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Frontonia: dissecting a ciliate appetite

The views expressed are those of the author and are not necessarily those of Scientific American.

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(I’m pretty sure every non-strictly-autotrophic thing has some form of appetite. Some ciliates and amoebae just tend to display it more prominently — as in this cool video of a Climacostomum devouring a flatworm!)

Frontonia is a genus of predatory ciliates who feed on anything from algae and bacteria to fellow ciliates (and anything else that can fit through the cytostome, or its “throat”). Some can get quite large, to the point of making yours truly momentarily wonder once whether an observed big hairy thing was a ciliate or an animal. It was bigger than the rotifers. Bigger than most animals in there — I try to avoid inverts while sampling as they have a tendency to eat everything, or at least whatever you’ve found a particular interest in. Like that time I tried to culture a dense swarm of Oxyrrhis (a dinoflagellate relative) and a couple rotifers got in… but I digress. This thing was huge, at least 400um. And it was a ciliate. The mouth identified it as Frontonia. Stuffed with all sorts of cyanobacterial filaments, diatoms, and anything else unfortunate enough to find itself in the way.

Frontonia is relatively plain morphologically, as far as ciliates go — though some species can get quite big (even some around 600um have been reported!). This specimen is about 90um long. It has the characteristic mouth for its group (Peniculids), along with a suture where the rows of cilia meet from both sides. The mouth can open significantly wider when it needs to swallow things it probably really shouldn’t, as far as table manners go. This specimen may be F. angusta, though it’s notoriously difficult to confidently identify microbes to species, especially without any sequence data or staining.

Here’s a couple more surface views, just for fun. Among the rows of cilia is the opening of Frontonia‘s contractile vacuole — the accumulated liquid has to escape somehow. In some protists, the contractile vacuole simply fuses with the membrane and expels its contents, but there are also plenty of variations on the subject. Just beneath the cilia are rows of extrusomes — stabby organelles the ciliate uses to hunt and protect itself.

Lots of cilia… the neat rows of them are called kineties. Kind of looks like the ciliate’s been farmed on.

The ciliate’s oral structure is one of the key clues for figuring out what it is, and what it does. Several groups of cilia look remarkably alike on the surface, and can most clearly be differentiated by their mouthes, which can be quite complex — perhaps somewhat unsurprisingly for such a gluttonous group of organisms. Frontonia‘s oral apparatus is far from being the most impressive in the ciliate world, but even in its modesty it still betrays remarkable structural organisation and complexity. There are rows and clumps of various kinetids — a general term for both ciliated and unciliated basal bodies (if feeling particularly inclined to be very confused, there’s a whole world of structural variation even within kinetids!). Kinetids have spawned a whole mess of similar-sounding ciliate-specific terms (for starters: kinetids, kineties and kinetosomes) that seem to be used differently within varying groups of ciliates, and sometimes inconsistently. That was a long-winded way of writing “I’m still confused and would like to avoid the topic altogether). I just found a helpful glossary of terms here, but will try to write as much as I can in English nonetheless…

Often, the oral apparatus has a dense row of cilia that appears (and acts) as a membrane until a closer inspection. It is visible as a sheet of cilia on the top part of the mouth in this image. Below it are three rows of oral kinetids

Let’s look inside the ciliate. In the centre lies a sizeable macronucleus, which stores the chromatin used for making the various genome products. Not shown here yet is a much smaller micronucleus, which stores the chromatin that actually gets passed on to subsequent generations. The macronucleus has to be recreated each time the ciliate has sex (or autogamy — sex with itself), rendering sex a recreational activity. Incidentally, the genome size is typically bigger in the micronucleus — the reason for this requires a post of its own (or several) — as swaths of macronuclear sequence get removed before ending up in the micronucleus. Ciliates seem to do almost everything weird. Also, note how the large ridged inclusions in the cell look suspiciously similar to the two Drepanomonas sp. ciliates below it.

And now for the pic I originally wanted to just show on its own (but, as usual, got distracted): a labelled close-up of the Frontonia, featuring some of its key cellular components: the oral apparatus, contractile vacuole, micro- and macronucleus (MIC and MAC, respectively), trichocysts and, of course, half-digested prey carcasses. Because you really don’t want to get anywhere near that little monster’s mouth. Note the varying textures of the MIC and MAC, reflecting their different chromatin organisation (remember: the MIC is transcriptionally silent).

(and here’s an unlabelled version, unspoiled by text, for fun)

So there’s our tour of this Frontonia representative. Of course, to really see cell structure one needs electron microscopy — and then we could get quite picky. But luckily for you, I don’t have access to an electron microscope yet, and am too lazy to dig in literature at the moment. ;-)

Next post, we’ll switch back to a little diversity. And there haven’t been any amoebae here in a while, which is a problem.

Psi Wavefunction About the Author: Psi Wavefunction is a graduate of the University of British Columbia working as a protist researcher (soon to be graduate student) at Dalhousie University in Halifax, Nova Scotia, and blogs about protists and evolution at The Ocelloid as well as at Skeptic Wonder. Follow on Twitter @Ocelloid.

The views expressed are those of the author and are not necessarily those of Scientific American.

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