Sunday Protist - Haptoglossa: Missile-growing spores

I totally just skipped a week, I know. But no one really noticed, right? Good! ^.^ Kind of busy with classes, setting up a seminar course, and...much neglected research. I'm beginning to forget to do treatments and such, too much stuff going on. Also, training new undergrads is a lot of work. Especially when they show up at various random times and you forget what you've explained to whom. And when they forget certain important details that you've spent a good 15min elaborating on, complete with hopefully memorable stories. Such as what happens when you hang a 2L flask on one of the plastic pegs on our drying rack, which tend to be a bit too flexible. Well, the flasks were ok, but the glass 1L cylinder - not so much... and I liked that cylinder! Oh no, I actually have 'favourite' pieces of lab equipment already...


Anyway, enough with the lab rants... remember the mystery micrograph from a good two weeks ago? The one which only one person actually bothered to guess, while the rest of you hid silently somewhere?

(Jane Robb, U of Guelph; TEM of projectile)
Admittedly, this one was kind of unfair. Turns out, it barely exists in the Encyclopedia of Life, imageless and under the very descriptive category "not assigned". And even after revealing it being an oomycete, most oomycete trees seem to utterly ignore this poor obscure organism. It's not completely obscure, as Elio at Small Things Considered beat me to posting about it a while ago, but they cover a lot of rather well-hidden life forms there. So let's welcome Haptoglossa as the guest of this week's Sunday Protist!

Before we proceed, however, let's orient ourselves phylogenetically a bit:

What are oomycetes?
Oomycetes are a non-photosynthetic fungal-like group of stramenopiles (Chromalveolata), which include Phytophthora, the potato blight 'fungus' responsible for the 19th century Irish Potato Famine, drastically impacting the course of history of both Ireland and the genetic makeup of the North American population. Oomycetes were long considered to be fungi due to the similarities in life style and morphology, thus they're still largely studied by mycologists. In fact, they were taught in the fungi section of this vastly polyphyletic course I took a couple years ago ('non-vascular plants', ha!), thereby leaving us rather confused.

One characteristic that made them stand out from their supposively fellow fungi was biflagellate zoospores - the motile spores of true fungi that produce them (ie. Chytrids) have a single posterior flagellum. Now it may seem like a trivial difference, but check this note on flagellar and basal body evolution for perspective. In short, flagellar loss (and an entirely different mechanism of flagellar propagation during cell division) is not a very common event, and can be argued to be a reasonably reliable morphological character. (Although relying on morphology alone tends to lead to complete and total chaos...) There are more fundamental differences between oomycetes and fungi, and some parallels that are quite fascinating, but I won't discuss them here.

Haptoglossa is a more or less basal oomycete, and has thus attracted attention for investigating the potential presence of relic plastid genes. (search for "Craig Bailey" in this report of the 2009 Int'l Crysophyte Symposium. Also, a whole conference full of Chrysophyte geeks - interesting thought ^.^) Here's one of the recent oomycete trees featuring Haptoglossa:

(Hakariya et al. 2007 Mycoscience; for a broader perspective, see this diagram from the Chromalveolate introduction post. This tree also shows the more 'famous' oomycetes like Phytophthora, Albugo and Peronospora, see if you can find them. Also, Cafeteria is a real genus name.)

They had three trees in that paper, but to a cell biologist they all look the same, so here's one with the highest bootstrap values. My "Phylogenetics Advisory Committee" is asleep right now. And I don't think anyone here would care much about the difference between maximum likelihood, maximum parsimony and neighbour joining analysis with respect to these particular trees. So let's move on to our guest organism.

(NB: For those interested in oomycete character evolution, there's a diagram in the appendix at the very bottom of this post. I thought it would be too distracting here...)

Haptoglossa - "Nature's ballistic missile"*
*Title of 1982 Science paper by Robb & Barron; now if only they published it in Nature instead...
You're a spore. You want to get inside a host. You really don't want to sit around hoping to get ingested or inserted by some vector. What can you do?

If you're Haptoglossa, you grow a cannon, load yourself into it, and fire away. The contraption involved looks something like this:

(Barron 1987 Mycologia; diagram of a gun cell)

Ready, set, FIRE!

