Wednesday, January 28, 2015

Crawling To The Top

Biology concepts – characteristics of animals, undulipodia, gametes, nematodes, roundworms,

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.
Sponges and birds – they’re both animals, but would you know it to look at them? Sponges are sessile (except for the exceptions), and birds can’t breathe under water (no exceptions). Birds eat worms and lay eggs – most people don’t know what the heck sponges do. Yet they’re both animals. Are there characteristics that all animals have in common?

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.

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
The roundworms, phylum Nematoda, are our exception for the day. Their male gametes can’t swim! But who cares, it hasn’t seemed to slow them down. Which makes us ask why everyone else goes to the trouble of producing gametes with flagella – it’s expensive. Shall we investigate?

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.

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.
Actin comes on a two main forms. G-actin is the globular form; it's the monomer. When the monomers are induced to form filaments, like tubulin monomers monomers form microtubules, it's now called F-actin. Quick assembly and disassembly of F-actin polymers from G-actin monomers allows for movement of selected parts of the cell membrane.

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.

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.
In strictly numerical terms, it’s amazing that we aren’t nematodes. In truth, four out of every five animals are Earth are roundworms! Long ago in classification, all the roundworms used to be lumped together; later on they were grouped according to head size. With the advent of molecular typing, there are more than 25,000 species, and estimates are for more than a million. Compare that to 5000 known mammal species.

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.
A. lumbricoides is the largest nematode by mass which infects humans. Females can be 40 cm long and the diameter of a No. 2 pencil. The worm takes quite the tour through you. From your stomach to your liver to your lung move the young larva. You cough them up and swallow them, and they mature in the gut. There they grow, fall in love, and mate. Yep—pretty gross.

The smallest nematodes are in marine sediment. Desmoscolex sp. and Greeffiella sp. are only 80 ┬Ám 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.

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.
The German beer mat nematode, Panagrellus redivivus, was first named Chaos redivivum by none other than Linnaeus himself. Its story is told in 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.

Smith HE (2014). Nematode sperm motility. WormBook : the online review of C. elegans biology, 1-15 PMID: 24715710

H. Ferris (2009). The beer mat nematode, Panagrellus The beer mat nematode, Panagrellus redivivus: A study of the connectedness of scientific discovery J. Nematode Morphol. Syst., 12 (1), 19-25

McKnight, K., Hoang, H., Prasain, J., Brown, N., Vibbert, J., Hollister, K., Moore, R., Ragains, J., Reese, J., & Miller, M. (2014). Neurosensory Perception of Environmental Cues Modulates Sperm Motility Critical for Fertilization Science, 344 (6185), 754-757 DOI: 10.1126/science.1250598

Morgan, E., Azam, D., & Pegler, K. (2013). Quantifying sources of environmental contamination with Toxocara spp. eggs Veterinary Parasitology, 193 (4), 390-397 DOI: 10.1016/j.vetpar.2012.12.034

Sepsenwol S, Ris H, & Roberts TM (1989). A unique cytoskeleton associated with crawling in the amoeboid sperm of the nematode, Ascaris suum. The Journal of cell biology, 108 (1), 55-66 PMID: 2910878

For more information or classroom activities, see:

Nematodes –

Characteristics of animals –

Cytoskeleton –

Wednesday, January 21, 2015

Evolving A Second Job

Biology concepts – protein moonlighting, undulipodia, evolution, basal body, centriole, GAPDH, intraflagellar transport

Today’s post is on a multitasking cell structure. This
would make Alton Brown proud, since he hates tools
that do only one thing. The University of Miami of
Florida football team runs through fire extinguisher
blasts when they enter the stadium – maybe Alton
can find a second use for his.
Alton Brown from Food Network hates a unitasker. He wants all his kitchen tools to have more than one function – I least I think it’s just his kitchen tools. But he might just as well be talking about biology. Nature hates a unitasker, that’s why some many things in our cells have multiple jobs.

This phenomenon is called protein moonlighting. The re-evaluation of the human genome (about 19,000 genes) suggests that many proteins have more than one distinct function. This would allow for a relatively small number of genes to provide a large functional proteome (the total number of protein functions).  As such, a 2014 study is showing the importance of moonlighting proteins in health and biology.

