TOEFL Quiz 9: Spiders

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I'm here to spread the word about the magnificence
of spiders and how much we can learn from them.
Spiders are truly global citizens. You can find
spiders in nearly every terrestrial habitat. This
red dot marks the Great Basin of North America,
and I'm involved with an alpine biodiversity
project there with some collaborators. Here's one
of our field sites, and just to give you a sense
of perspective, this little blue smudge here,
that's one of my collaborators. This is a rugged
and barren landscape, yet there are quite a few
spiders here. Turning rocks over revealed this
crab spider grappling with a beetle.

Spiders are not just everywhere, but they're
extremely diverse. There are over 40,000 described
species of spiders. To put that number into
perspective, here's a graph comparing the 40,000
species of spiders to the 400 species of primates.
There are two orders of magnitude more spiders
than primates. Spiders are also extremely old. On
the bottom here, this is the geologic timescale,
and the numbers on it indicate millions of years
from the present, so the zero here, that would be
today. So what this figure shows is that spiders
date back to almost 380 million years. To put that
into perspective, this red vertical bar here marks
the divergence time of humans from chimpanzees, a
mere seven million years ago.

All spiders make silk at some point in their life.
Most spiders use copious amounts of silk, and silk
is essential to their survival and reproduction.
Even fossil spiders can make silk, as we can see
from this impression of a spinneret on this fossil
spider. So this means that both spiders and spider
silk have been around for 380 million years. It
doesn't take long from working with spiders to
start noticing how essential silk is to just about
every aspect of their life. Spiders use silk for
many purposes, including the trailing safety
dragline, wrapping eggs for reproduction,
protective retreats and catching prey.

There are many kinds of spider silk. For example,
this garden spider can make seven different kinds
of silks. When you look at this orb web, you're
actually seeing many types of silk fibers. The
frame and radii of this web is made up of one type
of silk, while the capture spiral is a composite
of two different silks: the filament and the
sticky droplet. How does an individual spider make
so many kinds of silk? To answer that, you have to
look a lot closer at the spinneret region of a
spider. So silk comes out of the spinnerets, and
for those of us spider silk biologists, this is
what we call the "business end" of the spider.
(Laughter) We spend long days ... Hey! Don't
laugh. That's my life. (Laughter) We spend long
days and nights staring at this part of the
spider. And this is what we see. You can see
multiple fibers coming out of the spinnerets,
because each spinneret has many spigots on it.
Each of these silk fibers exits from the spigot,
and if you were to trace the fiber back into the
spider, what you would find is that each spigot
connects to its own individual silk gland. A silk
gland kind of looks like a sac with a lot of silk
proteins stuck inside. So if you ever have the
opportunity to dissect an orb-web-weaving spider,
and I hope you do, what you would find is a bounty
of beautiful, translucent silk glands.

Inside each spider, there are hundreds of silk
glands, sometimes thousands. These can be grouped
into seven categories. They differ by size, shape,
and sometimes even color. In an orb-web-weaving
spider, you can find seven types of silk glands,
and what I have depicted here in this picture,
let's start at the one o'clock position, there's
tubuliform silk glands, which are used to make the
outer silk of an egg sac. There's the aggregate
and flagelliform silk glands which combine to make
the sticky capture spiral of an orb web. Pyriform
silk glands make the attachment cement -- that's
the silk that's used to adhere silk lines to a
substrate. There's also aciniform silk, which is
used to wrap prey. Minor ampullate silk is used in
web construction. And the most studied silk line
of them all: major ampullate silk. This is the
silk that's used to make the frame and radii of an
orb web, and also the safety trailing dragline.

But what, exactly, is spider silk? Spider silk is
almost entirely protein. Nearly all of these
proteins can be explained by a single gene family,
so this means that the diversity of silk types we
see today is encoded by one gene family, so
presumably the original spider ancestor made one
kind of silk, and over the last 380 million years,
that one silk gene has duplicated and then
diverged, specialized, over and over and over
again, to get the large variety of flavors of
spider silks that we have today. There are several
features that all these silks have in common. They
all have a common design, such as they're all very
long -- they're sort of outlandishly long compared
to other proteins. They're very repetitive, and
they're very rich in the amino acids glycine and
alanine. To give you an idea of what a spider silk
protein looks like, this is a dragline silk
protein, it's just a portion of it, from the black
widow spider. This is the kind of sequence that I
love looking at day and night. (Laughter)

So what you're seeing here is the one letter
abbreviation for amino acids, and I've colored in
the glycines with green, and the alanines in red,
and so you can see it's just a lot of G's and A's.
You can also see that there's a lot of short
sequence motifs that repeat over and over and over
again, so for example there's a lot of what we
call polyalanines, or iterated A's, AAAAA. There's
GGQ. There's GGY. You can think of these short
motifs that repeat over and over again as words,
and these words occur in sentences. So for example
this would be one sentence, and you would get this
sort of green region and the red polyalanine, that
repeats over and over and over again, and you can
have that hundreds and hundreds and hundreds of
times within an individual silk molecule.

