Male mouse embryos have been turned into females by a procedure that involves eliminating the Y chromosome. The technique relies on CRISPR gene editing and has been used to create healthy and fertile female and male mice, both of which are clones of the father.
鈥淲e thought if we could remove the Y chromosomes immediately after fertilisation, we can change the sex,鈥� says at the University of Yamanashi, Japan. 鈥淚t is essentially sex reversal.鈥�
The method may become useful for conserving animals that are so close to extinction they are down to just one male or only a few.
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In animals that reproduce sexually, running out of males or females can mean the end of the line for a population or species. However, some species can turn to asexual reproduction, through budding off new organisms, such as jellyfish-like, or by parthenogenesis, in which unfertilised eggs of animals including lizards, stick insects and sharks can develop into a new animal. This produces offspring that are identical clones of the parent and the same sex. Clones created in labs, such as Dolly the sheep, are made through a similar process.
A few animals, like Okinawa rubble gobies (Trimma okinawae), can change sex to ensure sexual reproduction can continue. Inspired by these colourful marine fish, Ishiuchi and his colleagues came up with a method to change the sex of a male clone embryo, so sexual reproduction between individuals could take place 鈥� albeit exceedingly closely related ones.
when the male-determining Y sex chromosome was deleted, leaving the specimen with just one X chromosome (known as XO), rather than an XY pair.
Ishiuchi and his colleagues developed a CRISPR-based tool called Y-CUT to reliably eliminate the Y chromosome from male mouse embryos.
First, they inserted DNA from the blood cells of a male mouse into mouse egg cells that had already had their own DNA removed 鈥� a technique called somatic cell nuclear transfer. They treated some of these embryos with Y-CUT and transferred them to surrogate female mice.聽
All of the mice born after the use of Y-CUT were female clones 鈥� genetically identical to the father apart from lacking the Y chromosome. Other offspring, which hadn鈥檛 undergone Y-CUT, were all male clones.
鈥淲e call this dual-sex cloning,鈥� says team member at the RIKEN BioResource Research Center in Tsukuba, Japan.
Introducing the father mouse to his clones was a very special moment, says Ishiuchi. 鈥淗e could see the female version of himself, which is like sci-fi.鈥�
The clones were fertile, and when they mated together, they produced offspring that seemed healthy. 鈥淭his means sexual reproduction was initiated from a single male genome,鈥� says Matoba. 鈥淭he kids have survived over a year so far with no clear defects.鈥�
Ishiuchi hopes that the Y-CUT technique could help revive species that are highly endangered or extinct, and for which only frozen tissue remains in cell banks.
鈥淚t would be incredibly valuable if this were successful in species other than mice,鈥� says , also at the University of Yamanashi, who wasn鈥檛 involved in the study.
at wildlife conservation organisation Revive & Restore, which has been involved in cloning efforts with endangered species including and the , thinks similarly. 鈥淭hey are laying the foundations for a future of manipulating chromosomes that is probably going to be important in many cases for conservation,鈥� he says.
Novak agrees the technique may be useful for frozen tissue samples from endangered or extinct species, where we have samples from only one or two individuals.
A previous study generated both , but it used a method that is unlikely to work in other species, unlike somatic cell nuclear transfer, which has been successfully applied to many mammal species and works with living or stored donor material.
One issue is that, though an XO female mouse is healthy and fertile, that isn鈥檛 the case in many other species.
鈥淵-deleted offspring are not fertile in humans and horses, and therefore this is likely to be the case in multiple wildlife species,鈥� says of SEZARC, a group of zoos and conservation centres based in the US. She also says there could be ethical concerns with producing Y-deleted animals.
Ishiuchi accepts this, but notes that there is research showing that , so it might be possible to do this in combination with Y-CUT to make resulting females fertile in a wider range of species.
Penfold also warns that breeding from clones of a single animal would create genetic issues. 鈥淭here is a lack of genetic diversity, which is critical for healthy populations,鈥� she says.
Novak is more positive about that aspect. 鈥淪exual reproduction with one individual is certainly better than no sexual reproduction,鈥� he says, but he adds that we should do everything we can now to never have to use such tools. 鈥淲hile they鈥檙e awesome, their use indicates that you鈥檝e gotten to a situation that鈥檚 so dire that you need sex reversal,鈥� he says.
Habitat protection and reducing hunting will always be much more effective at conserving species, says Penfold.
bioRxiv