There are rules for a protein to have a legitimate second job. It’s only moonlighting if the two functions are unrelated,  the functions can't be carried out by two different domains of the protein either. This would suggest a gene fusion event. The two functions must be independent, so ablating one doesn’t affect the other.

There are hundreds of examples of moonlighting proteins in the literature now, and more are sure to follow. There are examples aplenty within the glycolysis pathway; you know, the breakdown of sugar for energy. No fewer than seven of the ten glycolytic enzymes are known to have other jobs.

Crystallin proteins (alpha and beta) make up the
majority of the lens and cornea of the eye. They are
transparent, but they do more than that. Recent
studies show that they have enzymatic activity in other
tissues. Aldehyde dehydrogenase and transketolase are
enzymes that turn out to be moonlight crystallins.
The king of the moonlighting proteins is glyceraldehyde -3-phosphate dehydrogenase (GAPDH). Sure, it's one of the enzymes that breaks glucose down to pyruvate, but it does so much more – like helping to maintain the ends of our chromosomes (telomeres), working to move tRNAs out of the nucleus, controlling the expression of some genes, especially those involved with gamma-interferon, repairing our DNA when it is damaged. And apparently GAPDH is crucial for helping cells to bring in particles from outside (endocytosis). That’s a full day.

But as amazing as GAPDH is, today’s example of a multitasker is even more rare, in that the moonlighter is a complete structure, made of many proteins, and has two distinctly different jobs. What’s more, each function has its own exceptions. What's our structure of interest? The basal body, or perhaps I should call it the centriole.

Let’s talk about the basal body first. This is the base of the eukaryotic undulipodia (cilia and flagella). These moving tails come in two parts; the basal body and the axoneme. We talked at length about the axoneme a few months ago, with its nine doublet microtubules surrounding two singlet microtubules (9[2]+2, see picture below). Undulipodia movement, as opposed to the motor driven prokaryotic flagella, is achieved by sliding the different doublets forward and back past one another.

But in those previous posts we didn’t talk much about the basal body. It too is a ring of microtubules, although these are shorter polymers that in the axoneme. Instead of doublets, there are almost always nine sets of triplet microtubules, and there are no center microtubules (9[3]+0). Of course, there were a couple of exceptions, and we talked about them.

The basal body has a 9(3) + 0 structure, while the
axoneme is 9(2) + 0. While this cartoon shows the
complexity of the axoneme, our post today highlights
the complexity of the basal body. One thing this cartoon
does show, the axoneme is sheathed in the plasma
membrane, it isn’t a protein structure sticking out through
the membrane.
The basal body is about 100 nm wide and 150 nm long, and serves as the base of the flagellum or cilium. If a cell has hundreds of cilia, like the male gametes of the cycads we talked about last week, then it has hundreds of basal bodies as well – one per cilium.

The basal body serves as the nucleation site for mictrotubule growth into the axoneme. It’s like a skyscraper, the microtubule girders are built vertically on the basal body foundation; only here, the basal body sparks a self-assembly of the microtubules. You don’t need fearless guys climbing the beams to build an axoneme.

If we turn our attention to the centriole, we find that it’s used in mitosis. When a cell divides, each progeny cell receives one centrosome. Don’t confuse the centrosome with a centriole or a centromere (a near center point of a chromosome, it holds the two chromatids together). The centrosome is more of an area, it contains two centrioles, a mother and a daughter, and the pericentriolar matrix (PCM) amorphous group of proteins that help the centrioles do their job.

Each centriole is a complex microtubule formation in a 9(3)+0 arrangement, about 120 nm wide and 175 nm long. This is exactly the structure of the basal body – they’re the same thing! Well, almost.... a centriole has to mature into a basal body.

During S phase of the cell cycle (when the chromosome are replicated) the centrosome will duplicate. Each centriole grows another one from its side, at a right angle. The mother is a mother again, and the daughter becomes a mother for the first time. The two mother/daughter pairs then gather their own PCMs and move to the sides of the nucleus. When mitosis time comes, microtubules grow toward the chromsosomes, but from the mother centriole only. This is the spindle and will help pull the chromatids apart during cell division.

