Death is inevitable...at least for the individual.
Whether you’re getting consumed, or your environment poisons you, or you fall
apart as time marches on, we die.
But our genes, fellow traveler, our genes often survive.
We living things...we reproduce.
And while humans, with our experience of plants and animals, are of course most familiar
with sexual reproduction, that is absolutely not the only way it can be done.
By this point on your journey, you should already know that nothing is usual in the
microcosmos.
Most unicellular reproduction is asexual, meaning one organism, on its own, can reproduce itself.
The most common form of asexual cellular reproduction, and the one you are most likely to
know about, in the microcosmos, is binary fission.
One organism simply divides into two.
Unicellular organisms of course do not have genders, but by custom, the dividing cell
is called the “mother” while the cells formed after division are called “daughters.”
This is a Heliozoa, an amoeboid cell with stiff arms called axopodia radiating from
its spherical body.
Here it undergoes binary fission to create two daughter cells.
Watch as these two future daughters move in opposing directions with the help of axopodia
until the cytoplasmic bridge that was formed between them is finally cut off.
Now, which of these two cells is the original and which is the offspring?
In the microcosmos, that’s not always how it works.
Both of these Heliozoa are daughters, and the mother has ceased existing...either that,
or mother is now...both of them.
And, if that is the case, every single-celled organism is, itself, the same organism that
has continued surviving since the very first cell reproduced itself billions of years ago.
But, like, we don’t want to hurt your brain too early in the episode, so it’s probably
best not to think about it too much.
The process of binary fission happens fairly quickly, all things considered, but of course
we have to speed up the tape a bit for you to enjoy it.
Here a photosynthetic flagellate called Euglena deses is dividing.
The nucleus, however, has already divided, and each of these halves has its own control
center, even though the cytoplasm is still joined, which makes you wonder...are these two separate
organisms now?
Or are they only individuals once their membranes separate?
Now, we’re pretty sure this organism is Cyphoderia, but it’s a little hard to be
sure, especially during the reproduction process.
But it’s definitey some kind of Testate Ameoba, which are ameoba that build shells
for themselves to live in, and that protect them from predators and, just, the world,
which can be a rough place.
You might think that they, like snails or clams, build these shells around themselves after
they are born, and that is true from some Testate Ameobas.
But not for this one.
During cell division, Cyphoderia actually builds a second shell and then squirts new
cell material into that shell to create its daughters.
Ciliates like Paramecium and Stentor actually have two types of nuclei; macronucleus and
a micronucleus.
The macronucleus controls the non-reproductive cell functions, like eating, movement, and
digestion and the micronucleus is necessary for reproduction.
These nuclei actually divide differently during reproduction.
And for certain cilliates, the micro-nucleus plays a truly bizarre role...but to talk about
that, we need to move away from asexual reproduction.
Which, so far, is all we’ve talked about.
And it’s fine, we guess.
But in all of these cases each daughter cell is a clone of the mother.
In sexual reproduction, genes can flow much more easily through populations, which has
a huge evolutionary advantage.
But sexual reproduction in the microcosmos is not like sexual reproduction for the rest
of us.
Conjugation requires two cells.
First two ciliates become attached to each other by their oral surface.
Micronuclei in the ciliates then divide several times in each cell.
And then one micronucleus from each cell moves into the other cell and fuses with the micronucleus
there.
The macronuclei of both cells then dissolves into the cytoplasm.
Each cell now has a brand new genome.
It is as if you gave someone a kiss, and walked away a genetically different person.
There are no children...the gene transfer happens to the existing organism.
Now, after separation, the cells produce new micro and macro-nuclei and then reproduce
asexually until each daughter cell has the correct number of macro and mirco nuclei for
their species.
Conjugation is thus considered a sexual phenomenon which is immediately followed by repeated
asexual reproduction.
This can be hard to get your brain around because, to us, the exchange of genetic information
is so deeply tied with the creation of new offspring.
But in the microcosmos, you can see, genetic exchange is possible without reproduction.
If I were to tell you that this is only the very surface of the iceberg of complexity
of reproduction in the microcosmos, I hope that, by this point, you would believe me, because it is.
These beautiful bags of chemicals have to continue existing, and they have done so elegantly
using strategies that to us might seem counter-intuitive,
but are nevertheless the simplest and most effective ways for keeping their genes alive.
And it’s hard to fault them for it...it’s been working for them for billions of years
while you and I, fellow traveller, have only been here for a blink.
So, thank you for coming on this journey with us as we explore the unseen world, that surrounds us.
If you want to see more from our master of microscopes, James, check out Jam and Germs
on instagram, and if you want to see more from us, there’s always a subscribe button
somewhere nearby.