Silks made by the same spider can have
dramatically different repeat sequences. At the
top of the screen, you're seeing the repeat unit
from the dragline silk of a garden argiope spider.
It's short. And on the bottom, this is the repeat
sequence for the egg case, or tubuliform silk
protein, for the exact same spider. And you can
see how dramatically different these silk proteins
are -- so this is sort of the beauty of the
diversification of the spider silk gene family.
You can see that the repeat units differ in
length. They also differ in sequence. So I've
colored in the glycines again in green, alanine in
red, and the serines, the letter S, in purple. And
you can see that the top repeat unit can be
explained almost entirely by green and red, and
the bottom repeat unit has a substantial amount of
purple. What silk biologists do is we try to
relate these sequences, these amino acid
sequences, to the mechanical properties of the
silk fibers.

Now, it's really convenient that spiders use their
silk completely outside their body. This makes
testing spider silk really, really easy to do in
the laboratory, because we're actually, you know,
testing it in air that's exactly the environment
that spiders are using their silk proteins. So
this makes quantifying silk properties by methods
such as tensile testing, which is basically, you
know, tugging on one end of the fiber, very
amenable. Here are stress-strain curves generated
by tensile testing five fibers made by the same
spider. So what you can see here is that the five
fibers have different behaviors. Specifically, if
you look on the vertical axis, that's stress. If
you look at the maximum stress value for each of
these fibers, you can see that there's a lot of
variation, and in fact dragline, or major
ampullate silk, is the strongest of these fibers.
We think that's because the dragline silk, which
is used to make the frame and radii for a web,
needs to be very strong.

On the other hand, if you were to look at strain -
- this is how much a fiber can be extended -- if
you look at the maximum value here, again, there's
a lot of variation and the clear winner is
flagelliform, or the capture spiral filament. In
fact, this flagelliform fiber can actually stretch
over twice its original length. So silk fibers
vary in their strength and also their
extensibility. In the case of the capture spiral,
it needs to be so stretchy to absorb the impact of
flying prey. If it wasn't able to stretch so much,
then basically when an insect hit the web, it
would just trampoline right off of it. So if the
web was made entirely out of dragline silk, an
insect is very likely to just bounce right off.
But by having really, really stretchy capture
spiral silk, the web is actually able to absorb
the impact of that intercepted prey.

There's quite a bit of variation within the fibers
that an individual spider can make. We call that
the tool kit of a spider. That's what the spider
has to interact with their environment. But how
about variation among spider species, so looking
at one type of silk and looking at different
species of spiders? This is an area that's largely
unexplored but here's a little bit of data I can
show you. This is the comparison of the toughness
of the dragline spilk spun by 21 species of
spiders. Some of them are orb-weaving spiders and
some of them are non-orb-weaving spiders. It's
been hypothesized that orb-weaving spiders, like
this argiope here, should have the toughest
dragline silks because they must intercept flying
prey. What you see here on this toughness graph is
the higher the black dot is on the graph, the
higher the toughness.

The 21 species are indicated here by this
phylogeny, this evolutionary tree, that shows
their genetic relationships, and I've colored in
yellow the orb-web-weaving spiders. If you look
right here at the two red arrows, they point to
the toughness values for the draglines of nephila
clavipes and araneus diadematus. These are the two
species of spiders for which the vast majority of
time and money on synthetic spider silk research
has been to replicate their dragline silk
proteins. Yet, their draglines are not the
toughest. In fact, the toughest dragline in this
survey is this one right here in this white
region, a non orb-web-weaving spider. This is the
dragline spun by scytodes, the spitting spider.
Scytodes doesn't use a web at all to catch prey.
Instead, scytodes sort of lurks around and waits
for prey to get close to it, and then immobilizes
prey by spraying a silk-like venom onto that
insect. Think of hunting with silly string. That's
how scytodes forages. We don't really know why
scytodes needs such a tough dragline, but it's
unexpected results like this that make bio-
prospecting so exciting and worthwhile. It frees
us from the constraints of our imagination.

Now I'm going to mark on the toughness values for
nylon fiber, bombyx -- or domesticated silkworm
silk -- wool, Kevlar, and carbon fibers. And what
you can see is that nearly all the spider
draglines surpass them. It's the combination of
strength, extensibility and toughness that makes
spider silk so special, and that has attracted the
attention of biomimeticists, so people that turn
to nature to try to find new solutions. And the
strength, extensibility and toughness of spider
silks combined with the fact that silks do not
elicit an immune response, have attracted a lot of
interest in the use of spider silks in biomedical
applications, for example, as a component of
artificial tendons, for serving as guides to
regrow nerves, and for scaffolds for tissue
growth.

Spider silks also have a lot of potential for
their anti-ballistic capabilities. Silks could be
incorporated into body and equipment armor that
would be more lightweight and flexible than any
armor available today. In addition to these
biomimetic applications of spider silks,
personally, I find studying spider silks just
fascinating in and of itself. I love when I'm in
the laboratory, a new spider silk sequence comes
in. That's just the best. (Laughter) It's like the
spiders are sharing an ancient secret with me, and
that's why I'm going to spend the rest of my life
studying spider silk. The next time you see a
spider web, please, pause and look a little
closer. You'll be seeing one of the most high-
performance materials known to man. To borrow from
the writings of a spider named Charlotte, silk is
terrific.

Thank you. (Applause)

(Applause)