A CRISPR cut to the Y chromosome produces female clones of male mice

A team based in Japan has used a CRISPR-based method to remove the Y chromosome from male mouse cells and produce female clones of male mice ^[1]. The consequence, if the result survives scrutiny, is that a breeding program no longer strictly requires animals of both sexes to be present at the start.

The work is a preprint posted to bioRxiv this month and has not been through peer review ^[2]. Monika Ward, a reproductive biologist at the University of Hawaii who was not involved, told MIT Technology Review that "no one has done this before" ^[3]. Takashi Ishiuchi of the University of Yamanashi, who co-led the work, said the field holds "a fixed concept" that both females and males are needed for reproduction, and that he thinks the concept could change ^[4].

The tool, which the researchers call Y-CUT, targets a section of the Y chromosome that plays an important role in making sure daughter cells inherit the Y each time a cell divides ^[5]. Applied to early-stage mouse embryos, it eliminated the Y; treated XY embryos transferred to surrogate mothers produced female pups, an effect the researchers describe as sex reversal ^[6]. Those pups carried XO chromosomes, one X rather than the usual two, and according to Ishiuchi and his colleague Shogo Matoba of the Riken BioResource Research Center in Ibaraki, the animals grew up healthy and fertile ^[7].

The cloning experiment is the part with operational teeth. Standard cloning inserts the nucleus of an adult cell into an egg that has had its own DNA removed, producing an animal genetically identical to the donor ^[8]. The team treated some of these cloned cells with Y-CUT before transferring them to surrogates ^[9], yielding females that Matoba says are genetically identical to the original male apart from the missing Y ^[10]. They also report producing female clones from male cells that had been cryopreserved ^[11].

That matters because cloning only copies what it is given: clone a male and you get males ^[12]. Cloning has already been used in efforts to pull species back from the brink, including black-footed ferrets and Przewalski's horse ^[13], but it is described in the same account as tricky and inefficient, and it produces genetically identical individuals whose offspring may be more vulnerable to disease ^[14]. Ishiuchi and Matoba hope the technique can help rescue endangered species, particularly where only a few individuals remain ^[15]. Ben Novak, lead scientist at Revive & Restore, who was not involved, said he is confident there will be plenty of applications, particularly for conservation ^[16].

There is a thin precedent. In 2009, researchers reported the accidental birth of one female pup in a batch of 27 clones made from male mouse cells ^[17], roughly 4 percent of that batch ^[18]. Y-CUT is the deliberate version of that accident, but the available account does not report how often it worked, in either the embryo or the cloning experiments ^[19]. The researchers say they were inspired by the Okinawa rubble goby, a fish in which a female can become male when no males are present, and males can also change to female ^[20].

Two things to watch. First, peer review, and with it the efficiency numbers that decide whether this is a supply route or a demonstration ^[2][19]. Second, whether XO females are viable outside mice: healthy and fertile is currently a mouse result reported by the authors ^[7], and a conservation case for a species down to its last males rests entirely on that generalising. The nearer-term reading, on our part, is quieter than de-extinction: a co-author sits at a bioresource center ^[7], and the cryopreserved-cell result ^[11] points at frozen male lines already sitting in freezers.