[♪ INTRO] 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 chromsome, 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 ]
