6 | New Scientist | 17 August 2019THE finding that some chemicals
slow down sperm that carry the
X chromosome could lead to gels
for home use that make a couple
less likely to conceive a girl,
scientists have warned.
“I am concerned about the
social impact of this,” says Alireza
Fazeli of Tartu University in
Estonia. “It’s so simple. You could
start to do it in your bedroom.
Nobody would be able to stop
you from doing it.”
It was thought that the sperm
of mammals that lead to male
and female offspring are identical
except for the DNA they carry. But
Masayuki Shimada of Hiroshima
University in Japan and his
colleagues have found that500 genes are active in sperm that
carry the X chromosome, which
give rise to female offspring, that
aren’t active in sperm that carry
the Y chromosome, which lead
to male offspring.
Of these genes, 18 code for
proteins that stick out from the
sperm cell’s surface. The team
has found that chemicals that bind
to two of these proteins can slow
down the movement of X-carrying
sperm without affecting the
Y-carrying ones.
This discovery makes it simple
to separate sperm according to
the sex of the offspring they could
produce. When the researchers
used this method on mouse
sperm, they found that selecting
the fastest swimmers for
conception led to 90 per cent
of the resulting pups being
male. When they used slowed-
down sperm, the pups were 81 per
cent female (PLoS Biology, DOI:
10.1371/ journal.pbio.3000398).The researchers focus on
livestock, and they have found
that the technique works in cattle
and pigs (see “Why sort sperm?”,
below). They haven’t tried it on
human sperm, but Shimada
says he thinks it would work.
“It’s fairly convincing,” says
George Seidel of Colorado State
University. It could take up to
a decade to turn this into a
commercial method for sorting
human sperm, he says.
However, Fazeli thinks it may
not be necessary to select the
fastest or slowest swimmers
before insemination or IVF to
influence the sex of an embryo.
He says that, if the chemicals were
added to a gel or foam applied
inside the vagina before sex,
this could be enough to greatly
increase a couple’s chances of
conceiving a boy. Seidel agrees.
There is likely to be an appetite
for such products, especially in
countries where sex ratios have
already been distorted in favour
of boys. “In countries where there
is already a skewed sex ratio, it is
clear that if there are more easy,
cheap, accessible technologiesthey will be used,” says bioethicist
Wybo Dondorp of Maastricht
University in the Netherlands.
Several methods for sex
determination already exist.
The most reliable is to analyse
the chromosomes of IVF embryos
prior to implantation. This pre-
implantation genetic diagnosis
(PGD) method is almost 100 per
cent effective, but it is expensive.Another method, which Seidel
helped develop, is to add a
fluorescent dye to semen that
binds to DNA. As Y-carrying sperm
have less DNA, these will be less
bright and can be separated one
at a time. This “flow cytometry”
gives an 82 per cent chance of
having a boy if desired, or a
93 per cent chance of a girl.
Both IVF PGD and flow cytometry
require specialised equipment
and expertise.
Sex selection is banned for
non-medical uses in the UK,
Australia, Canada, China and
India, but is legal in most other
countries, including the US, where
a small number of people pay to
use it for “family balancing”.
Shimada’s work could make
sperm sorting much simpler
and more widely available – and
more prone to misuse. “The
sexing is much easier than the
conventional technique,” he says.
“Therefore, I am worried about
the problem.” ❚“I am concerned about the
social impact of this. It’s so
simple. You could start to
do it in your bedroom”
News
FertilityAlarm over sex selection
Scientists warn that sex ratios could be skewed even further by the
discovery of physical differences between sperm. Michael Le Page reportsIn some countries, more
than 107 boys are born
for every 100 girlsMI
KH
AILPOCH
UYEV
/TASS
VIAGETT
YWhy sort sperm?
While choosing the sex of children
is an ethically fraught issue, the
ability to choose sex in livestock
can lead to more humane farming.
Masayuki Shimada of
Hiroshima University, Japan, has
tried his new method for sorting
sperm (see main story) in cattle,
where there is much demand for
sex determination. Bulls are of
limited use to dairy farmers, while
beef producers prefer males.
In the US, many farmers
inseminate cows with semen
sorted using a technique called
flow cytometry. This gives arounda 90 per cent chance of each
offspring being of the desired
sex, and leads to fewer unwanted
calves being killed.
Shimada has also tested his
method in pigs. Male pigs are
usually castrated to prevent an
unpleasant odour in pork known
as boar taint, but the European
Union aims to phase this out on
animal welfare grounds.
However, flow cytometry
doesn’t work as well in pigs.
If Shimada’s approach proves
commercially viable, it could
become the preferred technique.