Biology concepts – characteristics of animals, undulipodia,
gametes, nematodes, roundworms,
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Yes,
a sponge is an animal – just like a barracuda, a
platypus
or a that weird nephew of yours. They are
multicellular,
loosely organized into a couple tissues,
and
eat other organisms. You can see how they filter
feed
in this demonstration. Not so different from
that
nephew.
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Yes there are, thanks for asking. Animals are all eukaryotic
and diploid (2 of each chromosome). For the most part, our cells have nuclei
and organelles and all but our gametes have two copies of each chromosome.
Animals are all multicellular - a unicellular organism that
acts like an animal is still called a protist. Because they are multicellular,
animals have the capability to have cells of different types that organize
themselves into tissues and organs, like we have discussed before as a
characteristic of life.
Another attribute of animals is that they can move. True,
sponges are sessile when attached to rocks or coral reefs, but they do have
motile cells and motile life cycle stages. Birds are very motile unless dead.
One other thing animals have in common is how the male
gamete finds the female egg. Male gametes have a flagellum that allows them to
swim toward the chemical signals that show them where the egg is located. True,
we have learned that protists and some lower plants also have gametes that swim
with flagella or cilia, but animals characteristically have flagellated male
gametes. But of course, given the nature of this blog, there must be an
exception.
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Nematodes
are round worms; helminthes are just
one
group of parasitic roundworms. They represent a
turning
point in animal development. They sort of have
an
internal body cavity (they are pseudocoelomates),
they
sort of have a head (start of cephalization) and
they
sort of have body symmetry (starting to be
bilateral).
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In every other phylum of animals, male gametes use the
eukaryotic flagellum to swim their way to the egg. Using exactly the same
structure that we have talked about before, male gamete flagella have basal
bodies and axonemes made of microtubules. The microtubule filaments slide past
one another to produce their beating movement.
Look as hard as you want, but round worms don’t have basal
bodies or flagella. They do have centrioles and centrosomes used for mitosis,
but none of them mature into basal bodies for flagellar assembly, In fact, the
male gametes of nematodes carry one centrosome (with its centriole pair) to the
egg and form the basis of all centrioles in the baby roundworm. Weird - why no
basal bodies? – I have no idea, but evolution approved it.
Instead of flagella for male gamete swimming, nematodes use
an amoeboid movement to crawl to the
egg. O.K., so they crawl instead of swim. That’s exceptional, but is it really
that weird? Well… yes, considering that they don’t contain the most important
protein that most cells use to make amoeboid movements.
Actin is one of
the major proteins of the cytoskeleton.
Actin works mostly in protrusion and contraction of parts of the cell, while intermediate filaments hold the cell’s shape
and give it rigidity and microtubules
are primarily for movement of proteins and structures throughout the cell.
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This
short video shows you the movement of C.
elegans
male
gametes. They are shaped like typical animal male
gametes,
they don’t move like typical male gametes, and
they
don’t have the same proteins as typical ones. Yet, the
nematode
is the most numerous type of animal on Earth.
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So amoeboid cells use F-actin as the way they extends and
retracts its pseudopodia. Thus, they crawls along. Nematodes do have cells with G-
and F-actin, but the male gametes don’t have any (or very little). But
it’s the male gametes that need it to move! What gives?
Instead, male gametes of nematodes use the MSP protein (major sperx protein; my
posts get blocked by schools if I use the whole word, so I use gamete whenever possible). A 2014 study shows that MSP
proteins are abundant in the male gamete (40% of total soluble protein), and
change its distribution and volume as the gamete matures and is activated. When
fully activated and in the female oviduct, the MSP of the male gamete assembles and creates
pseudopodia just as actin would in any other amoeboid cell. Another 2014 study shows how it then senses the egg.
Does the inability of nematode male gametes to swim to the
egg cost them in terms of reproductive advantage and evolution? Heck no.
Nematodes, ie. roundworms, are the most
successful animals on Earth. They live inside every other living thing, and just
about everywhere on Earth. There are free living worms, parasitic worms, and worms
that eat decaying tissue. There are roundworms that eat nothing but other roundworms.
