How does this all-female species reproduce? - Susana Freitas and Darren Parker
In 2021, workers at a Sardinian aquarium
were stunned by the birth
of a smoothhound shark,
who they called Ispera.
What shocked them was that,
for the last decade,
Ispera’s mother had been living
only with other females.
But it’s actually entirely possible
that Ispera had no father—
and the reason why that is also explains
other biological curiosities,
like the existence
of an all-female lizard species.
Usually sexual species have
sex cells that contain
half the number of chromosomes
required to create a viable embryo.
So an egg cell must be
fertilized by a sperm cell
to form two full sets of chromosomes.
But some species that have
sex cells can undergo
a type of asexual reproduction
called parthenogenesis—
meaning “virgin origin” in Greek.
In parthenogenesis, an embryo
develops from an unfertilized egg cell
that doubles its own chromosome count.
In fact, some animals only ever
undergo parthenogenesis,
while others can reproduce both
sexually and parthenogenetically.
It's actually more common
than previously thought.
More than 80 different
sexual vertebrate species—
including Komodo dragons and certain
kinds of turkeys, pythons, and sharks—
have surprised us by occasionally
reproducing this way.
These discoveries were usually made
when females
unexpectedly gave birth in captivity.
Ispera’s birth, for one, may have been
the first account
of parthenogenesis in smoothhound sharks.
Scientists also confirmed
that parthenogenesis was taking place
in some wild snake populations.
But just how many fatherless creatures
are running, slithering, and swimming
around out there is unknown:
it’s a tough thing to track without
population-wide genetic analyses.
So, why is it happening at all?
Scientists think parthenogenesis could
be evolutionarily beneficial
in some contexts because, well,
sex can be a drag.
Mating and its associated demands and
rituals can be time- and energy-intensive,
leave individuals vulnerable
to predators, and even be fatal.
Parthenogenesis, meanwhile,
requires only one parent.
Mayflies can sometimes
default to parthenogenesis
if there are no males available,
which is especially handy because
they’ve only got a day or so
to reproduce before dying.
It can also help rapidly
expand a population.
In the summer, when food is abundant,
pea aphids can rely on parthenogenesis,
allowing their population to explode
under favorable conditions.
And in the autumn,
they switch back to sex.
But some aphids, katydids, lizards,
geckos, and snakes
only ever reproduce
via parthenogenesis.
So, why do other animals bother with sex?
Scientists hypothesize that sex makes up
for its shortcomings with long-term gains.
It allows individuals to mix their genes,
leading to greater genetic diversity.
That way, when the going gets tough,
beneficial mutations can be selected
and harmful ones can be removed
without ending the entire population.
In a parthenogenetic population,
on the other hand,
individuals can only reproduce
using their own genetic material.
According to a theory called
Muller’s ratchet,
that’s not good.
The theory predicts that parthenogenetic
lineages will accumulate harmful mutations
over time and eventually,
after thousands of generations,
will reach a point of so-called
mutational meltdown.
At this stage, individuals will be so
compromised that they can't reproduce,
so the population will nosedive,
leading to extinction.
We haven’t yet seen this entire process
unfold in nature.
But scientists have observed
an accumulation of harmful mutations
in parthenogenetic stick insects that are
absent in their sexual relatives.
Only time will tell whether this
will cause their extinction.
Otherwise, some parthenogenetic species
appear to have ways of circumventing
a mutational meltdown.
New Mexico whiptail lizards came about
when two different lizard
species hybridized,
creating this new all-female species.
As hybrids, their genome is a combination
of the different sets of chromosomes
from their two parent species.
This gives them a high level
of genetic diversity,
which may allow them to survive
long into the future.
Bdelloid rotifers, meanwhile,
have been reproducing parthenogenetically
for 60 million years.
They might have managed this by taking
in foreign genetic material.
Indeed, about 10% of their genes comes
from other organisms,
like fungi, bacteria, and algae.
How exactly they do this is unclear,
but whatever the trick is,
it seems to be working.
To totally untangle
the mysteries of reproduction,
we’ll need more research—
and probably a few more surprises
like Ispera.
