So, what is the point of sex?
I'm sure that some, if not all of you have some ideas.
Maybe you’re thinking up a number of original, ingenious witticisms to answer my question with.
And why not?
In the millennia upon millennia of human sexuality, the act itself has become no less fascinating
or complicated or fraught.
It has shaped our societies and informed our art.
And yes, it can also be absurd and it can make you laugh.
We made a whole episode about tardigrade sex and there were a non-zero number of puns.
While not all sex is reproductive, today, we’re going to focus on sexual reproduction.
What's important to remember in that context is that sexual reproduction is not just some
mechanical process between individuals, it is also cellular.
And while that can get lost in the mating habits of metazoans, protozoans--or single-celled
eukaryotes--contain many of the keys to understanding how sexual reproduction came to be.
Cuz it’s important to remember that sex was an evolutionary innovation, a thing that at
one point, did not happen and then began to happen, and the advantages of that caused
sexual reproduction to be carried over throughout the eons up until today.
But the evolutionary history of sexual reproduction is complicated and it is mysterious.
And for an illustration of why that is the case, we're going to turn to amoebas, those
seemingly simple and sexless eukaryotic blobs.
Because, it turns out that amoebas aren't actually that simple.
Or even that sexless.
Amoebas are single-celled eukaryotes, united by the use of pseudopodia (or "false feet")
that they use to move around and eat.
They're distributed around various eukaryotic lineages, and we've talked about just how
tangled the taxonomic classification of amoebas has become in a previous episode.
And for a large part of our scientific understanding and our popular imagination, amoebas have
been decidedly asexual.
In 1932, The New Yorker published a poem called "Ode to the Amoeba" composed by Arthur Guiterman,
a writer known for his humor.
In the poem, he writes:
Now, there are a lot of scientific details in there that we could nitpick.
But Guiterman's poem helpfully and delightfully lays out what has long been the pervading
image of amoebas: an ancient being that has sustained its own individual identity by dividing
into clonal daughter cells.
Those daughter cells continue the process, propagating into other identical amoebas or
serving as the evolutionary basis for other species.
That image of the amoeba as primitive organism is itself linked to what seems like an equally
primitive process: asexual reproduction.
After all, if the existential goal of any organism is to propagate their genetic information,
then division seems simple.
It's fast, convenient, and your new clones should contain 100% of your genome.
But there's a pretty big evolutionary limitation.
With more and more divisions, an asexual organism will start to rack up harmful mutations that
will eventually kill off the species, this process has a name.
It’s called Muller's ratchet and one way to avoid this outcome is to take in foreign
DNA, a strategy that bacteria and archaea pursue in their own recombination processes.
Eukaryotes, as we'll talk about, have sex.
Muller's ratchet poses a conundrum for our understanding of amoebas.
If the theory holds, then any asexual lineage of organisms shouldn't last very long.
So how do we square that with the image of amoebas as an ancient line of organisms?
The answer is simple: we weren’t wrong about Muller’s ratchet.
We were wrong about amoebas.
Let's start by defining sexual reproduction at the cellular level, turning to a 2011 paper
with the wonderful title:
In this work, the authors define sex as "the presence of a meiotic reduction of the genome
complement followed eventually by karyogamy."
That is, ya know, for us, not the set of words that we might use to describe sex.
What it means is that sexual reproduction involves two key processes: meiosis and nuclear fusion.
In meiosis, a cell divides, distributing its genetic material between its progeny cells
and reducing the number of chromosome sets each cell contains.
After those progeny cells find their mate, the next step is karyogamy.
In this process, the cells fuse all the way down to their nucleus, thus increasing the
number of chromosome sets and providing the basis for a whole new member of the family.
In plants and animals, that new cell will then divide and divide and develop into a
multicellular organism.
But in protozoans, that new cell is the organism.
Like the recombination methods employed by bacteria, sexual reproduction provides a mechanism
for genetic variability, avoiding Muller's ratchet.
But think of how complicated it must have been to evolve this process, which is really
the culmination of several processes.