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Nematodes
are famous for the parasitic infections they
cause.
On the left is a root knot worm. Nematodes are
responsible
for more than 15% of crop loss each year.
On
the top is one of the filarial worms that cause river
blindness.
On the bottom is a grasshopper worm (Mermis
nigrescens) that grows to fill the
entire body cavity.
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Sure there are many species, but that number is dwarfed by
the number of individuals of some species. One 2013 study from England gives us
a clue. In just the city of Bristol, dogs drop about four tons of doo-doo each
day. That four tons holds an astounding 3.7 billion Toxicara eggs. Every two
days the dogs of that one city squat out the equivalent of the human population
of the entire world. Man, is that a bizarre visual.
This isn’t useless information, considering that the eggs become worms that can
cause blindness in people who accidentally eat contaminated dirt, or those who eat dirt on
purpose for that matter. Indeed, many nematodes are parasites of humans and
cause much disease, but this isn't our focus today. If you like that sort of weird
disease stuff (and I most certainly do), I suggest you Google ascariasis, hookworm,
onchocerciasis, strongyloidiasis, filariasis,
or trichinosis.
Nathan Cobb of the U.S. Bureau of Plant Industry gave a very
apt description of the numbers and distribution of nematodes in 1915. He said
that if you eliminated every bit of matter on Earth other than nematodes, an onlooker could still recognize our world.
There are enough nematodes in the dirt that we could
distinguish mountains and valleys. There are more in the cities, so we would
know where they had been. Nematodes are numerous enough in living things that
we could identify where every living thing had once stood. And yes, the onlooker
could see humans, we ingest billions over our lifetime and more than two
billion people are infected with Ascaris
lumbricoides at any one moment.
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At
least 2 billion people are infected with this worm
(Ascaris lumbricoides) at any one time.
The pictures of
the
infection are just too gruesome, so I show his smile
instead.
Look can up the pictures for yourself if you
haven’t
eaten recently.
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The smallest nematodes
are in marine sediment. Desmoscolex sp. and Greeffiella sp. are only 80 µm long, which means that if 30 of them stood on each other’s shoulders, they would only be as tall as
a dime is thick.
At the other end of the spectrum, the largest known nematode
is Placentonema gigantissima, which
can reach around 30 feet long in the placenta of its host, the sperm whale.
The placenta of a whale, tree root balls, in water, mud,
fruit, nematodes literally live everywhere except in the skies – even though
they do find themselves in the sky every day - inside birds. Roundworms have
been found in the crevices of South African gold mines two miles below the
Earth’s surface – at 48˚ C (118.5˚ F) and 1000x atmospheric pressure. No other
animal has been found living in stone at these depths and conditions.
Many roundworms live in the soil, and perhaps the greatest
number live in the sediment of ocean floors. Because there are some many
different kinds of nematodes, it isn’t surprising that many have very developed
specific niches.
Biologist Colin Tudge stated in his book, The Variety of Life that half the animal
species on Earth have a nematode that lives only
in that species. Even beyond animal hosts, there is evidence of a nematode
species that lives one place on Earth – in the felt of German beer coasters.
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The
German beer mat worm doesn’t just
live on the
bottoms
of beer soaked coasters. But they do like yeast
for
dinner. I like this one for Apostelbrau in Germany
because
the brewery has been located in Worms, a city
between
Frankfort and Stuttgart, since 1713.
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Nematodes can be political was well. The giant kidney worm, Dioctophyma renale, is found in many
different mammal species, such as dogs, cats, minks, humans, etc. But the
infection is almost always just in the right kidney. Since this worm is usually
ingested via contaminated fish, the right kidney might be more
susceptible simply because it's closer to the liver and stomach
– or maybe they’re Democrats.
All this talk about undulipodia and nematodes has been
perhaps a little misleading. Nematodes do have cilia on a very small subset of
the their neurons, but they aren’t motile cilia. These are sensory cilia, also
called primary cilia. They are our topic for next week.
For
more information or classroom activities, see:
Nematodes
–
Characteristics
of animals –
Cytoskeleton
–
