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 A
giant snake in a Dutch zoo has achieved two world-firsts, and it's all
down to her remarkable ability to reproduce through so-called ‘virgin
birth'. It's the first time that parthenogenesis has been seen in the
great snakes (pythons and boas) and it's the only known case of exact
clones being produced naturally in any snake species.
Lying
coiled up in a shiny, mottled heap against the vivarium
window, a four-metre long, nameless Burmese Python (Python
molurus bivittatus, also known as a Dark Tiger Python)
at Artis Zoo in Amsterdam is going about her normal routine,
unaware of the excitement she has been causing in the
scientific world of reproductive biology.
Every
year for the past six years she has been laying her annual clutch of
leathery eggs, just like most other captive female snakes. And, as with
all the animals kept in the reptile house at Artis, every year her eggs
have been removed and examined by staff, because they can reveal a lot
about the condition of the females.
 But
this specific snake proved to be very special, as the Curator of
Reptiles, Fish and Invertebrates, Eugène Bruins, reveals: "She always
lays some eggs that are nicely big and white, about 9cm long, but about
60% are much smaller and more yellow. These are always infertile, but
we did find in some of the [big] eggs some embryos."
Storage story
To
begin with, Bruins and his colleagues assumed that the python must have
mated with a male during the first two years of her life, before she
arrived at Artis Zoo. If this had happened, she could have been using
sperm stored in her body over a number of years to produce fertilised
eggs – a well-known phenomenon in several species of reptiles including
some snakes. But after five years had passed and viable embryos were
still being found in her eggs, Bruins started to wonder whether he had
a world record for the longest known sperm-storage, or whether
something else was going on.
 Bruins
called in the help of his friend, Tom Groot, then a Masters degree
biology student at the University of Amsterdam, who did genetic
fingerprinting on both the snake and her embryos. "Fingerprinting uses
a random set of characters from the entire genome," he explains. "If a
father was involved you'd expect to find characters in the embryos that
don't relate to the mother. We didn't find that, so the first thing to
conclude was that there couldn't be any sperm-storage."
The
alternative explanation was that the embryos had resulted from
parthenogenesis, or so-called ‘virgin birth', where eggs develop into
new individuals although they are unfertilised by a male. "Virgin birth
is quite common in some lizards and is known in a few snake species,"
says Bruins, "But it was never yet known in the family of pythons or
boas, so that was a world first already. But even more special was that
all other snake species reproducing this way produce only males and our
embryos were all females."
Identical copies
 The
sex of the offspring was confirmed by anatomy specialists at the
University of Utrecht in the Netherlands and, in fact, further analysis
of their genetic material showed that they were all exact clones of the
mother – a phenomenon never before seen in snakes at all. "It's really
exciting to see it and know it is possible," enthuses Tom Groot, who is
now studying parthenogenesis in mites for his PhD. "We don't know if it
can happen in these pythons in the wild. But with this mechanism a
single female would be able to start an entirely new population which
makes it, in the evolutionary sense, very interesting."
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How parthenogenesis works in snakes
Unlike
humans, where women have two sex chromosomes of the same type (XX) and
men have two different (XY), in snakes it's the female that has the
different combination (ZW) while the male has two the same (ZZ).
During
normal egg production in any species, the number of chromosomes is
divided in two, because during sexual reproduction half of the genetic
material for the offspring comes from the male sperm. In humans this
means that all eggs contain one X-chromosome, whereas in snakes,
egg-cells can contain one Z- or one W-chromosome.
In parthenogenesis the genome must be duplicated to achieve the full complement of chromosomes without any input from a male.
In
previous cases in snakes, single W- and Z- egg-cells were doubling up
to form either WW eggs (an invalid and therefore infertile combination)
or ZZ eggs, which were viable and by definition male.
The Artis
python seems to be first doubling her chromosomes to ZWZW – then going
through the division process to produce viable ZW embryos which are
females and identical clones of herself. This happens in about only 40%
of her eggs. |
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Hatching a name
Now
that Artis Zoo is aware of the very special embryos appearing in its
reptile house every year, there are plans now to try incubating this
snake's eggs for the very first time. Some of the offspring may be
given away to other zoos or to researchers specialising in
parthenogenesis in snakes, but some would be kept in Amsterdam. Eugène
Bruins: "We want to see if they're as healthy as the mother and whether
they have the same spots in the same place! And in four years time we
want to see if they can reproduce parthenogenetically as well." As for
the mother snake herself – according to Bruins she may finally be given
a name: "Some people including myself have suggested Maria, which is
perhaps not surprising!" |