Start at the top left. During the cycle, a pair of centrioles
will separate and each will grow another from the proximal
end. The daughter centrioles then have to mature, with
proteins added toward the distal end. During mitosis, they
two pairs separate and form the spindle body, which pulls
the chromatids apart.
If you remove the centrioles after S phase, most cells can still go through mitosis just fine. In fact, there many cell types that don’t have centrioles at all (higher plants, some protists, most fungi). Even in cells that are supposed to have centrosomes, destroying centrioles with a laser after S phase doesn’t always affect mitosis negatively.

A commentary published in 2010 talks about how many cell types, including some oocytes can undergo meiosis without centrioles, while centriole numbers than are re-established after fertilization. However, in other tissues, loss of centrioles leads to genetic instability over time, even if the spindle will develop without the centrosome. To this point, we still haven’t resolved the issue of whether centrioles are necessary for proper cell division.

In the vast majority of cases, centrioles come from centrioles. One serves as a template to form the second. The process through which this occurs has just begun to be revealed in the past few years. There is a linking fiber from the proximal end of the mother centriole that acts as a seed point to start aggregation of the daughter centriole at a 90˚ angle to the first. 

The process is very complicated, but is carried out spontaneously, without specific gene products to guide it. It was first believed that if centrioles were lost, then they could not be regained. Of course, they also thought that centrioles had their own DNA. Now we know that in cases where centrioles are lost, they can form de novo, and function just fine in mitosis or as basal bodies. In several studies, removal of centrioles or cells without centrioles to begin with allows for new centrioles being formed from aggregated microtubules.

Intraflagellar transport is the method by which the
axoneme grows, adding tubulin monomers to the end.
The proteins are carried up the axoneme by walking
proteins. These are important not for just cilia assembly,
but also for cell signaling and sensing that occurs in
the undulipodia.
After centriole duplication by prescribed pathways, the basal body matures. A 2011 review shows that there are many proteins involved in the process. First the distal end of the centriole is capped, then it migrates to the cell membrane and docks. A transition zone is produced that allows for selective movement of molecules up and down the inside of the axoneme (intraflagellar transport, IFT).

Finally, there is attachment of accessory bodies like rootlets to anchor it to the membrane, transition fibers to move to the axoneme and distal appendages. Only after all this maturation of the basal body can the axoneme be built by IFT. This is how the process happens in all species EXCEPT fruit fly male gametes and the microbe Plasmodium yoelii, where the axoneme grows BEFORE plasma membrane docking of the basal body. 

What’s more, you can always reverse it and go from basal body back to centriole, so their functions must somewhat overlap. The basal body and centriole both serve as microtubule organizing centers (MTOCs). So what makes it a moonlighting structure?

Prokaryotic flagella just whip around in a circle, but
eukaryotic undulipodia have more of a beating motion.
The side view shows how the flow is one direction, while
the top view indicates that the motion is circular, but not
like a propeller.
The basal body does a different job by controling the direction of movement of the cilium or flagellum! The prokaryotic flagellar motor spins the prokaryotic flagella as we have described, but eukaryotic cilia or flagella beat, rather than flop around, and the direction in which they beat is important. Cilia on a cell beat in concert in one direction, and the basal body is asymmetrical enough to drive this directional beating. Believe it or not, this is controlled by a protein called disheveled.

That’s a little weird, but what’s really weird is the kids. In a 1991 paper, part of the oviduct of a quail was reversed, so that the cilia beat toward the ovary instead of toward the uterus. When cells divided through embryo and chick development, the cilia of the progeny cells had the same orientation as those in the parent! They continued to beat in the wrong direction. Since basal bodies are duplicated from basal bodies, the mother basal bodies gave rise to daughters that beat the same direction.

The evidence presented shows that we have an ancient, yet complex, structure that has a couple of important jobs. The question for evolutionary biologists is which came first, the basal body or the centriole? The last common eukaryotic ancestor (LECA), the cell from which all eukaryotic cells descend, had undulipodia, so the basal body is an extremely old structure. But all eukaryotic cells undergo mitosis or meiosis, so the centriole must be important too.

