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It’s time for another edition of “The world is more interesting and  nuanced than you might think.”   In biology classes, we’re often  taught that developing animals become one sex or another because of their chromosomes. Chromosomes are packages of genetic material.

And usually, we’re taught that female  animals get their traits because they have two X chromosomes, and males get  theirs from having one X and one Y. This is the system most mammals  use, so many lessons don’t go into more detail than that, and this XY  thing is kind of implied to be universal. But really, there are all kinds  of other ways this can work.

We’re talking things like Z and W  chromosomes, sex that develops based on temperature, and sex determination based  on whether an egg is fertilized at all. Oh, and also, in the grand scheme of things, how sex is determined in different  species isn’t even stable. Now, to take a step back, for  the purposes of this episode, we’re defining “females” as animals  that produce sex cells called eggs, and “males” as animals that  produce sex cells called sperm.

This isn’t the only possible definition, but  it’s the one that’s going to be most helpful for understanding what’s going on here. That said: These egg and sperm cells  are collectively called gametes. Typically, they contain a half copy of the  parent’s genome, and they’re all unique.

So, when two gametes get together, the offspring’s genome generally differs  from both its parents and its siblings. One basic reason for this is that  it makes a species more resilient:. When you have genetic diversity, it also  leads to individuals with different traits.

As an environment changes,  this variation helps some individuals adapt generation after generation. So, this ensures that at least some  of a group will survive long-term. It took a lot of hits and misses before people figured out how sex determination works, though.

In fact, before the early 20th century, there had been around 500  documented wrong ideas about it. For instance, Aristotle thought  male humans were a product of excess heat and dryness produced  by the male during intercourse. Meanwhile, females were supposedly a product of excess cold and wetness from the female parent.

Others thought that sex was  determined by whether the embryo was on the right or left side of the uterus,  or which testicle the sperm came from. And ideas like this survived  for thousands of years. But around the turn of the 20th century,  a scientist named Nettie Stevens uncovered one of the first right answers.

While doing research at Bryn Mawr College,. Stevens began researching  sex determination in insects. She was convinced that it  had something to do with the genetic material in eggs and sperm cells.

Partly, she was influenced  by Mendel, another scientist who had shown how pea plants could  pass traits to their offspring. But also, her ideas were  informed by a recent discovery: the identification of chromosomes. Scientists had just figured out that  these rod-shaped structures were the packages of genetic material responsible  for carrying traits from a parent to child.

So, with these ideas in hand,  plus plenty of her own research,. Stevens published a breakthrough  paper in 1905 on sex determination in various insects, including mealworms. While looking at mealworm sperm and eggs  through her microscope, she saw that the eggs all had ten large chromosomes,  but the sperm were different.

Half of them had ten large chromosomes, but the other half had nine large  chromosomes and one small one. Similarly, cells from other  parts of the female mealworms had 20 large chromosomes, but cells  from other parts of the males had 19 large ones and one small one. Ultimately, Stevens concluded  that this small chromosome must have something to do with sex.

And she was right! In fact, this “small chromosome” would  later become known as the Y chromosome, one of the two sex chromosomes you might  have learned about in biology class. Since 1905, though, we’ve learned a lot more about just how variable sex  determination actually is.

The most talked-about system  is that XX and XY one, which humans, most other  mammals, and lots of insects use. Not all of these systems are identical, in fact, scientists think  Xs and Ys probably evolved independently in several species. But generally, in this case, a single egg  carries one large sex chromosome, or an X.

And a single sperm either carries a  large sex chromosome or a small one, so, an X or a Y. This means sperm cells determine an animal’s sex. If an egg is fertilized with a sperm carrying  a Y chromosome, the offspring will be.

XY and male, again, using that very  narrow definition of “males make sperm.” On the other hand, if an egg is  fertilized with an X-chromosome sperm, it will be XX and female.   But also, this is nowhere near  the only way this plays out.   Like, the sex chromosomes of  some birds and some reptiles work the opposite way: In their system,  it’s usually the egg that calls the shots.   In these species, males have  a pair of Z sex chromosomes, so their sperm all have one large Z.   Meanwhile, females have one Z,  plus a smaller W sex chromosome, so eggs can have one or the other.   Then, there are animals that  completely throw a wrench into this neat, two-chromosome system. One of them is the platypus, because of course.   On top of being semi-aquatic, egg-laying  mammals of action, platypuses have ten sex chromosomes, ten Xs for females,  and five pairs of Xs and Ys for males.   When platypus sperm cells form,  the sex chromosomes form a chain. And although scientists don’t  quite know how this works yet, this results in sperm that either  carries five Xs or five Ys.   Overall, though, platypuses are  at least similar to other mammals.

