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Recently, we did an episode about sperm, and in it we talked about the awkward history
of how, at times, humans have overestimated the role of sperm.
It is true that sperm is essential to human reproduction.
But the spermist school of thought maintained that the sperm was where all of the important
information encoding our traits was located.
And the egg, well…the egg was just there to provide some nourishment.
That point of view was wrong about how genetic information gets passed along.
But also…it really just underestimated the engineering marvel that is the egg.
Now, it’s easy to focus on eggs in sexual reproduction because for humans and many other
animals, the fertilization of the egg by a sperm is the key step that drives the development
of a new embryo.
And the meeting of sperm and egg is an incredibly valuable one, allowing organisms to mix and
match genes between individuals so that the population can better evolve and adapt.
But not all sperm/egg pairings introduce variation.
For example, some flatworms are hermaphroditic, and under certain conditions, they might fertilize
their own eggs.
In one species, that process involves injecting sperm directly into their own head.
But also, while all sperm needs an egg, not all eggs need sperm.
There are quite a few organisms, like many gastrotrichs and rotifers, that reproduce
by parthenogenesis.
In these species, the female makes an unfertilized egg that develops and eventually hatches into
a miniature version of its singular parent.
Both self-fertilization and parthenogenesis come along with the advantage that you do
not have to find a mate, you can just go ahead and get your own egg going.
And that’s great if you want to grow a large population or there just aren’t very many
viable mating options around, but it does come at the cost of genetic diversity.
As an egg develops and the cells within it divide and differentiate, there is a flurry
of activity to support them.
Developing an organism is an involved process.
It’s not like you have all your materials laid out in front of you, and all that’s
left is to assemble those pieces.
It’s more like starting out with one brick and being told to make more bricks from it.
And as you make more bricks from those bricks, you have to also make them more and more specialized
until you get, like, a castle.
Now, coming up with a comparison inevitably sells the complexity of the whole process
short.
In rotifer eggs, scientists have found an uptick in genes involved with cell proliferation,
adhesion, and communication—all of which makes sense because there is so much coordination
needed to make sure that as cells divide, they’re talking to each other and assembling
themselves correctly into one of the most complex structures the universe has ever seen,
a living, multicellular organism.
Now, in addition to everything going on inside the egg, there’s everything going on in
the world around it.
An egg that has been released into the microcosmos is subject to the environmental whims of its
surroundings, which makes it all the more remarkable how many animals produce eggs that
are crafted to wait and survive those conditions for long periods of time.
Tardigrades, for example, can produce eggs capable of withstanding extreme heat and extreme
cold, as well as doses of radiation that would kill a human being.
Those eggs however are different from the ones that are currently on your screen, which
are hydrated and thus not able to survive those extremes.
They’re only good at surviving these wild circumstances when they’ve been desiccated,
or dried out.
Those hardy eggs are called dormant or “resting” eggs, which makes them sound inert.
And that’s kind of the point: the eggs are inactive.
Their metabolism essentially shuts down, as does synthesis of any new DNA, RNA, or proteins.
What the eggs take with them, going into that state is all that they have to survive, like
an internal desert island.
But it takes a lot of work to make that inactivity possible.
There are a lot of animals that produce dormant eggs, and the conditions that drive that dormancy
and the way they emerge from it vary according to that animals’ lifestyle and environment.
For many rotifer species, resting eggs are the result of sexual reproduction6.
And an important thing for us to note as you look at these rotifer eggs: there are morphological
features specific to rotifer resting eggs, but we are not familiar enough with those
traits to be able to make the call on whether or not the eggs we’re looking at are resting
or non-resting.
Now, if we had some decades, we might be able to tell you because that’s how long some
resting eggs can survive.
Some have even hatched after over 100 years of rest, but alas we definitely do not have
that long to wait.
Even if we’re having a hard time telling resting and non-resting eggs apart from the
outside, they are very different on the inside.
These protections can be straightforward, like an abundance of proteins that protect
the egg from heat and oxidative stress.
But on top of protecting cells from external factors, the resting egg has to be protected
from itself.
In animals, apoptosis--or programmed cell death-- is a necessary part of our survival.
It helps our bodies clear out cells that are no longer healthy or necessary.
Without it, you and I would have already died of cancer.
But a cell in the dessicated, dormant environment of the resting egg is not going to look very healthy.
So some animals like Artemia have devised molecular strategies that interfere with the
pathways that lead to apoptosis so that the cell can be left alive
And alongside the protections are stores of messenger RNA, also kept at rest and stabilized
while the egg is in stasis.
Those stores of RNA contain the instructions the egg needs to move quickly when it emerges
from its resting state.
Because, after decades or even a century of waiting, once it’s time to get going, it’s
TIME TO GET GOING and those instructions will allow the egg to jump start the synthesis
of the proteins that it will need to grow and divide and bring new life back to the
microcosmos wherever they have ended up.
Thanks for coming on this journey with us as we explore the unseen world that surrounds us.
We would like to say thank you again to Skillshare for supporting this video.
Did looking at those eggs make you hungry?
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