Notice the cilia that are the upper most in the animation.
They show best the coordinated beating that pushes fluid
in one direction.

Cells without either centrioles and basal bodies suggest that basal body function came first; since you can’t have undulipodia without basal bodies, but our discussion above shows you can have mitosis without centrioles.

On the other hand, centrioles have to mature to become basal bodies, wouldn’t this suggest that they came first? Or perhaps the basal body was the primary product and nature managed to find a use for one if its precursors. What do you think, is the basal body the chicken or the egg?

Next week let’s look at one of the animal kingdoms great exceptions in terms of cilia, even though primitive and higher animals have them, round worms don’t cilia or flagella!

Henderson, B., & Martin, A. (2014). Protein moonlighting: a new factor in biology and medicine Biochemical Society Transactions, 42 (6), 1671-1678 DOI: 10.1042/BST20140273

Kobayashi, T., & Dynlacht, B. (2011). Regulating the transition from centriole to basal body The Journal of Cell Biology, 193 (3), 435-444 DOI: 10.1083/jcb.201101005

Debec, A., Sullivan, W., & Bettencourt-Dias, M. (2010). Centrioles: active players or passengers during mitosis? Cellular and Molecular Life Sciences, 67 (13), 2173-2194 DOI: 10.1007/s00018-010-0323-9

Boisvieux-Ulrich E, & Sandoz D (1991). Determination of ciliary polarity precedes differentiation in the epithelial cells of quail oviduct. Biology of the cell / under the auspices of the European Cell Biology Organization, 72 (1-2), 3-14 PMID: 1756309

For more information or classroom activities, see:

Protein moonlighting –

Centriole –

Basal body –

Centrosome –

Wednesday, January 14, 2015

Everybody In The Gene Pool - Plants That Swim

Biology concepts – botany, taxonomy, alternation of generations, cycad, gametophyte, sporophyte, gametes, motility, ginkgo, archegonium, antheridium

In the LOTR The Two Towers we find the tree herders
that can move on their own despite being plants. Today’s
exception is a version of this, if on a much smaller scale.
There are a few types of trees that have motile parts; they
don’t rely on wind, gravity, insects or anything else to
move from one place to another.
Plants are divided up into many categories, and few people agree completely on the groupings. I’ve got a new grouping – swimming plants versus non-swimming plants! It’s as good as anyone else has come up with – let’s investigate the plants that can move on their own.

Plants are most often called by their binomial system names (genus, species), but this is just naming, not categorizing in a large way. Animals, protists, fungi, etc. are all divided into different phyla based on their similarities ad differences. But for some reason (I hope there’s a reason) botany uses divisions instead of phyla.

There are 10-12 divisions of plants (we’ll use 10), covering everything from the mosses to the flowers. Compare that to 21 phyla of animals and it seems like the plants will be easier to classify – but not so fast.

Group plants according to various characteristics and you start to muddy the waters. You might divide them up according to whether they have vascular tissue or not. Non-vascular plants are short because they don’t have vessels to move water very high – these are the Anthocerotophyta (hornworts), the Bryophyta (mosses), and the Marchantiophyta (liverworts). Together they are called the Bryophytes.

The vascular plants are all seven of the other divisions – the Lycopodiophyta (the spikemosses and clubmosses), Pteridophyta (ferns and horsetails), Coniferophyta (conifers), Cycadophyta (cycads), Ginkgophyta (just one species, Ginkgo biloba), the Gnetophyta (a weird group), and Angiospermae (flowering plants).  The conifers, cycads, ginkgo and gnetophytes are often grouped together as the gymnosperms – you’ve probably heard of them.

On the other hand, you might divide the plants up into the seed plants and non-seed plants. In that case, you lump the club mosses and the ferns with the bryophytes, since they all reproduce using spores, not seeds.

