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Inbreeding with Yourself
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Duration: | 14:57 |
Uploaded: | 2024-04-26 |
Last sync: | 2024-12-12 04:45 |
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MLA Full: | "Inbreeding with Yourself." YouTube, uploaded by SciShow, 26 April 2024, www.youtube.com/watch?v=yFnNyC72FGY. |
MLA Inline: | (SciShow, 2024) |
APA Full: | SciShow. (2024, April 26). Inbreeding with Yourself [Video]. YouTube. https://youtube.com/watch?v=yFnNyC72FGY |
APA Inline: | (SciShow, 2024) |
Chicago Full: |
SciShow, "Inbreeding with Yourself.", April 26, 2024, YouTube, 14:57, https://youtube.com/watch?v=yFnNyC72FGY. |
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**At the time this video was made, it had been reported that Charlotte the round ray was pregnant. However, new information has come out that she was never pregnant with parthenotes, and in fact had a reproductive disorder, which is a bummer. Everything else in this video is accurate and unchanged by this news, but we wanted to be transparent that the introduction of this video contains outdated and incorrect information.**
Reproduction usually takes two parents, at least in most sexually reproductive species. But there's a few different species that have decided to go solo, so to speak. From whiptail lizards to California condors, here are a few of the coolest single moms in the animal world!
Hosted by: Savannah Geary (they/them)
The Virgin Births Of The Animal Kingdom
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Support SciShow by becoming a patron on Patreon: https://www.patreon.com/scishow
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Huge thanks go to the following Patreon supporters for helping us keep SciShow free for everyone forever: Adam Brainard, Alex Hackman, Ash, Benjamin Carleski, Bryan Cloer, charles george, Chris Mackey, Chris Peters, Christoph Schwanke, Christopher R Boucher, DrakoEsper, Eric Jensen, Friso, Garrett Galloway, Harrison Mills, J. Copen, Jaap Westera, Jason A Saslow, Jeffrey Mckishen, Jeremy Mattern, Kenny Wilson, Kevin Bealer, Kevin Knupp, Lyndsay Brown, Matt Curls, Michelle Dove, Piya Shedden, Rizwan Kassim, Sam Lutfi
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Sources: https://drive.google.com/file/d/1hLwQQtdh1soO93ihoV-z6pzSCNiaJVj4/view
**At the time this video was made, it had been reported that Charlotte the round ray was pregnant. However, new information has come out that she was never pregnant with parthenotes, and in fact had a reproductive disorder, which is a bummer. Everything else in this video is accurate and unchanged by this news, but we wanted to be transparent that the introduction of this video contains outdated and incorrect information.**
Reproduction usually takes two parents, at least in most sexually reproductive species. But there's a few different species that have decided to go solo, so to speak. From whiptail lizards to California condors, here are a few of the coolest single moms in the animal world!
Hosted by: Savannah Geary (they/them)
The Virgin Births Of The Animal Kingdom
----------
Support SciShow by becoming a patron on Patreon: https://www.patreon.com/scishow
----------
Huge thanks go to the following Patreon supporters for helping us keep SciShow free for everyone forever: Adam Brainard, Alex Hackman, Ash, Benjamin Carleski, Bryan Cloer, charles george, Chris Mackey, Chris Peters, Christoph Schwanke, Christopher R Boucher, DrakoEsper, Eric Jensen, Friso, Garrett Galloway, Harrison Mills, J. Copen, Jaap Westera, Jason A Saslow, Jeffrey Mckishen, Jeremy Mattern, Kenny Wilson, Kevin Bealer, Kevin Knupp, Lyndsay Brown, Matt Curls, Michelle Dove, Piya Shedden, Rizwan Kassim, Sam Lutfi
----------
Looking for SciShow elsewhere on the internet?
SciShow Tangents Podcast: https://scishow-tangents.simplecast.com/
TikTok: https://www.tiktok.com/@scishow
Twitter: http://www.twitter.com/scishow
Instagram: http://instagram.com/thescishow
Facebook: http://www.facebook.com/scishow
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Sources: https://drive.google.com/file/d/1hLwQQtdh1soO93ihoV-z6pzSCNiaJVj4/view
Thanks to Brilliant for supporting this SciShow list show.
Brilliant is offering a 30 day free trial and 20% off an annual premium subscription when 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, computer science, and math. Brilliant offers case studies, puzzles, and tons of unique ways to help you engage with each topic.
And that includes really complicated and cutting edge topics like how large language models work. That’s not the kind of thing you can get a good grasp of through memorization. So Brilliant created an immersive AI workshop to experience the tool for yourself.
First, you learn how LLMs work by exploring how real language models build vocabulary and choose their next word. Then, you get to train your model on data like Taylor Swift lyrics, cookbooks, and Big Tech's Terms and Conditions. Through that process, you’ll see how much of a difference training data makes.
And once you have a handle on the basics and training, you can tune your LLM to generate different kinds of output, from poetry to a cover letter. You can’t find interactive learning opportunities like that just anywhere. But you can find them at Brilliant.org/SciShow or the link in the description down below.
That link also gives you 20% off an annual premium Brilliant subscription. And you’ll get your first 30 days for free! Thanks to Brilliant for supporting this SciShow video! [♪ OUTRO]
Brilliant is offering a 30 day free trial and 20% off an annual premium subscription when 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.
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