In nature, you will notice that the vast majority of sexually reproducing organisms have got two
sexes - not one, and very rarely more.
Biologists love to ponder about why sexual reproduction evolved in the first place, but
today we're asking a very different question - why do we need sexes for sex, and why specifically,
TWO of them?
[Intro by Caro Waro & Cristina de Manuel]
Firstly, to clarify, in this video the term sex is used interchangeably with gametes types
or mating type.
Gametes are reproductive cells, and one gamete may only fuse with one of a different type
to give rise to a zygote.
Given there are generally two gametes or mating types, they are usually referred to as male
and female.
Male gametes are generally smaller and motile, and female gametes are generally larger and
sessile, and carry no further political connotations than that.
If you think about it, having two sexes may seem like a very weird set up, because it
effectively cuts the population a gamete could potentially mate with in half.
You would expect more sexes to appear, such that gametes can mate with those of a different
type, and in this situation, the rarer sex will have the highest advantage and therefore,
you would expect more sexes to appear over time.
However, this is NOT what we observe.
Although there are cases of multiple sexes in certain fungi species, in almost every
other case of sexually reproducing organisms, there are only two sexes, which raises the
question - how did two sexes evolve in the first place?
This is a fascinating question for which there are multiple potential theories.
One of the more simplistic models is based on size.
In the ancestral state, the gametes don't have separate mating types, and they are isogamous,
which means they are all the same size, and they can all fuse with each other.
The larger a gamete, the more resources it can provide to the future embryo, and this
is something that we see that in seeds.
Large seeds can generally last a long time without germinating and produce a robust plant
when they do finally germinate, but are also costly to produce, plants that produce large
seeds produce far fewer than those that produce small seeds.
Small seeds may only germinate if they land in soil under the right circumstances.
They are much cheaper to produce, so whilst the majority may never germinate, they rely
on numbers for one to land under the right conditions.
A similar set-up could be observed in gametes.
When two large gametes fuse, the young will be very robust.
However, there may be smaller gametes, cheats if you will, who may attempt to fuse with
the larger gametes.
The zygote won't have as many resources as if two large gametes had fused, but still
sufficient for it to have a decent start, and less energy would have been invested in
producing the smaller gamete.
If two small gametes fuse however, the young would likely be unviable or of low fitness.
Therefore, these smaller cheats create conflict, which could be resolved by allowing larger
gametes to only fuse with smaller gametes, thus leading to the establishment of two mating
types.
However, in my opinion, this explanation does not provide a full story of how sexes evolved
- but it does provide some good foundations and an example of conflict resolution.
So let's scrap that theory and move onto the theory which I believe holds a lot more
strength - the Hurst-Hamilton hypothesis.
Much like the previous scenario, the ancestral state has isogamous unisexual gametes in which
all gametes can fuse with all, in a haploid population, and like the previous scenario,
sexes evolve as a means to resolve conflict, in this case, genetic conflict.
You see, when gametes fuse, they both contribute the same amount of genetic material from the
nucleus due to the way meiosis and gamete fusion operates.
This, however, is not the case with organelles, like mitochondria, which are the powerhouses
of the cells, or the chloroplasts, which enable photosynthesis to occur.
Organelles have their own separate DNA, and their number within a cell is variable.
This means that when two gametes fuse to form a zygote, it is highly likely that one will
contribute more organelles than the other, and by extension, one gamete will be supplying
more genetic material than the other - and this leads to genetic conflict.
As is the fashion in evolution, it's only a matter of time before an organelle-destroyer
gene evolves, which would wipe out the organelles contributed by the other gamete, so that their
organelles and respective genetic material are the only ones passed onto the next generation.
And as you may imagine, this mutation would spread like wildfire as it would quickly eliminate
all organelles without said mutation.
However, by the time this mutation has reached fixation, it generates a new problem: when
two gametes which both carry the organelle-destroyer gene fuse, the organelles would wipe each
other out, which would be devastating for the zygote, so this would prompt the evolution
of a suppressor genes.
The suppressor gene would prevent an organelle from destroying organelles from the other
gamete, but would also make their own organelles prone to destruction.
The evolution of suppressor genes leads to a situation of heterozygote advantage, whereby
the fusion of a gamete with the suppressor gene and one without yields the highest fitness.
