Biology concepts – characteristics of animals, undulipodia, gametes, nematodes, roundworms,
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.
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.
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.
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.
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.
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.
a nice 2009 commentary. While nematologist Cobb was aware of this worm only from felt beer mats, in truth they live in rotting peaches, in book binding paste and in other places as well.
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:
Characteristics of animals –