That altering of the number of chromosomes, the meiosis, the nuclear fusion--not to mention
the identification of which organism to mate with.
Meiosis itself is so complex that it is hypothesized to have evolved a single time in the history
of life on earth.
Based on genetic evidence, that last eukaryotic common ancestor--that mysterious evolutionary
predecessor to amoebas and all other eukaryotes--relied on sexual reproduction.
So if we know that amoebas must be descended from a sexual microbe, and that the odds are
low on asexual amoebas lasting for long, then why has this image of the asexual amoeba persisted
for so long?
This misunderstanding is a product of our own observational challenges when it comes
to microbes.
The fact is that the gold standard in confirming sex is the ability to actually catch the organisms
in the act.
But that poses some clear problems with amoebas and some other microbes.
First, and most obvious: they are tiny.
They can also be difficult to cultivate in lab conditions, particularly in circumstances
that might lead to sex.
The slime mold Dictyostelium, for example, was thought to be asexual until the 1970s,
when the scientists realized their samples were just waiting for the right mate to show up.
Now, aside from direct observation, we now have developed genetic methods to determine
whether an organism might engage in sexual reproduction.
This was the case for Cochliopodium, an amoeba that was long considered parasexual because
it followed some of the steps required for sexual reproduction, but it had never been
observed going through meiosis.
Instead, in cochliopodium, multiple cells fuse together to make a big cell with several
polyploid nuclei.
This mass then breaks up to create cells with only one nuclei.
So just imagine six people fused together into a ball of goo and then once again break
apart into six more individual people, now all the children of those original six.
It’s a little more complicated than the way we do it, but I’m not here to judge.
But in studying Cochliopodium further, researchers found that they do have meiosis genes, meaning
they probably do reproduce sexually, we’ve just never been able to see it yet.
Scientists have also found extensive genetic evidence of sex in the Amoebozoa group of
amoebas, further challenging the assumption that amoebas are asexual.
And this introduces a strange challenge.
We can't assume that just because we haven't observed any particular species having sex
that it must therefore be strictly asexual because that’s clearly not the case.
But, at the same time, we cannot assume that all eukaryotic life engages in sexual reproduction
because to do so would mean blinding ourselves to other evolutionary innovations to avoid
Muller's ratchet.
If there are strictly asexual amoebas, then that's a trait they evolved further away from
their ancestors, perhaps finding an alternative to sex that can negate some of its downsides,
like needing to find a good mate or risking the transmission of disease.
And they wouldn't be alone.
Bdelloid rotifers are multicellular, and also asexual.
Instead of relying on sexual reproduction for recombination, they can acquire foreign
DNA during their resting anhydrobiosis state, which is wild!
Some scientists have hypothesized that asexual amoeba might have their own strategies, like
relying on having many many chromosomes to reduce the harm of any mutation.
So what’s the point of sex?
Well, if we look at amoeba, it seems to be both simple and unclear.
Sex is a way to create genetic variability, but it is not the only option.
Amoebas employ a wide range of sexual and asexual life cycles, some of which we've been
able to observe and understand.
Some of which we have not.
Arthur Guiterman wrote another poem, called appropriately enough "Sex."
It begins:
We appreciate that Guiterman was seemingly as preoccupied with the reproductive habits
of amoebas as we are.
Of course, now we know that the science underlying this stanza might not be quite correct, that
amoebas were not the beginners of sex but rather the product of it.
Just like sex itself is not just a driver of evolution, but a result of it as well.
And that the loss of sex might also be an evolutionary strategy.
If anything, looking at sex through this lens seems like a reminder that the amoebas at
the start--the ones we might desognate "primitive"--are often much more complex than we think.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
If you’re glad that there’s a channel on youtube that is able to dive into the weird
mating habits of amoeba, well the people on screen the folks you want to thank for that.
They are our patrons on Patreon.
And if you want to join them, You can check us out at patreon.com/journeytomicro.
If you want to see more from our Master of Microscopes James Weiss, check out Jam & Germs
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And if you want to see more from us, we are at youtube.com/microcosmos