Today’s word is fairly hardcore. A couple of months ago, I talked about the different ways scientists (biologists, taxonomists, zoologists, botanists, etc.) categorize life on Earth into five kingdoms (the Prokaryote superkingdom consisting solely of Bacteria, and the Eukaryote superkingdom, which contains the remaining 4 kingdoms: Protoctista, Animalia, Fungi, and Plantae).
Since then, I’ve been reading a lot of fun stuff (Tolkien, Herbert, comic books, Elmo board books, find the truck/train/car/plane/boat books), and hadn’t gotten back to my Margulis and Chapman until today. After all, it has a Lot of Hard Words In It.
But I knew I had to come back to it at some point, because I was curious about that first eukaryote kingdom, Protoctista. I mean, what the heck is that, anyway? Well, according to M & C,
Kingdom Protoctista comprises the eukaryotic microorganisms and their immediate descendants: all algae, including the seaweeds; undulipodiated mastigote molds, water molds, the slime molds and slime nets; the traditional protozoa; and other even more obscure aquatic organisms. Its members are not animals (which develop from a blastula), plants (which develop from maternally retained plant embryos), or fungi (which lack unulipodia and develop from fungal spores). Nor are protoctists prokaryotes. Protoctist cells contain microtubules, nuclei, and other characteristic eukaryotic features…. Many photosynthesize (have plastids), and most are aerobes (have mitochondria)…. (120)
I warned you that there were A Lot of Hard Words. This is just part of the first paragraph of the introduction to this kingdom. Keep plugging away. The real juicy bit is right ahead:
All protoctists evolved from symbioses between at least two different kinds of bacteria—often many more than two.
Cool, huh? Just like us, these leetle teensy-tinsy organisms (well, seaweeds can get pretty big… oops! don’t want to give away the point of this post!) evolved from bacteria.
But why do we have to call them protoctists? What’s wrong with the good old shorter word protists? I’m glad you asked. I’m even gladder that M&C have the answer, so I don’t have to do too much more research:
Why “protoctist” rather than “protist”? Since the nineteenth century, the word protists, whether used informally or formally, has come to connote a single-celled or few-celled tiny organism. In the past three decades, however, the basis for classifying single-celled organisms separately from their multicellular descendants has weakened. Multicellularity evolved many times in unicellular organisms. Many multicellular beings are far more closely related to certain unicells than they are to other multicellular organisms….
We adopt the concept of protoctist propounded in modern times by Californian botanist Herbert F. Copeland in 1956. The word was introduced by English naturalist John Hogg in 1860 to designate “all the lower creatures, or the primary organic beings;—both Protophyta, … having more the nature of plants; and Protozoa … having rather the nature of animals. Copeland recognized, as had several scholars in the nineteenth century, the absurdity of referring to a giant kelp by the word “protist,” a term that had come to imply unicellularity and, thus, smallness. (122)
In other words, calling this kingdom Protoctista allows us to avoid lumping most of these organisms in with the other three eukaryote kingdoms,* all of which are inherently multicellular, but still allows us to include large multicellular organisms in as needed (like those giant seaweeds I was talking about earlier).
All protoctists are aquatic: marine, freshwater, terrestrial but needing moist soil, parasitic in moist tissues of other organisms. In fact, according to M&C, “nearly all animals, fungi, and plants—perhaps all—have protoctist associates” (123).
The best part about protoctists, though, as with almost all the organisms in M&C, is the pictures. These things have some amazing body plans. Too bad you have to shell out the money for the book to get a really good feel for what I’m talking about.
Maybe at some point I’ll be able to put this book down and move on into the other kingdoms, but for now, Rhizopoda, Granuloreticulosa, and 34 other phyla of protoctists have my attention. They don’t have much of a plot, though…
* Bonus points to those of you who can name those other three kingdoms without Googling! (All you have to do is read this page again; this is not a hard quiz, even if it does have Hard Words.)
Welcome to Protista! ^_^ Beware of the giant amoebae, vicious Didinium (ciliates) and harmless-looking dinoflagellates (eg. Protoperidinium) that can eat long filamentous things 10x their size! May I recommend you a vacant glass lorica while you settle in this surreal alien world of a foreign scale?
