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. 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.
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 Hydra, 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 – albeit exceedingly closely related ones.
Some attempts at cloning male mice have, by chance, produced a female 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 – a technique called somatic cell nuclear transfer.
They treated some of these embryos with Y-CUT and transferred them to surrogate female mice. All of the mice born after the use of Y-CUT were female clones – genetically identical to the father apart from lacking the Y chromosome. Other offspring, which hadn’t undergone Y-CUT, were all male clones.
Introducing the father mouse to his clones was a very special moment, says Ishiuchi. Arguably the greatest bioengineering feat yet 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. Ben Novak at wildlife conservation organisation Revive & Restore, which has been involved in cloning efforts with endangered species including Przewalski’s horse and the black-footed ferret, thinks similarly.
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