Every plant alternates between versions of itself that are
diploid (sporophyte) and haploid (gametophyte). Different
types of plants spend different amounts of time as one or
the other. Mosses and other bryophytes are almost always
in their gametophyte state, while trees are sporophytes with
microgametophytes (antheridium and archegonium) located
in their flowers only.
Lest you think that all those divisions that are seed plants are the same, you can divide them all differently based on whether their seeds are naked (the gymnosperms) or those with fruit (parts of the ovary or accessory organs that overgrow – all in the angiosperms).  Or you could divide them on the basis of how many leaves the embryonic plants have; monocots have one while dicots have two. Angiosperms play by these rules, but gymnosperm seeds don’t, their embryos may have none, one, two, or dozens of cotyledons (embryonic leaves).

Here’s one classification method you may not have heard before – gametophyte dominant vs. sporophyte dominant plants. This has to do with the cycle of life of plants.

Every plant has two lives. Part of its life is spent as a haploid gametophyte (produces haploid gametes) while another plant of the species is a diploid sporophyte.  The sporophyte produces spores that grow into the gametophyte, then the gametophyte produces gametes that join together during fertilization to become a new sporophyte.

Some plant types (like bryophytes) exist mostly in the gametophyte stage and are therefore called gametophyte dominant. Other plants (like trees and flowers) spend all there time as sporophytes and only small parts of them become gametophytes (like pollen or cones).

One way you shouldn’t classify plants is based on their movement. Sure, some plants can grow in a certain direction, toward or away from some stimulus (tropisms, see this post), but plants aren’t motile. They don’t pick up and move themselves from one place to another under their own power.

The Himalayan Balsam is an invasive viny flower that has
become a problem in Europe and is invading the US as well.
Their seed pods contain fins under great strain when fill
with water. The slightest provocation will cause them to
explode, sending seeds more than 25 ft (7.5 m).
Plants also have many ways of moving their seeds and this is sort of a way of plants moving, but I think it’s cheating a bit. Seed dispersal mechanisms can rely on the wind (maples, dandelions) or water (cranberries, coconuts). They can use animals that eat them (many), or just grab ride on them (devil’s claws), or they can burst out (peas) or be shaken out (poppies) and use gravity. But this isn’t really a plant moving by it’s own power.

Plants disperse seeds as new plants, but they also disperse their male gametes in order to find the egg on another plant of their species. You have to get the pollen of seed plants (containing male gametes) or the male gametes themselves to the egg. Like seeds, pollen grains can be moved by insect, by wind, by rain, etc. These are the ways most plants get their male gametes to the egg in order to create a new plant, either in a seed or without a seed.

But there are exceptions, and this is weird exception. Some plants have male gametes that are motile. They swim to the egg! No big deal for animals, they pretty much all have motile male gametes (we’ll look at the exceptions to that), but it’s quite the stunner in plants.

Who knew SpongeBob cartoons were science lesson. Plankton
is green, a phytoplankton – an algae to be precise. Those two
antennae? Probably his two flagella, the way he would swim
around. The feet and the single eye-not so scientific.
Algae are probably the ancestors of all land plants, and we know their gametes have flagella for swimming from our post on them. But some land plants retain this method of male gamete dispersal, but they do include some weird twists. The plants that have motile male gametes cross many of the classes that we described above. There are some seed plants and some spore plants, some vascular and some non-vascular, some are gametophyte dominant and a few are sporophyte dominant – but no flowering plants do this.

Bryophyte males gametes are swimmers. The mosses, liverworts and hornworts  live close to the ground and must have standing water for the make gamete to reach the female gametophyte and egg. The haploid gametophytes are the moss that we usually see. The antheridium grow on the top of the male plants to produce male gametes, while the archegonium on the female plant tip produces the ovule with the egg. When the water is high enough the antheridium releases the male gametes and they swim to the egg using two flagella. The sporophyte (diploid) plant grows from the top of the female gametophyte.

Ferns, horsetails, and club mosses are taller than mosses because they are vascular, but they still require water for their male gametes to swim to the egg. The gametpophyte is a heart shaped leaf that lies near the ground. At one spot the archegonium grows the egg, while the male gametes in the antheridium grow nearby. Water resting on the leaf allows the male gametes to swim across the leaf to the egg (or from leaf to leaf). The sporophyte grows from the heart-shaped gametophyte and is the fern we usually think of.

