Thanks to Brilliant for
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you sign up at Brilliant.org/SciShow. In early 2024, an aquarium in North
Carolina shared some happy news. Their round stingray named Charlotte was pregnant! But there was one thing missing
from this family portrait. There were no male stingrays in the tank, which are typically required
for stingray reproduction. Sadly, Charlotte is not on track
to start a new underwater religion. Instead, science has another
explanation: parthenogenesis. Parthenogenesis is a form of
reproduction where a female is able to make her own fertilized embryo all
on her own, no genetic father needed. So we thought this was the perfect time
to talk about the single moms of nature, and a surprising form of reproduction that keeps popping up where we don’t expect it. [♪ INTRO] First, let’s talk about sex. Or more accurately, the lack of it. Animals that reproduce sexually
make new reproductive cells through a process called meiosis, which
is a way of getting organisms that have two sets of chromosomes to end up with
one set in each reproductive cell. A sperm brings one set, an
egg brings another, and boom! You have two sets of chromosomes
and you’re off to the races. In technical terms, a haploid
egg and a haploid sperm come together to make a diploid embryo. But making those haploid cells
is a complicated process. Cells don’t just split in two and give one
set of chromosomes to each daughter cell. Oh no, they had to get funky with it. First, the parent cell duplicates its chromosomes so it has four sets of chromosomes. Then it splits, and then it splits again, so you end up with four
haploid reproductive cells. For males, that just creates four
sperm cells per round of meiosis. But for females, it’s even more complicated. One of the four haploid cells becomes the egg, and the other three smaller,
sad little sisters die off. Well, usually, anyway. Sperm meets egg in whatever way is
usual for your given species, and bang! You’ve got a diploid bun in the oven. But now let’s talk about what happens
when animals take a different approach. Parthenogenesis is the term
for all the different ways that an individual can reproduce without any
other parent’s DNA entering the mix. And there are a few ways that it can work. For instance, in a lot of cases,
the egg actually fuses with one of those sad sisters I mentioned,
which is how they make a diploid embryo. In some other species, the chromosomes
get doubled twice before the splitting stages of meiosis, so they make the typical
four cells, but they’re all diploids. And neither of these processes
is the same as cloning. Cloning is when the reproductive
cells double up on chromosomes but then only divide once,
resulting in two egg cells with full sets of chromosomes
that are identical to mom’s. In any of these cases, you end up with a baby with just a female parent, AKA a parthenote. But, why do females do this? And also, like how? Well as with everything in science,
that answer is “it depends”. Let’s start with a classic - sharks. We’ve observed parthenogenesis in
sharks and their close relatives, the rays, both in captivity and in the wild. Normally, both male and female sharks are
involved in the making of baby sharks. So it’s not totally clear
why female sharks or rays might opt for the parthenogenetic route. Being able to choose between the two
is called facultative parthenogenesis, and the species that can pick and choose usually only go the parthenogenetic route
when they can’t find a mate. So, for example, if a female shark is living
in captivity with no males in her tank, she can basically say, fine,
I’ll just do it myself. And a single female shark can reproduce
both ways over the course of her lifetime. In one case, researchers
observed a captive zebra shark who had given birth to several litters
of shark pups fathered by a male, who then went on to give birth
to another litter of pups two years after daddy shark was relocated. But at least in sharks, parthenogenesis
is really a desperate times, desperate measures situation, because their parthenotes tend to not be super healthy. One of the advantages of sexual reproduction
is that it increases genetic diversity. So without those genes from a second
parent, the offspring lose out on a lot of potential diversity, which can harm
future generations of their offspring. And for sharks, the parthenote
babies tend not to be able to reproduce on their own, with
or without a male in the picture. However, in 2016, scientists published the first known case of a
second-generation shark parthenote. A female whitespotted bamboo shark
had nine pups via parthenogenesis. And just a few years later, one of her parthenotes had her own litter of parthenote-pups. So researchers think that for some sharks, parthenogenesis might play a bigger role
in overall reproduction than they thought. It seems like a way to make the best
of a bad situation, at least sometimes. Speaking of bad situations, let’s talk
about a species that’s recovering from the absolute brink of
