zhu, or “Little Bamboo” – was the world’s first mammal with fully reprogrammed genes, the scientists said, referring to a process where researchers break chromosomes into various segments and put them back together in different combinations to create a new package of genes.

“This means that, for the first time in the world, we have achieved complete chromosomal rearrangement in mammals, making a new breakthrough in synthetic biology,” Li was quoted by Science and Technology Daily as saying.

“This research is a breakthrough in bioengineering technology, helping to understand the impact of large-scale remodelling of mammalian chromosomes, and to gain a deeper understanding of the molecular mechanisms behind growth and development, reproductive evolution, and even the creation of a species,” he added.

That’s one of the worst bits of hype I’ve ever seen.

The paper (at least the abstract) says nothing about complete rearrangement. It’s testing the feasibility of fusing two chromosomes–one fusion failed, another succeeded. It’s cool, but not that cool:

Here, we report programmed chromosome ligation in mice that resulted in the creation of new karyotypes in the lab. Using haploid embryonic stem cells and gene editing, we fused the two largest mouse chromosomes, chromosomes 1 and 2, and two medium-size chromosomes, chromosomes 4 and 5. Chromatin conformation and stem cell differentiation were minimally affected. However, karyotypes carrying fused chromosomes 1 and 2 resulted in arrested mitosis, polyploidization, and embryonic lethality, whereas a smaller fused chromosome composed of chromosomes 4 and 5 was able to be passed on to homozygous offspring. Our results suggest the feasibility of chromosome-level engineering in mammals.

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Hey, at least it’s another piece of evidence that shows that chromosome fusion is possible. We already knew that, but there it is again.

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Indeed. At least one human with a stable arrangement of 44 chromosomes is known to exist:

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Could those mice with a fused chromosome produce viable offspring with a wild type mouse? Would that person with only 44 chromosomes?

Yes squared.

Well, you know there are wild populations of Mus musculus that differ in chromosome count, and chromosome count is polymorphic within some populations too. So all they’ve really done is duplicate something that’s already there.

Interesting. I didn’t know that about variation in mouse chromosome number. They are the same species and can successfully interbreed?

Yep. Happens in other species too. Some shrew, if I recall. Doubtless others. Google on “Chromosomal polymorphism” or perhaps “Karyotype polymorphism” ought to turn up plenty of examples.

This seems like a good review:
Chromosomal polymorphism in mammals: an evolutionary perspective

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They can, but with reduced fertility. Half of the products of meiosis will produce monosomic or trisomic progeny IIRC.

This can be a driver in sympatric speciation:
https://www.pnas.org/doi/10.1073/pnas.1920415117

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I don’t think that’s correct, as long as we’re talking about acrocentric fusions.

As @Mercer mentions, there are stable chromosomal fusions that don’t preclude fertility. In many cases, 50% of fertilized eggs will not be viable and will not come to term. Differences in chromosome counts have to be considered on a case-by-case basis.

For example, domesticated horses have 64 pairs of chromosomes, the Przwalski’s horses have 66 pair, and the donkeys have 62 pair. Mules are the offspring of domesticated horses and donkeys have 63 pairs of chromosomes, and they are nearly all sterile. However, offspring from domesticated horses and Przwalski’s horses have 65 pairs of chromosomes and they are viable and fertile.

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Do you mean Robertsonian translocations?

I do indeed; that’s another name for it, since it’s the fusion of two acrocentric chromosomes.