Sunday Protist -- Trichotokara nothriae: Guitar-shaped gregarine

This post turned into a bit of a hodgepodge of various gregarine-related trivia. Proceed with caution.

Gregarines are a group of apicomplexans (='Sporozoa', a vastly diverse group famous for the malarial parasite Plasmodium and the behaviour-altering Toxoplasma) characterised by a monoxenous (single host) lifestyle that is quite different from that of other 'apis'. Christopher Taylor wrote a nice post about them here.

Apicomplexa are alveolates along with ciliates and dinoflagellates; you can find them on the left side of this tree . The apicomplexan phylogeny is a complete mess at the moment; the old coccidian-haematozoan-gregarine divisions aren't too well-supported and the relationships of stuff within them are even murkier. As an aside, many apis have an 'apicoplast', or a relic plastid of red algal origin -- their ancestors were once photosynthetic! In fact, a paraphyletic group of organisms basal to apicomplexa (Chromera et al.) are currently photosynthetic, further supporting the photosynthetic ancestry of these weird mostly-intracellular parasites, most of whom rarely ever see the light of day!

Gregarines are typically invertebrate parasites and unlike other apicomplexans, tend to spend most of their lives extracellularly; in fact, their cellular penetration consists of attaching themselves to a cell via the mucron (holdfast-like structure). You can read more about their biology and life cycle on their ToLWeb page. (also a review in Tr Parasitol: Leander 2007) If you want to see some for yourself, kidnapping and slicing up an earthworm is an easy way to do so: Monocystis is a parasite of earthworm seminal vesicles (feeds on sperm), and a rather abundant one. It may actually be quite easy to find various apicomplexan parasites in insects -- it is estimated that most of them may have an api specialised in parasitising them, which hints at the total apicomplexan diversity being something outrageously vast! Such a project would also be a good excuse to learn insect anatomy, which I find to be quite complicated.

Right, you wanted to see a new genus of guitar-shaped gregarines:

Trichotokara from the intestine of an onuphid tubeworm. a-e: trophozoites (feeding forms). M - mucron ('holdfast'), CB - cell body. Arrow - junction between mucron and cell body, which can be seen extending further into the mucron in (e; arrowheads). f: gamonts in syzygy, or gregarine sex. Scalebars: a-e 10um; f 25um. (Rueckert & Leander 2010 J Invert Pathol)

By the way, if anyone asks you for a six-letter word in English 'devoid of any vowels', keep 'syzygy' in mind. Technically it does have vowels, as any phonologist would tell you, but most people insist on equating letters with sounds, and y 'is not a vowel'. Regardless, it's still a really awesome word. Syzygy!

More gregarine awesomeness. Note how the cell surface seems to strive for increased surface area, especially in the mucron which gets inserted into a cell:

SEM of Trichotokara. b - close-up of hair-like projections of the mucron. c - junction between mucron and cell body. d - folds along the cell body. Scalebars: a - 10um; b-d - 1um. (Rueckert & Leander 2010 J Invert Pathol)

This gives me an excuse to mention a paper on proximate vs. ultimate convergence by the senior author on the above gregarine paper: Leander 2008 JEM (free access). Among several other examples of ultimate convergence between multicellular and unicellular organisms inhabiting similar environments, gregarines and nematodes are compared in terms of their structural organisation. While nematodes have longitudinal muscles just beneath the elastic epidermis, gregarines have subpellicular bands of longitudinal microtubules running just underneath the elastic cortex (although used differently -- see gliding motility below). Curiously, in both cases the result is a sinusoidal (wiggling) pattern of movement. Additionally, tapeworm and Haplozoon (dinoflagellate) surface morphology are noted to be similar (covered with microvili), for the obvious purpose of increasing surface area. It's probably not much of a stretch to add gregarine surface structure to that list. (see Leander et al. 2003 J Parasitol for more gregarine surfaces)

Interesting case of structural ultimate convergence between nematodes and gregarines. Purple - bands of muscle and microtubules, respectively. Blue - elastic epidermis and tri-layered cortex, respectively. The three cortical layers consist of the plasma membrane at the very surface, with two alveolar membranes immediately below. (Leander 2008 JEM)

Before we proceed to a digression on apicomplexan motility, oblicatory phylogeny of Trichotokara and relatives. Note its extremely diverged SSU sequence resulting in a hellishly long branch:

ML tree of SSU rDNA sequences. Probably wouldn't trust its exact placement among the gregarines just yet... (Rueckert & Leander 2010 J Invert Pathol)

Apicomplexans are generally aflagellate in their trophic stage (I say 'generally' just in case...) -- their motility is an interesting topic, as they can't exactly extrude pseudopodia either. Nor do they have any spirochaetes doing the work for them as in Mixotricha, nor do they wildly thrash about an internal bundle of microtubules like Saccinobaculus. So how do they do it? Just like pennate diatoms: by gliding motility.

