Your screen is currently filling up with clouds of an algae that resembles a unicellular flagellate
known as Chlamydomonas, though we’re not quite sure that that’s definitely what it
is.
The algae are swimming towards the lit part of the stage, and as they amass, they form
a blanket over the slide that looks like a kind of very festive green television static.
Meanwhile, on the more orderly end of things, we have this Volvox.
It’s much larger than the tiny algae we just saw, and it’s also much more serene.
Just a nice chill green ball in the middle of your screen.
It doesn’t seem to have much in common with the Chalmydomonas.
I mean, yes, they’ve got some green in common.
But that’s not particularly difficult to come by in the microcosmos.
But these two organisms are both part of the same family of algae called volvocine algae.
And when you look at a more extended family portrait, maybe you will start to see the
resemblance.
You’ve got your gonium, looking like several of those little green algae stuck together.
And then there’s this active pandorina, also packing together several green cells
in its colony body as it moves around.
And then we see the Eudorina….and then the pleodorina, which looks increasingly like
the orderly sphere of the Volvox In fact, this combination of footage feels
less like a family portrait and more like a series of yearbook photos taken from kindergarten
through senior year.
Except instead of the striking passage of time documented through the changes in a child’s
face, this is the striking passage of multicellular evolution documented through the changes in
one algal family.
Multicellularity is one of those things that is so entwined with notions of complexity
and the forward marching of evolution that it might feel like it has to be one of those
singular evolutionary events: a moment that occurred once or maybe twice, and was maybe
too difficult to ever replicate again.
But it turns out that making the jump from unicellular to multicellular life has actually
happened quite a few times in our planet’s history.
It’s maybe surprisingly not that difficult a transition.
But finding a way to study it is.
Many of these jumps happened too long ago, and evolution has removed the organisms whose
bodies contain the story of how they or their ancestors went from living on their own to
teaming up with others.
But fortunately, the volvocine algae went about this transition much more recently--likely
around 200 million years ago.
And that leaves us with a gradient of cellular complexity that starts with the unicellular
Chlamydomonas, transitions to the colonial gatherings of cells that make up the gonium
through pleodorina, and then leads us to the multicellular Volvox.
That ordering though is a little misleading in its simplicity.
Evolution isn’t nearly so neat to work in a straight line across a whole family.
But laying out the organisms in this way helps us to understand that steps are involved in
taking the species from unicellular to multicellular.
One of the most apparent differences when we compare those single-celled algae to Volvox
is how much bigger the Volvox is.
The world is made up of large unicellular organisms and small multicellular organisms.
But in this family, there is an increase in size that is connected to how many cells an
individual contains.
So, how to do that?
Well, volvocine algae primarily reproduce asexually.
But where many microbes divide through binary fission--dividing into two daughter cells
that then grow and divide again--most volvocine algae are a little more chaotic.
They divide through multiple fission, which means that any cell that’s going to reproduce
starts by growing...a lot.
If they want two daughter cells, they grow twice as big.
If they want to make four, they grow four times as large.
And once they get as big as they need to, the algae goes through a rapid series of divisions
to produce all those daughter cells in quick succession.
For Chlamydomonas, you might get around 4 - 8 daughter cells.
A gonium colony might have 8 - 16.
And with Volvox, you can end up with thousands.
And more than just the sheer increase in numbers is what the daughter cells do when they are
newly made.
In the Chlamydomonas algae, they separate completely through a process called cytokinesis,
becoming their own individual self.
But with the colonial algae and on up, that cytokinesis is incomplete, leaving the cells
connected.
Those connections are further supported by an extracellular matrix made up of glycoproteins.
And as the number of cells increased through evolution, so too did the volume of that matrix.
At certain points in a Volvox’s life cycle, that transparent scaffolding can make up 99%
of its volume.
But multicellular is more than just getting big.
The big step to taking the volvocine algae from colony to multicellular life involves
putting all those cells to work--specialized work.
You can see the early days of specialization in the colonial algae through the development
of organismal polarity, which is a fancy way of saying that we can define a front and back
of the colony.
The cells in front are generally large with big eyespots.
And as you go to the back, the cells and eyespots get smaller.
These differences create a gradient not just in characteristics, but in function.
The larger eyespots in front mean those cells will be more sensitive to light, making them
responsible for directing the colony.
With the Volvox, this specialization transforms into a full division labor that you can actually
see reflected in the cells themselves.
Its globe-like shape features thousands of tiny cells dotting the exterior, all orbiting
a few large cells held in the middle.
Those are two distinct cell types: the thousands of cells on the outside are somatic cells,
decorated with flagella to help the organism move around.
And there may be thousands of these somatic cells, but they can’t do the most important
thing an organism needs to do, which is reproduce.
Instead, that role is given to those large cells called gonidia.
At some point in the Volvox’s life, those gonidia will each divide to produce a new
Volvox, which is essential to keeping the Volvox lineage going.
The one thing the gonidia can’t do is swim.
They don’t have the flagella for it.
And that’s why they need somatic cells.
It is a full division of labor, one that you’ll find in many forms of multicellular organisms
all over the planet.
But why?
Why go through all of this?
Well, there’s a lot of possible answers.
But one of the simplest answers is probably captured by this tardigrade idly grazing on
algae, looking a bit more like a “water cow” than a “water bear” here.
This Chlamydomonas algae does not stand a chance.
But you know who does stand a chance?
A pandorina.
It’s simply too large to get into the tardigrade’s mouth.
And in this moment, hundreds of millions of years of evolution--of cells dividing, of
being held together through incomplete cytokinesis, of biologically defining front and back--all
of that comes together to save this little guy’s life.
Thank you for coming on this journey with us as we explore the unseen world that surrounds
us.
We have some very exciting news to share with you!
You know that unseen world I just mentioned?
Well we want to help you see that world and share it with us and with others!
We’ve worked together with our Master of Microscopes James Weiss to design a microscope
with content creation in mind, and you can now help us make that microscope through a
Kickstarter.
We want people to be able to easily film, photograph, and share their discoveries with
the world, so we are creating a microscope designed to interface with the amazing cameras
that we all have in our pockets these days.
We’re going to be doing this through a Kickstarter campaign so if you would like to help us make
these microscopes a reality and get one of your own you’ll need to visit the link in
the description and pledge during the dates you’re seeing on your screen right now.
We’re also creating some materials to help you in your journey into becoming a microscopist
because, of course, using the microscope is one part, but also you’ve got collecting
samples, keeping them healthy, identifying what you’re seeing, and we want to help
with all of that!
Finding the right microscope can be complicated and expensive, and we want to help with that
so it’s as easy as possible for you to start your own journey.
If you’re interested in picking one up or if you just want to know more details about
the microscope itself, you can head on over to the link in the description.
Thank you as always to all of the people on screen right now.
They are our patrons on Patreon.
And they are as always the ones who have enabled us to go on this journey.
So, if you like it, these are the people to thank.
If you want to see more from our Master of Microscopes James Weiss, check out Jam & Germs
on Instagram.
And if you want to see more from us, there is always a subscribe button somewhere nearby.