extinction – California condors. At its lowest point, the California
condor population was just 23 birds. But today, they’re on the
upswing, hovering in the mid-500s. In 2021, researchers reported
that two California condors had hatched chicks through parthenogenesis, which hadn’t ever been seen
in this species before. We actually talked about that story
back when it was breaking news. That 2021 report announced that back in 2013, researchers doing a population wide genetic
study on their whole California Condor pedigree had identified two outlier birds,
both males and both who had died young. And they stood out because they
were each only genetically related to their respective moms,
with no genetic father at all. Two different female California
condors had parthenote babies, despite the fact that both of them were
living with male mates in their enclosures. They’d even had offspring with those males before, so it wasn’t like they didn’t get along. What’s extra cool about this
is that because of the way bird sex chromosomes work, both of
these parthenote babies were males. Having heterozygous sex
chromosomes makes a bird a female, not a male like it does for us. In birds, the sex chromosomes for
females are WZ and males are ZZ. And WW isn’t a thing, the same
way that YY isn’t a thing for us. So if a mom wants to make parthenote babies, the end result is a ZZ offspring,
and a nest full of mama’s boys! But there’s a pretty major downside to
parthenogenesis, at least for this species. Like we said, the California condor
population is already extremely small, which means they have really
low genetic diversity. And having only one parent means
even less genetic diversity, which can lead to poor health. In this case, we know that at
least one of these parthenotes was pretty unhealthy, since it never
qualified to be released into the wild. That one lived for eight years in total, and the condor that did get released
was found dead at just two years old. Which is really short, given that a healthy
condor lifespan is around sixty years. The captive-kept parthenote was
also small and had scoliosis, and while it survived to sexual maturity, it wasn’t particularly interested in
courting the female birds he lived with. We don’t know for sure if the parthenote
that did get released died due to health issues, or just because it wasn’t able to
integrate socially with the wild condors. Nor do we know how often parthenogenesis may have happened in California condors
before their near-extinction crisis. Conservation efforts right now are focused on keeping the condor gene pool as diverse
as possible to help them recover. So understanding the circumstances that
made these two choose parthenogenesis is going to be a major piece in the puzzle, especially in light of maintaining as
much genetic diversity as possible. On the subject of male parthenotes,
let’s talk about animals that use parthenogenesis specifically
to make more males: honeybees. There are three categories of honeybees
that live in a hive: female queens, female workers, and male drones, whose
main job is to mate with the queen. And while they do have two reproductive sexes, honeybees don’t have two types of sex chromosomes. In fact, they don’t have sex chromosomes at all. Instead, they differentiate sexes using a gene called the complementary sex determiner gene. And specifically, it’s about
the number and kinds of copies of this gene a honeybee gets. Here’s how that works. Queen bees make their haploid egg cells, as usual. And after mating, they store
sperm inside their abdomen, and release it sort of as needed
when they’re growing new eggs. If the queen releases sperm
and makes a fertilized egg, that egg will grow up to
become a female bee. Asterisk. But if the egg isn’t fertilized,
it will still hatch into a male. So female bees are all diploid,
and male bees are all haploid. And that means the males
only have one genetic parent, which means that every male
honeybee is a parthenote. But the weird thing about this sex
determining gene is that technically, it’s not exactly the number of copies that makes a honeybee embryo into male or female. They do get one copy each from
the egg genes and the sperm genes, but there’s more to it than that. The gene has about fifteen
different versions, or alleles, that are floating around
in the honeybee population. While they’re all different from each other, they don’t necessarily correlate to a
different function in the individual bees. But here’s why all that variation matters; a honeybee embryo has to inherit two
different alleles to become a female. If an embryo inherits two
copies of the same allele, that embryo becomes a diploid male bee instead. Hence my asterisk a second ago. And apparently their sisters don’t
like having diploid brothers, because these males are always killed
after they hatch. And then eaten. So the only male bees that get to hang around for any extended period of time
are haploid males, and parthenotes. Now, we’ve talked a lot