(‘protist’ is fine, nobody uses ‘protoctist’ in the field, besides Marguilis)
Protista is daunting upon first sight. After all, it encompasses ~99% of eukaryotic diversity, and features everything from walking ciliates with scrambled genomes to trypanosomes with their RNA editing from hell to rotifer-eating testate amoebae to Acanthometra with spines of strontium sulfate(!) to gigantic unicellular foraminiferans up to 30cm in diameter to oomycetes that have changed the course of human history on several occasions to… it’s a big place. =D
The definition of Protista/Protoctista(ewww at the latter) given there is unnecessarily long and convoluted. It can be stated much more simply: Protist = A eukaryote that is neither a land plant, animal, or fungus. The latter three groups themselves do have fairly agreed-upon definitions, so this may be the easiest to go by. (all three are by definition share a multicellular common ancestor, with the exception of plants, which also require embryos to form on the parent (hence the alternative name, Embryophyte).
See here for more on a brief history of protistology and here for a few trees to put it into some perspective. Apologies for shameless self-promotion…
You might want to be a little careful with your source of introduction, however. Ahhh…Marguilis. See, sadly the only non-ancient comprehensive introduction/overview of protists comes from someone who’s turned into a complete nutcase. Back in the day, she was right about mitochondrial and plastid endosymbiosis (although she did not come up with the idea herself, but was a good populariser), but she just didn’t quite figure out where to stop.
“All protoctists evolved from symbioses between at least two different kinds of bacteria—often many more than two.”
Wrong, just wrong. While the evidence for plastid + mitochondrial endosymbiosis (as well as serial endosymbioses thereof) is overwhelming and undisputed, her theories of spirochaete flagellar endosymbiosis and whatever-the-hell else she has (the mere diagrams are enough to overwhelm the simple-minded biology student here) have no support whatsoever.
Furthermore, while it is pretty much a given that eukaryotes evolved from prokaryotes/bacteria, it is more likely that a proto-eukaryote evolved before mitochondrial and plastid endosymbioses. Phagocytosis (eating things) is a necessary prerequisite for endosymbiosis, and bacteria are incapable of it. Thus, the proto-eukaryote first evolved the cytoskeleton and endomembrane trafficking before any genomic fusions or massive gene transfers became even possible. If you’re up for some hardcore reading, may I recommend Cavalier-Smith 2009 Int J Biochem & Cell Biol on phagocytic/predatory origins of eukaryotic evolution? A word of warning, however — Cavalier-Smith isn’t exactly light reading. But I find his ideas definitely worth the struggle! Or just Google the Neomuran Hypothesis, it’s quite fascinating! (I’ve been planning to write up a post on it for a while now…)
I’ve looked through some of Marguilis’ books, they are indeed beautiful and captivating. Quite unfortunately, her science is shaky, and it’s a shame that often good science promoters tend to value communication over accuracy. This spills over into the other realms of the scientific community as well — the sheer volumes of endosymbiotically obsessed hypotheses and reviews published by non-protistologists is breathtaking, and very sad that all this work goes into theories that are very likely to be completely wrong (as opposed to developing ‘tamer’ ideas). Marguilis’ stories are much more likely to hit Nature and Science than Cavalier-Smith’s Neomuran Hypothesis, perhaps because massive genome hybridisations are cooler and easier to understand than scientifically-feasible alternatives.
So that’s my rant on Marguilis. Rumour has it, if you apply to her department for grad school and indicate an interest in working with her, they automatically reject you. Partly so that you’d still have a chance at an academic career afterwards… (and her recent PNAS kerfuffle doesn’t exactly help her reputation — she basically snuck in this ridiculous paper wherein the author claimed that caterpillars were a result of genomic hybridisation between velvet worms and some non-larval ancestors of butterflies! It only got worse from there…)
Enjoy the book and its illustrations, but take the endosymbiosis stuff with a HUMONGOUS grain of salt! ^_~
Again, welcome to the interesting eukaryotes! =P
Cheers,
-Psi-
Hah! I started reading this post and immediately thought of Psi’s blog before getting to the comments.
I really need to get back to the book I’m reading on mitochondria.
Also, I didn’t realize Margulis was implicated in the velvet worms paper. Ars technica mentioned it recently here.
Regarding the velvet worms/butterflies paper, I guess all I can say is that it has some prestigious science fiction precedents. I love the end of David Brin’s second Uplift trilogy when the sole surviving humans get assimilated into this giant bacterium before they all exit the doomed galaxy they’re in. Brin’s a physicist, so he can be forgiven his flights of macrobiological fancy…