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Paramecium: The White Rat of Ciliates – Transcript
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- Paramecia are unicellular eukaryotes covered in tiny hairs called cilia that help them swim, and they have been widely used by scientists and students as a model organism.
- Although paramecia lack protective cysts, scientists have confirmed the presence of hitch-hiking paramecia on the wet fur of raccoons and on snail bodies.
- Scientist Tracy M. Sonneborn researched the killer trait in paramecium and proposed the plasmagene theory, which was later proven incorrect when kappa particles were found to be bacteria.
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This paramecium is not having a particularlygood day. You wouldn’t be either if the contents ofyour body were being slowly consumed by a ciliate. But as you, hopefully, are watching this froma comfortable position, while not being consumed, we would understand if your attention is divertedfrom the microbial drama and instead focused
on all the shiny things, like looking downon a city at night from an airplane, the moving lights a sign of life at a distance. These shiny colors are actually crystals,visible this way under polarized light.
So, we have to use a special filter to getthese shots and they seem to be connected to the paramecium’s metabolism. We wanted to start with these crystals because,like, we think they’re stunning. And stunning is not necessarily the firstword that comes to mind when you think of paramecia.
Of the early microorganisms discovered byscientists, paramecia have gone on to have perhaps the most distinguished career. Studied by scientists and students alike,these protozoans have been so useful as a model organism that one scientist called themthe “white rat” of ciliates.
But while paramecia have played an importantrole in our understanding of genetics, microscopic movement, and more, importance does not alwaysimply excitement or enthusiasm. After all, if we compare paramecia to theirprotozoan brethren, what does a hairy oval have next to the shifting shapes of the amoebaor the strange movements of the euglenoid?
That doesn’t stop paramecia from wantingsome attention though: there have been plenty of times where we’ve set up a recordingof some beautiful organism, only to have paramecia come by and photobomb our precious composition.
Their reliability in labs and frequent appearancein classrooms might make paramecia seem like a known quantity. The things we know, we seem to have a goodgrasp on. We know that this is a paramecium, a unicellulareukaryote covered in tiny hairs called cilia that help it swim.
We know that when scientists first began describingparamecia, they divided them into two groups based on their morphology: Paramecium aurelia,which have a long, tapered oval shape; and Paramecium bursaria, which are shorter androunder.
We also know that they have multiple nuclei:one macronucleus where transcription happens, and a number of micronuclei that vary across species that become important during sexual reproduction. We’ve talked about p aramecia in a previousepisode, describing the endosymbiotic algae that reside inside of some species, providingsugar produced through photosynthesis in exchange for the movement and shelter of the surroundingcell.
So much of their knownness rests on just howwidespread they are. You can find paramecia in freshwater, brackish,and marine environments around the world, though the distribution, of course, variesby species. The ones that we’ve come to know best seemto be the ones that are considered cosmopolitan, that’s a word we use for things that arefound seemingly everywhere.
Recently, scientists studying Paramecium biaureliacollected from 92 spots around the world over 62 years of time found very little geneticvariation between the different strains.
This kind of genetic similarity indicatesnot only that these organisms have travelled the world, but that they continue to, keepingtheir populations spread out and not diversifying into different species. But the ubiquity that makes paramecia so knownto us is itself a question: how did paramecia manage to find their way to all these differentlocations, especially bodies of water that
are isolated from others? Unlike some of the other aquatic organismswe’ve discussed on this channel, paramecium don’t form protective cysts that keep themsafe during a dry spell, which makes it that much harder for them to cross the globe andits many dry spots.
Through experimentation and observation, scientistshave confirmed the presence of hitch-hiking paramecia on the wet fur of raccoons and onthe bodies of snails, showing that they might be using the surface of animals as moist taxis,but whether that explains the entirety of their travels remains to be seen.
Paramecia are also subject to what you mightstart to notice is a recurring theme when it comes to the microcosmos. While our early classification methods reliedon what we could see of an organism’s morphology, the development of various molecular techniquesis uncovering new patterns and complications in how we define the relationships betweenits species.
These complications were hinted at by earlyobservations, like the fact that the broad, morphologically-defined label of P. aureliacouldn’t capture the strange mating subdivisions that occured in the species, groups of P.aurelia seemed reproductively isolated from each other.
In time, as scientists gathered more observations, P. aurelia stopped referring to one species of paramecia. Instead, it was redefined as a complex offourteen siblings named P. primaurelia, P. biaurelia, P. triaurelia…all the way upto P. quadecaurelia.
And a fifteenth subspecies named P. sonneborniwas discovered later. With more observation and genetic techniques,the Paramecium genus has come to encompass 19 morphospecies, comprised of what are sometimescalled “cryptic species” these are species who morphological similarities belie theirunderlying differences.
When we look at them in the microscope, thereis no way for us to tell which species it is. We would need to sequence their genome. With words like “cosmopolitan” and “cryptic”used as technical terms to describe paramecia, they start to seem less like a textbook organismand more like an international microbe of mystery.
It might be fitting then to learn from theexperience of the scientist Tracy M. Sonneborn, who began working with protozoa in the 1930s. Sonneborn developed many of the techniquesthat turned paramecia into a model organism, his work so influential that it has been memorializedin the name of that 15th P. aurelia subspecies, P. sonneborni.
While researching paramecia, Sonneborn becameinterested in the so-called killer trait, a subset of paramecia that seemed able toproduce toxins that killed other paramecia.
These killer paramecia all shared a trait:the presence of mysterious cytoplasmic particles called kappa. Sonneborn tracked how these kappa particlespassed on through generations, proposing the “plasmagene theory” to explain the patternshe saw, a hypothesis that these kappa particles
were some special thing coded in the paramecium’sDNA, but that could self-replicate on their own in the cytoplasm. And he was wrong. It turned out that these kappa particles aren’tparticles, they are bacteria: another example of endosymbiosis, except that instead of providingthe nourishing promise of photosynthesis,
these bacteria turned their paramecium homesinto killers. It took time for Sonneborn to accept thathis plasmagene theory didn’t explain kappa particles.
But if paramecia have come to model so muchof what we learn in science, then looking more broadly, this microscopic organism alsomodels us and our own use of the scientific process. In a letter to another researcher, Sonnebornreflected on this, on what it means to set aside his own ego for the reality of the microbe,to know that studying nature requires an understanding
of our own selves. He wrote, “‘It was awful of me to be soattached to a pet idea. That was an ordeal between my mind and myheart and it took a while for the mind to win and the heart to accept.
Impersonal scientific objectivity is a goalto be sought by hard self-discipline; we are not born with it.’’ Thank you for coming on this journey withus as we explore the unseen world that surrounds us.
Journey to the Microcosmos is produced byComplexly. We produce over a dozen different shows, includinga new one, Ours Poetica, it’s a co-production between Complexly, The Poetry Foundation,and the poet Paige Lewis. Ours Poetica brings you a new poem three timesa week, read by poets, writers, artists, and sometimes unexpected, yet familiar, voiceslike... like mine for example.
You can check out a video of me reading EdgarAlan Poe's "The Raven". We think there are times for YouTube to behigh energy and times for it to be a little bit more relaxing. Journey to the Microcosmos and Ours Poeticaare both going for that second thing.
I also want to say thank you for listeningto my extra gravely voice today. I have a cold. We haven’t talked much about viruses onthis channel. They are very difficult to observe, they’retiny, probably aren’t really even alive. But they have colonized much of my upper respiratorysystem right now and I’m just pushing through.