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This is not an organism. At least, it’s not yet anyway. What we are looking at are a lot of
zoospores packed together, getting ready to be released so that they can find
the right place to grow into themselves. And this? Well, this is a male tardigrade
filled from head to many toes with sperm. Just chock full of it.
We did not start this channel with the intent of becoming so familiar with tardigrade mating
habits, but tardigrades are easy to find, and their mating habits are straightforward enough
to track, so we keep getting drawn back into the details and the drama of tardigrade reproduction.
In the past, we’ve talked about the particulars of their mating habits. And today, we’re going to
use this tardigrade as a convenient starting point to talk about the history of our understanding of
sperm. Because as absurd as it might seem to be looking at tardigrade sperm, the theories
people have had about sperm are much weirder.
Humans have been theorizing about the contents of seminal fluid long before the invention of the microscope.
We did not, as a species, need to work out all of the scientific mechanisms underlying sexual reproduction to make babies.
But the observation of children who carried traits from their parents naturally raised questions
about how those traits were passed on. And those questions inevitably led
to some hypotheses. And look, not all hypotheses
need to be good. In fact, many of them are terrible. It’s just that when you’re trying to
understand the world, you need to start somewhere, and as long as you’re willing to work with your
ideas and challenge them, then hopefully some day, you go from somewhere, to somewhere better.
Some of these early ideas about sperm weren’t just bad though, they were, maybe as we should expect,
a little self-serving to the male ego. Masturbatory, even, you could say.
Like the theory now known as spermism, advocated for by one Pythagorus, yes, that Pythagorus.
According to spermism, the information driving heredity came entirely through the semen.
Whether that was the color of someone’s hair or their height or whatever other trait—
all of that came through the father, and the father’s father, and so on. The mother’s role
was just to provide nourishment to the sperm as it developed and manifested those traits.
It’s easy enough to find flaws with this idea. After all, our observed reality, which Pythagorus
just chose to ignore, shows us that traits pass from mother and father alike. But this would not
be the last theory to overestimate the sperm. Even with the advance of scientific tools over
the ages, there was plenty to get wrong. One of those key scientific tools was, of course,
the microscope, which allowed scientists to observe the cells within semen for the first time.
But just because microscopists could see sperm now, that did not mean they could make out the
details. And this invited speculation, including the idea that we are essentially developed from
a much smaller version of ourselves. This theory was called preformationism.
At the end of the 17th century, the Dutch scientist Nicolaas Hartsoeker sketched out
his spermist vision of preformationism: inside the head of the sperm cell, he speculated, was a very,
very, very small human--later called a homunculus--just waiting for a womb to grow in.
And look, we have the advantage of centuries of advancements to know that the homunculus
was wrong. But maybe take a moment to think about how weird that would be if it were right.
Look at this tardigrade, full of sperm. And then just imagine if we zoomed in further on
those sperm cells, and we could see little tiny tardigrades inside of them. And then,
if we could zoom in even more on those tinier tardigrades, we’d see even more tinier
tardigrades inside of them, and so on and so on. Now, to be clear, that is not what tardigrade
sperm looks like. Their sperm is made up of a slightly globular head connected to a flagellum.
But as wrong as both spermism and tiny people inside of sperm turned out to be, they’re not that
far off the mark if you think about the zoospores we showed at the beginning of the episode.
They are a tool of asexual reproduction in a lot of different organisms, including algae,
fungi, and bacteria. They all use zoospores in different ways.
And because we don’t know exactly what organism these zoospores come from, there are specific
details of their lives we just don’t know. But take, for example, the Phytophthora,
a group of oomycetes most notorious for their role in the 19th century potato blight
that led to the Great Famine in Ireland. Phytophthora can reproduce sexually, but they mostly rely
on making asexual zoospores, which pack together in a biological enclosure called the sporangia
until the conditions are right for their release. And when they emerge from the sporangia,
the zoospores start swimming, using their flagella and chemical sensors
to find the ideal place to settle and germinate. All of the hereditary information in those
zoospores came from one parent, fashioning an organism that resembles the infant form of that
parent, and that seeks only nourishment from its new host. The outlines of that
process sound pretty similar to how early philosophers and scientists envisioned their
sperm-centric ideas of sexual reproduction. But what those scientists missed—what
separates the sperm from the zoospore—is the nature of sexual reproduction itself,
of the genetic variation it drives by combining and recombining traits from different parents.
Sperm cannot accomplish that on their own. Sperm needs an ovum to fertilize,
which scientists would eventually uncover in the 19th century with experiments on rabbits,
sea urchins, and starfish eggs that helped to further our understanding of the molecular and
cellular collaboration that set the stage for inheritance. Yes, my friends, as Napoleon drove
to conquer Europe, we had no idea that sperm and eggs came together to make the next generation.
There are a lot of different types of spores out there, used for both sexual
and asexual reproduction. And in contrast, sperm might seem more narrow and focused,
both in diversity and purpose. But the function of sperm is so specialized
that it requires its own diversity. Even among tardigrades, there are different morphologies to
the head and middle structures of the sperm, small differences that distinguish between species.
And that’s because however weird our theories about sperm have been,
the reality is also very strange. Of all the cells in the body, sperm cells
are maybe the only ones that are expected to navigate and survive in two different beings.
The homunculus was wrong, but the idea of there being an image of our life passed on within
sperm and in spores, it isn’t absurd at all. It’s just too literal. Instead,
that image is abstracted and tucked away into genetic information. And zoospores and sperm
alike are just packets of that information, swimming off to new and different futures.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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