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When it comes to making more of themselves, single-celled eukaryotes—or protists—have a few options.
They can go the asexual reproduction route, dividing and making nearly exact copies of
themselves.
And this is a pretty nice way of life.
The euglenoids have taken to it so strongly that they’ve never been observed reproducing
any other way.
I mean, think of the advantages.
You don’t have to waste time trying to find a mate.
And when the process is done, the end result is more of yourself.
Now, that might sound narcissistic, but when it comes to survival, it’s also pragmatic.
If your genes work, why not just make more of them?
But the world is always changing and testing the organisms within it.
Temperatures can become uncomfortable, nutrients can become scarce, and enemies can become
plentiful.
Adapting to these changing conditions requires change in the organism and its progeny.
That change can come about in many different ways and manifest in many different forms,
but one of the tried and true methods is sexual reproduction.
The principle seems straightforward enough, whether you’re talking about protists or
plants or people.
You take two parents, mix and match their genes, and produce a set of offspring with
the hopes that they will better survive the hardships both the present and the future.
Of course, in practice, sexual reproduction takes on many different forms, some of which
we’ve documented and discussed already on this channel.
Today we’re going to focus on one particular method of ciliate reproduction: my personal
favorite, conjugation.
We’ve actually shown ciliates like these mid-coitus before.
Sometimes they seem to be frantically chasing each other like these two are.
And sometimes, they’re more chill, like two blobs that are just very, very…close.
To explain what makes this specifically conjugation, we need to first take a moment to explain
a different kind of sexual reproduction.
For some protists, like Chlamydomonas, sexual reproduction is a fusion of two gamete cells.
It will spend much of its life as an asexual cell, reproducing through binary fission,
or just splitting in two.
But when the environment gets tough, it will turn into a gamete that belongs in one of
two mating types.
When a gamete finds its opposite mating type, the two fuse to create a zygote that then
divides to create the progeny.
Conjugation, in contrast, is not the fusion of two gamete cells.
It is the fusion of gametic nuclei.
What is that?
Well, when you watch ciliates mid-conjugation, you are watching two individuals that have
fused together, but it’s a temporary connection forged by a mutual desire for the other’s nuclei.
Now, ciliates have two different kinds of nuclei: the macronucleus and the micronucleus.
The macronucleus works like our own, housing the genetic template that gets transcribed
and translated into the molecules the organism needs for its daily life.
But in conjugation, the micronucleus becomes the dominant player, dividing through meiosis
to create a series of pronuclei that contain a single set of the organism’s genetic material—though
all but one of the pronuclei will get promptly destroyed.
The one that remains divides by mitosis so that the organism now has two identical copies
of the pronuclei: one for itself, and one for its partner.
In the meantime, their original macronucleus will fall apart, which would normally be catastrophic.
But in this case, it is deliberate.
The two ciliates will then exchange one of their pronuclei with the other through their
connected mouths, and each will fuse their pronucleus with their partner’s to create
a zygotic nucleus.
This new nucleus combines and rearranges the genetic material inherited from both parents,
replacing the old macronucleus with this new remixed version.
So instead of having sex and producing offspring, these two ciliates have sex and BECOME the
offspring.
They are both GENETICALLY DIFFERENT after the conjugation than they were before.
When they’re done, the conjugants separate, and to pass those new genes on to another
generation, the organisms will have to eventually divide, producing daughter cells that now
carry their own copies of this new nucleus.
So what sets the mood for conjugation?
Well, it can be a few things.
For Paramecium tetraurelia, it can be a matter of age.
Scientists have documented that Paramecium tetraurelia can only survive around 200 rounds
of asexual reproduction unless it undergoes self-fertilization or conjugation to patch
up the genetic damage that can accumulate over multiple rounds of division.
Scientists have also observed that a lack of nutrients can set other organisms off to
conjugate.
But of course, environmental or aging considerations aside, the most vital step is just finding
a partner.
Think of how difficult this must be for microbes.
They’ve got to navigate a world of debris and predators without sound or sight or any
of the large scale mating displays you often find in the macro world.
And even if you do find someone to mate with, they might be a bit occupied.
Like these two hypotrich ciliates that are trying so hard to rudely interrupt a couple
that is already mid-conjugation.
They are expending so much effort, though the solution to their predicament seems so
obvious: they should just channel that energy into conjugating with each other.
So why are they so preoccupied with a couple that’s already taken?
Well, for one possible answer, let’s look at a different ciliate: the Blepharisma.
Within a few hours of being starved, sexually mature Blepharisma will pair up and begin
conjugation.
But the process is more than just stumbling around the microcosmos.
Blepharisma come in two mating types, called mating-type I and mating-type II, which will
only mate with the opposite type.
And to find their partners, Blepharisma rely on chemical signals.
When Type I Blepharisma are starved, they begin to secrete a chemical called gamone
1, which alerts any nearby Type II Blepharisma to the fact that there are some Type I’s
waiting around and interested.
The Type II cells transform and release a chemical called…yes…gamone 2, which then
finds its way to the Type I cells.
These gamones do more than help the different mating types find each other, they act upon
them to get them ready for conjugation, kind of like microscopic flirting with a hidden
chemical language.
For our poor unloved hypotrich ciliates, what’s likely happened is that they are the same
mating type as each other, drawn to the same location because of the secretions of one
of the conjugating pair.
But alas, they both showed up too late to the party, and they’ve remained too chemically
preoccupied with the object of their conjugational affections to divert their attentions to each other.
So while conjugation may be advantageous for the species, it doesn’t always work out
so well for the individuals.
But as always with biology, the underlying mechanisms are often more mysterious than
the simple image we originally lay out.
Blepharisma and other ciliates with mating types can sometimes spontaneously switch their
mating type, though we don’t fully understand how or why this happens.
This is but one moment in these hypotrich’s attempt at conjugation, who knows what the
next one will look like And as for the conjugating pair, well, we
wish them and their progeny the best.
Because when the camera is gone and the day is done, these two organisms may never see
each other again.
But they’ve shared this moment and their nuclei, and they’ve made the microcosmos
all the richer for it.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
Patrick Olson is a composer and musician and science publisher who recently released his
album Music for Scientists.
An album made in tribute to those that have dedicated their lives to science-drive work.
And the music itself is inspired by the beauty of science and the knowableness of the universe.
One track on the album is called ‘The Idea’.
The song touches on how difficult it is to form correct ideas, and how for every /right/
answer, there are an infinite number of wrong ones.
The music video for that track was produced with the help of artist Jon Todd and the team
over at ThoughtCafe and it's a creative hybrid of analog painting and artificial intellegence.
If you think this sounds like something you might enjoy, click the link in the description
to see the video or stream the music on all major music services.
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If you want to see more from our Master of Microscopes James Weiss, you can check out
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