Biologists in Britain have reversed the sex of a female mouse embryo
by genetic engineering. Their method involves implanting a single gene from
the Y chromosome of male mice into fertilised mouse eggs that carry two
X chromosomes, the usual complement for females. They report their results
in today’s issue of Nature.
‘We find that some chromosally female embryos carrying the (extra) gene
develop into normal males,’ says Peter Koopman of Robin Lovell-Badge’s team
at the National Institute of Medical Research (NIMR), London. The NIMR researchers
collaborated with Peter Goodfellow’s team at the Imperial Cancer Research
Fund, London. The results provide the best evidence yet for the researchers’
theory that the gene, known as Sry in mice and SRY in humans, acts as a
‘master switch’ for male development.
The researchers stress that they have no intention of applying the technique
to humans. ‘The experiments were done to prove a scientific point,’ says
Godfellow, adding that similar experiments on humans would be ‘morally wrong
and highly impractical’.
To get their sex-reversed embryos, the researchers injected DNA containing
the Sry gene into hundreds of eggs. As is usual in such experiments, only
a fraction of the eggs incorporated the DNA into their own genetic material,
so the researchers obtained only a handful of ‘transgenic’ embryos. Of these,
three were sex-reversed, and one sex-reversed mouse was born. This mouse,
like all males born with two or more X chromosomes, was sterile.
The study is part of a long quest to identify exactly which gene, or
genes, on the Y chromosomes cause embryos to develop as males rather than
females. Many biologists believe that a single gene is in control: a hypothetical
‘testes-determining factor’, which somehow signals young gonads to develop
into testes rather than ovaries.
The researchers behind the latest study bolstered this notion last year
when they discovered the Sry gene at the tip of the Y chromosome (New Scientist,
Science, 21 June 1990). Circumstantial evidence made Sry a strong candidate
for the long-sought testes-determining factor, and newspaper headline writers
were quick to call Sry the gene that ‘makes a man of you’.
But without direct evidence of the gene’s involvement in sex determination,
the researchers refrained from calling it the testes-determining factor.
The sex-reversal experiments make them more certain. ‘We feel this clears
up the whole issue of whether Sry determines sex,’ says Lovell-Badge.
However, not all the ‘female’ embryos carrying the Sry gene developed
as males. Most developed as perfectly normal females instead. Two transgenic
mice even developed as females despite having ‘many copies’ of the Sry gene.
According to Koopman: ‘Why the gene doesn’t work in all cases, we don’t
yet know. But that doesn’t change our conclusions.’
One possibility is that the potency of the gene may vary according to
where it integrates after injection, say the researchers. When DNA is injected
into an egg, it merges with the egg’s chromosomes in a more-or-less random
fashion. Some chromosomal environments, argue the researchers, could ‘strait-jacket’
the gene, making it unable to produce enough protein at the right time to
switch on the development of testes.
Another reason for the failure of the Sry gene to cause sex reversal
in most embryos could be a phenomenon known as ‘mosaicism’, which occurs
when a fraction only of the cells in a transgenic embryo incorporate the
The researchers are adamant that one adult sex-reversed mouse is enough
to prove that Sry is the only gene on the Y chromosome needed to initiate
male development. Ursula Mittwoch, a developmental biologist at Queen Mary
and Westfield College in London and opponent of the ‘single gene’ theory,
is not convinced. She says the testes of the adult sex-reversed mouse are
‘exceedingly small’, a sign that the Sry gene may induce only partial masculinisation.
The researchers see the activation of the Sry gene as the first step
in a cascade of genetic events which eventually leads to testes formation.
Sry codes for the DNA-binding protein whose likely function is to regulate
the activity of a second gene on another chromosome; this gene in turn could
regulate a third gene, and so on.
The presence of inherited defects in the later genes in the cascade
could influence sex determination in some individuals, say the researchers.
This possibility could explain an otherwise awkward fact: a small number
of humans develop as males even though they have no Y chromosome, and by
implication no Sry gene.