about haploids and diploids, but get ready for a whole
new -ploid to enter the mix. And to talk about that, we’re going to get to some of the most famous parthenotes
out there: whiptail lizards. The species we’re talking about are polyploid, which means they have three
copies of their genome. The polyploid whiptail lizards
are the product of hybridization, meaning that members of two separate species interbred and made a whole new thing. And while we used to think that hybrids
between two species were always sterile, we’re learning more and more
that this isn’t always true. Whiptails are a great example of this, because they can still have offspring, just not the same way their parent species would. Specifically, they rely on a version
of parthenogenesis called thelytoky, which always results in female offspring
that are clones of their mothers. So whiptail lizards are an
entire species of all females. And they aren’t alone! There are at least eighty species of
vertebrates that literally do not have males. And it really does seem to be useful for these whiptails to be able
to take on solo parenting. See, parthenogenetic species
of whiptails are more common in areas where there’s a
lot of habitat disturbance, while their sexually reproducing cousins
tend to stick to areas that are more stable. So the idea is that if you can reproduce solo, you can scamper off when your
old territory is disturbed, without having to consider what the dating
scene will be like in your new home. But clearly, there’s a downside to a species
that just clones itself all the time, and that’s genetic diversity. So having those extra copies
of their chromosomes may be a way for parthenote species to keep
extra genes in the mix, just in case. And there’s somehow yet another
way to do parthenogenesis, as demonstrated by Amazon mollies. Like whiptails, they’re a
polyploid, all-female species. But to reproduce, they still need sperm. So, that complicates things. This is a form of reproduction called gynogenesis, and it means that the mother
single-handedly produces eggs that contain all of the
genetic material they need, but those eggs can’t develop into
embryos without the help of some sperm. The sperm doesn’t fuse with the egg
in the way that it would in sexual reproduction, so the genes don’t
end up part of the embryo’s genome. It’s like the egg needs a sperm cell’s
blessing, but not its genetic material. But there aren’t any male Amazon mollies. So they’re reliant on the sperm of other
species of mollies in their habitats. And that means that they’re not quite
as geographically free as whiptails are, since if they can’t find male
mollies, they can’t reproduce. So the benefit of being a parthenote
species that’s still locked into living with its parent species isn’t
as clear as it is with the whiptails. One hypothesis is that
they’re able to specialize on really specific food sources or
habitats within a larger biome. That’s called niche partitioning,
and it means that Amazon mollies can stay neighbors with their parent species without competing directly
with them for resources. So the Amazon mollies end up as specialists, while their parent species’
can be more generalists. It’s an evolutionary win-win. So that’s fish, birds, insects, and reptiles. But there hasn’t been a mammalian
parthenote on this list. And there probably never will be. There’s never been any observed
cases of parthenogenesis in any mammal species, thanks to a process
we all do called genomic imprinting. In placental mammals, meiosis includes
an extra step where some genes are chemically tagged with
something called a methyl group. Those tags basically alter how cells can
read whatever gene they're attached to. Female eggs and male sperm
tag different sets of genes, and those genes stay tagged even after they combine and an embryo begins to develop. We have about 100 genes that
are imprinted like this, which means that even though we
get copies from both parents, our cells can’t read them all the same way. So if a placental mammal female
doubled up her chromosomes and made a diploid egg, it would end up
with two tagged copies of some genes, and zero tagged copies of some others. Lots of these genes have to do
with how the placenta works, so when they aren’t working correctly, it can result in lots of problems
for the embryo’s development, and that’s just no good. So for better or worse,
placental mammal parenthood is always going to take teamwork. Reproduction is really complicated,
even if you’re doing it solo. So it’s cool that the animal kingdom has come up with so many ways to change up the
rules, for better and for worse. And clearly, we still don’t know everything
there is to know about parthenogenesis, since we’re still finding new
species that can do it all the time. It just goes to show that two heads
aren’t always better than one. And to make the most of the one head you’ve got, you can spend some time with Brilliant, the interactive online learning platform with thousands of lessons in science,
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