You see, the fusion of two gametes without suppressor genes would likely be lethal, whereas
the fusion of two gametes with suppressor genes would lead to more diverse organelles,
but the zygote also incurs the cost of suppression twice - so the ideal situation is for a gamete
with suppressor genes and one without to fuse.
And because the ideal state is for opposite types to mate, this then leads to the evolution
of a brand new "Chooser" gene!
The Chooser gene essentially ensures fusion only occurs with a gamete of the opposite
suppressor profile.
This is usually done through protein recognition, and also represents an additional cost to
the gamete, but as long as the benefits from ensuring a heterozygote advantage with each
fusion outweigh the costs of having a choosy gene - it will be worth it and the gene will
slowly become fixed in the population!
And the last and final step in this process is associated with the linkage between the
Choosy gene and the suppressor type gene - in this case linkage refers to the genes being
physically close within the organelle's DNA, which strengthens them being selected
together and therefore guarantees the fixation of both Chooser genes and the suppressor types.
The outcome of this is anisogamy, which means that the two mating types have a different
size.
In essence, the gamete with suppression will have its organelles destroyed by the opposite
type, so these won't be contributed to the zygote and will have the bare minimum of organelles
to ensure the gamete can find one of the opposite type.
This gamete is generally referred to as the male gamete, and are generally smaller in
size.
On the other hand, the female gamete is the one without the suppressor gene, and will
be passing on all of her organelles and cytoplasm to the zygote, and is thus, much larger in
size.
This actually matches what we observe in nature, as mitochondria are almost universally maternally
transmitted, and the female gamete contributes the majority of cytoplasmic resources to the
zygote.
You could almost claim this theory shares some parallelisms with the first one presented,
although the conflict occurs at the genetic level, and the difference in size reflects
the compensation with regards to the organelles contributed to the zygote.
Male gametes are generally smaller and cheaper to produce, but they also don't pass on
the genetic material from their organelles, whereas the female gametes contribute the
majority of the cytoplasm, which is costly to produce, but they also get to contribute
the entirety of their organelle genetic material to the next generation.
And boom - there you go, how sexes evolve!
Now, I'm aware there are two main criticisms that this theory may receive.
The first is, "well Inés, that theory is cool and all, but it's a theory out of at
least 8 others, how do we know this is how the sexes originated?" and the second is,
"if this theory explains why two sexes evolve by having two opposite binary types, then
how do you explain species with multiple sexes?"; and both of these are valid points.
I think an in-depth reply to both of these criticisms would require an entire video in
itself - so I will keep my answers brief.
The answer to the first question I believe to currently be a matter of personal opinion,
as different academics will hold different beliefs - after all, seven of them came up
with competing theories.
Personally, I believe the Hurst-Hamilton hypothesis provides the most plausible explanation based
on genetic conflict for the origin of the sexes, but doesn't always justify how two
sexes are maintained in all situations.
On the criticism of explaining multiple sexes, I actually think their existence is the exception
that confirms the rule, as they do lend support to the Hurst & Hamilton hypothesis.
Organisms with multiple mating types are relatively uncommon, and the vast majority are found
in different fungal species.
It appears that those species with multiple sexes either represent intermediate states
which are inherently unstable and will eventually collapse down to two sexes, or they are species
which have found alternative ways of resolving organelle genetic conflict, such that two
sexes would not be a requirement.
Nonetheless, regardless of their origin, without a doubt the origin of the sexes has been a
crucial contributor to the development and evolution of more complex life forms as we
know them today and is truly, a revolutionary biological event.
I hope you enjoyed this video - as always, I appreciate this is a more complicated topic
than usual, so feel free to ask any questions in the comment section below.
The other thing is… this video is part of a very exciting collaboration on the topic
of evolution - there are various other creators who have covered the evolution of homosexuality
(which you have requested many times in comments!), the evolution of facial expressions, selfies,
words and more - so go check that out in the linked playlist.
Likewise, if you've watched them all and you're in the mood for more evolutionary
biology, can I cheekily recommend this playlist which I'm aiming to expand over the next
year.
And as always, thank you so much for watching me - and I'll see you in the next one!
Bye!
[Translated into {Language} by {your name}]
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