While sharing some basic similarities with diatom gliding, the apicomplexan variant has an unrelated origin and is quite different. One annoying thing (to us) about alveolates is their alveolae, or little membranous sacs just underneath the plasma membrane. In apicomplexan cell biology literature, this is called the Inner Membrane Complex. Prior to explaining why this detail is particularly annoying, first let's go over the crude basics of gliding motility: First, you need something to anchor to the substrate. This material is usually discarded, leaving behind a trail of 'slime', if you will. Then, you need an adaptor protein [complex] that attaches to the anchoring substance and crosses the plasma membrane. This adaptor must have a way of reaching a cytoskeletal element, usually actin via myosins (eg. see Molino & Wetherbee 2008 Biofouling; also, that journal title is very WTF...). The problem (again, mostly for researchers, and students...) with apicomplexa is their tendency to have the Inner Membrane Complex in the middle of that. This means the mechanism looks roughly like this:

Complicated motility. (Soldati & Meissner 2003 Curr Op Cell Biol)

Or, in the language of Nature Reviews:

Cell biology: Always more fun with extra gene/protein names thrown in, especially those irrelevant to the point. Shall we look up some protein structures while we're at it?
Ignore the target cell part -- a similar process happens along other surfaces too. If I recall, the model with intra-IMC proteins reaching across between actin and microtubule systems is actually more up-to-date; the "rolling IMC conveyor belt" model was outdated. Don't quote me on this though! (Baum et al. 2006 Nature Rev Microbiol)

Remember cramming the molecular biology of amoeboid motion? Isn't it almost a good thing that traditional cell biology courses are so phylogenetically impoverished? So many things are much more complex than animal cells, so we actually get the easy (and [arguably] boring) option. In a nutshell, you have something like this:

anchor-adhesin-actin-myosin-[interamembranous particles?]-subpellicular microtubules

The myosins move to the opposite of cell motility (and actin polymerisation), thereby pushing the pellicular microtubule skeleton in the right direction. Look at the Soldati & Meissner figure again to see why. It's a rather convoluted process just to get a cell moving! Of course, that complexity is more of a problem for cell biologists than the organism, considering how abundant and efficient apicomplexans tend to be.

Another aside: Apicomplexans, as well as numerous other organisms, are capable of a cell divison process known as palintomy: they can undergo several rounds of mitosis without cytokinesis, resulting in multinucleate cells (helps to not have open mitosis), and then simultaneously undergo cytokinesis for each of those nuclei (cellularisation). In gregarines, this looks vaguely like budding, as the nuclei tend to congregate near the cortex during this process (Kuriyama et al. 2005 Cell Motility Cytosk). Drosophila embryos do something similar, so palintomy isn't that unusual, but still pretty cool.

Back to gregarines, there are some more species that seem to be on a morphological acid trip. Some of them have been described only once and never noted again, which makes me sad:

Aikinetocystis singularis. I really want an SEM of that! Too bad it's from an obscure burmese earthworm... (Gates 1926 Biol Bulletin)

So if you like finding new species and genera and describing them, may I recommend apicomplexan diversity. It's taking a while for entrail-hungry parasitologists to go through all the various invertebrate (and vertebrate) parasites out there, so there's still plenty of room for work. If there is truly one species of apicomplexa for roughly each species of insects (and other animals), that pie chart of diversity showing most life as insects (and protists but a tiny splinter) is truly laughable:

LOL. Simply hilarious! Looks like the "global biodiversity assessment" team was a bit short on microbiologists... (at least they admit to not knowing much bacterial diversity; at least they put 'protozoa' and 'algae' in quotation marks...) (Purvis & Hector 2000 Nature)

All hail microbial parasites -- the bane of biodiversity research!

References
Baum, J., Papenfuss, A., Baum, B., Speed, T., & Cowman, A. (2006). Regulation of apicomplexan actin-based motility Nature Reviews Microbiology, 4 (8), 621-628 DOI: 10.1038/nrmicro1465

G. E. Gates (1926). Preliminary Note on a New Protozoan Parasite of Earthworms of the Genus Eutyphœus Biological Bulletin, 51 (6), 400-404

LEANDER, B. (2008). Marine gregarines: evolutionary prelude to the apicomplexan radiation? Trends in Parasitology, 24 (2), 60-67 DOI: 10.1016/j.pt.2007.11.005

LEANDER, B. (2008). A Hierarchical View of Convergent Evolution in Microbial Eukaryotes Journal of Eukaryotic Microbiology, 55 (2), 59-68 DOI: 10.1111/j.1550-7408.2008.00308.x

Molino, P., & Wetherbee, R. (2008). The biology of biofouling diatoms and their role in the development of microbial slimes Biofouling, 24 (5), 365-379 DOI: 10.1080/08927010802254583

Purvis, A., & Hector, A. (2000). Getting the measure of biodiversity Nature, 405 (6783), 212-219 DOI: 10.1038/35012221

Rueckert, S., & Leander, B. (2010). Description of Trichotokara nothriae n. gen. et sp. (Apicomplexa, Lecudinidae) – an intestinal gregarine of Nothria conchylega (Polychaeta, Onuphidae) Journal of Invertebrate Pathology DOI: 10.1016/j.jip.2010.03.005

Soldati, D., & Meissner, M. (2004). Toxoplasma as a novel system for motility Current Opinion in Cell Biology, 16 (1), 32-40 DOI: 10.1016/j.ceb.2003.11.013