Cycads (left) are gymnosperms whose trunk are formed from
the bases of the leaves as they grow and are lost. There are
about 300 species of cycads known, with several added every
year. The Ginkgo biloba is the only extant species of the
division. Because its wood is insect resistant, some trees may
be over 2500 years old.
Angiosperms and other seed plants use pollen to send the male gametes to the ovule. When the pollen reaches the archegonium, a tube grows into the ovule and to the egg. The male gamete cells are carried along by the pollen tube right to the front door of the egg. This is when fertilization occurs.  But not all seed plants work this way. A few of the gymnosperms still use a modified version of swimming to the egg.

Cycads (about 300 species) and the lone extant ginkgo, Ginkgo biloba, do have pollen grains that represent the male gametophyte plant. They get blow or carried to the female gametophyte cone and then it gets weird.

The ovule produces a drop of liquid that sticks into the air. The pollen gets caught in this drop and then the drop and the pollen are pulled back into the ovule. The pollen tube grows into the female reproductive organ, but not right to the egg. When the pollen tube reaches the entrance of the archegonium, it ruptures and the gametes are released into a watery fluid that surrounds the eggs.

Cycad and ginkgo male gametes move on their own, the only
exceptions in the seed plants. They have hundreds to thousands
of cilia, as opposed to flagella in bryophyte gametes, which
pull the cell forward. Ginkgo male gametes are huge (0.3 mm),
larger than an entire Wolffia globosa plant.
The male gametes have about a thousand of cilia (not flagella) that pull the cell through the watery environment inside the ovule toward the egg. Fusion and fertilization occurs when the male gametes find the egg – as always. The cycad and ginkgo male gametes swim, but they swim in the indoor pool, not out in the old swimmin’ hole like the bryophytes.

The male gametes of cycads and ferns are very different, from where they swim, to their relative sizes – ginkgo male gametes are HUGE compared to those of ferns, to the use of thousands of cilia as opposed to a couple of flagella.  However, research shows that they are remarkable similar in structure and function.

A 2006 study looked at the proteins involved in gamete movement in ferns and ginkgo. Their results indicated that most of the proteins in both were homologous enough that it suggested a direct descent from bryophyte to gymnosperm, not a case of parallel evolution.

In Guam, there is a neurologic disease that looks a lot like
Alzheimer’s. Research in 2004 found that it was actually
coming from cycad trees. Here’s how it happens. Cyanobacteria
live in the tree roots and put the toxin BMAA into the tree
tissues. Bats eat the fruits and people eat the bats. Than BMAA in
the brain causes disease.
One more cycad exception while we’re here. The cycads can do something that I thought was reserved only for philodendrons and a few relatives. These thermogenic plants can raise the temperature of their male cones by several degrees when the pollen is mature. A 2013 study showed that they can raise the temperature of male cones 2-15 degrees above ambient temperature.

This is believed to attract more insects as pollen distributors, and the researchers did find that more insects visited the plant when the temperature was increased. The mechanism may involve volatilizing more attracting chemicals though the added heat, which would then attract more pollinators (usually weevils).  Pretty advanced for a plant with a so-called primitive reproduction mechanism.

Next week – the base of the undulipodia has a special story all its own. Is it another instance of bacteria evolving into one of our organelles? And it has two very different jobs – which came first?

Suinyuy, T., Donaldson, J., & Johnson, S. (2013). Patterns of odour emission, thermogenesis and pollinator activity in cones of an African cycad: what mechanisms apply? Annals of Botany, 112 (5), 891-902 DOI: 10.1093/aob/mct159

Vaughn, K., & Renzaglia, K. (2006). Structural and immunocytochemical characterization of the Ginkgo biloba L. sperm motility apparatus Protoplasma, 227 (2-4), 165-173 DOI: 10.1007/s00709-005-0141-3

Murch, S., Cox, P., & Banack, S. (2004). A mechanism for slow release of biomagnified cyanobacterial neurotoxins and neurodegenerative disease in Guam Proceedings of the National Academy of Sciences, 101 (33), 12228-12231 DOI: 10.1073/pnas.0404926101

For more information or classroom activities, see:

Alternation of generations –

Seed dispersal –

Pollen –

Cycads –

Ginkgo biloba –