Once you move into other branches of  the tree of life, things get wilder.   Like, in some animals that reproduce sexually, there are no obvious sex chromosomes at all.   Instead, sex is triggered by some  environmental factor that determines which genes are turned on  and what body parts develop.   The most common is  temperature-dependent sex determination, which occurs in reptiles like  alligators, sea turtles, and some fish.   For instance, in studies of  the European pond turtle, incubating eggs at temperatures above  30 degrees Celsius produces all females, while temperatures below 25  degrees Celsius makes all males.   And in-between, you get an equal  ratio of males to females.   In other cases, some birds seem  to determine their sex based on resources available to their parents, like  the nutritional content of their food.   And some invertebrates, like  certain worms and snails, determine their sex based on proximity  to some environmental factor.   For example, some female  marine worms release their larva into the surrounding water. If the larvae land on the ocean floor,   they develop into females. But they can also re-enter the parent’s  body, where they turn into males.

And produce sperm and mate with the  same female they emerged from. Yeah.   So, there are plenty of ways this can play out. And of course, there are animals  that throw all of these ideas straight out the window, too.

As a final example, more than 200  thousand species of ants, bees, and wasps use a sex-determination  system called haplodiploidy.   In this system, females can  lay unfertilized eggs that, all on their own, develop into males. But if the female chooses to mate with a male,   then the extra genetic material from her partner means the fertilized  eggs develop into females.   And the list just goes on from there! You  could talk all day about this kind of thing.   But really, it all follows one basic theme:.

In the end, you get one kind  of animal that makes eggs, and another kind that makes sperm. But here’s the real kicker: Whatever  system a species settles on, that system isn’t permanent. Instead, over long periods of time,   researchers think that what system a species uses, and whether it uses  sex chromosomes at all, is temporary.   Like, in a 2017 paper in Genetics,  researchers described a type of frog that lives in Japan.

In northern Japan, these frogs have a ZW system, and in the south, they have an XY system. According to their mathematical modeling, the authors suggest that these transitions  just happened by chance as these populations diverged into different areas. Other studies have suggested that in  some species, there can even be an intermediate period as a species  switches from XY to ZW or vice versa, where sex is determined by the  environment.

Like with those turtles. So not only are there more than just X  and Y chromosomes, the whole concept of sex determination is an evolving  thing, even for individual species. When they’re studying this, one way  scientists can try to learn how long a species has been using a particular system is  by looking at the small sex chromosomes, so, the Y or W, for species that have them.

The current thinking is  that these small chromosomes have degraded over many, many generations.   See, these small chromosomes carry what’s  called a master sex-determining gene, which is as important as it sounds:  This gene controls reproductive traits. Like, in humans, the process of  developing these traits is regulated by a gene on the Y chromosome called SRY. Small chromosomes might have picked up genes like this through some  kind of mutation or mutations.

But however it happened, the thinking  is that when a chromosome picks up a master sex-determining gene, something changes: either a region of these  chromosomes or the whole thing stopped recombining during meiosis. Meiosis is the process that makes eggs and sperm. In it, a cell duplicates all of its  chromosomes, and then pairs of chromosomes line up and exchange genetic  material, or recombine.

So, the thinking is that at least part  of structures like the Y chromosome just stopped recombining for some reason. That would have led the chromosome to  lose bits and pieces and erode over time. So, in this view, Y or W chromosomes that are significantly smaller than the  Xs or Zs are relatively old.

In fact, there’s even some evidence that these chromosomes can degrade so much, they go extinct. That’s what seems to have  happened with a certain group of rodents called mole voles. Studies of its genome show  that both sexes usually have one X chromosome and no  second sex chromosome at all.

The current leading explanation  is that it lost the Y chromosome, and that some other parts of the  genome took over sex determination. And for as dramatic as this sounds, it  isn’t bad. If this is what happened, it was just another part of the long,  changing arc of how sex determination works.

So, we’ve come a long way since Aristotle,  but there’s still a lot to figure out. Like, today, researchers are learning more  about genes that can influence sex and how animals develop, including genes  that aren’t on sex chromosomes at all. We also have more to learn  about why this is so complex.

Those answers could come from  studying how sex chromosomes evolve, and comparing the evolutionarily old  ones to their younger counterparts. But one thing is for sure:. The world is a lot more  nuanced than we often realize.

And that just makes it a  whole lot more interesting. Now, if you’re into the whole “the world is super interesting in  all kinds of super weird ways” thing, you might enjoy our podcast: SciShow Tangents. It’s a bunch of smart people in a  lightly competitive setting showing off various science facts, including a bonus  butt fact at the end of every episode.

If you’re interested, you can find SciShow Tangents  wherever you get your podcasts. [OUTRO ] .