(Barron 1987 Mycologia)
It seems that upon triggering, the basal vacuole expands rapidly, everting the projectile at rapid speed through the host cuticle. However, the exact mechanism of action still remains to be understood (one becomes so desensitised to this phrase in protistology...) Haptoglossa targets nematodes and rotifers, depending on the species. Once inside the host, depending on the species, some may grow into large multicellular parasites like this H.dickii:

(Hakariya et al. 2007 Mycoscience; a - spores, b - sporangium inside nematode)

To give an idea of the diversity within the genus, some species, like H.eumpens, H.heteromorpha and H.polymorpha have two distinct types of gun cells, one set of which has not been seen in action - it's exact function and behaviour remain but a mystery at the moment:

(Glockling & Beakes 2002 Fungal Genet. & Biol.; comparison of five Haptoglossa species at various life cycle stages; note the odd binucleate and 'atypical' infection cells.)

Here's the H.heteromorpha (one of the species with the mysterious alternative gun cells) life cycle in a diagram:

(Glockling & Beakes 2002 Mycologia; just follow the letters, the order is a bit counterintuitive.)
Again, it is rather puzzling why an organism would 'bother' to make two types of spores with slightly different, yet still sophisticated, firing structures. However, ridiculously off the wall complex life cycles aren't all that strange to parasites - many true fungi like 'rusts' are notorious for their particularly sadistic abuse of a mycology student's memory. Parasites can sometimes have multiple hosts if there is a seasonal cycle - ie. if you parasitise annual plants, it may actually be advantageous to be able to utilise different food sources throughout the year. However, nematodes aren't particularly seasonal, to my knowledge, so the reasons here may be something different. This could also be an ancestral feature, from the long gone days of making a living off plants, but that is rather far fetched. There may be a whole chunk of a life cycle that we are completely oblivious to at the moment...

Having quite a fetish for complex intracellular structures, I find the question of gun cell development rather fascinating. The entire structure does seem to form de novo, and a very exciting research project would be to examine the genetics and cell biology of the projectile development. While there have been observations of gun cell development, further details seem to be obscure at the moment. Here's a nice diagram based on microscopy observations:

(Glockling & Beakes 2000 Mycol Res; just sit back and savour the ability of a cell to generate seemingly limitless complexity within itself! Note that prior to this process, the spore shifts its contents into this newly generated gun cell compartment, essentially growing itself a cannon on the side.)

And another detail of the firing mechanism, and the aftermath: (the projectile on the right faces opposite of the gun cell on the left)

(Glockling & Beakes 2000 J Invert Pathol; I'm still thoroughly amazed by this mechanism!)

Now, recall that this process seems to rely on basal vacuole expansion, followed by a rapid inversion of the projectile inside out. Keeping that in mind, take a look at this sample of proximal convergence:

(source: Palaeos.org)
Microsporidia also have a clever mechanism for shooting themselves into their prey, via the posterior vacuole expanding and everting the polar tube outwards at an extremely rapid speed. There's a nice article about them on Palaeos.com, although I thought microsporidia were fairly certainly within fungi, between chytrids and zygomycetes - not sister group to Fungi. Again, my "Phylogenetics Advisory Committee: Microsporidian chapter" is unavailable at this hour...

There needs to be a protist battle game of some sort... can you imagine, firing off microsporidian and haptoglossan missles at your oponents? Only to be entangled and digested by a protoperidinian pallium? The microbial world is an endlessly fascinating place, surreal and alien to our innate perception of scale.

I've been challenged by someone to blog about Heterolobosea, which I shall do after I finish that long-overdue Neomura post. I'm not sure I can match the quality and depth of his Amoebozoa (and all the other) posts, but I'll try!

References
Barron, G. (1987). The Gun Cell of Haptoglossa mirabilis Mycologia, 79 (6) DOI: 10.2307/3807689

Hakariya, M., Hirose, D., & Tokumasu, S. (2007). A molecular phylogeny of Haptoglossa species, terrestrial peronosporomycetes (oomycetes) endoparasitic on nematodes Mycoscience, 48 (3), 169-175 DOI: 10.1007/s10267-007-0355-7

Glockling, S. (2000). An Ultrastructural Study of Sporidium Formation during Infection of a Rhabditid Nematode by Large Gun Cells of Haptoglossa heteromorpha Journal of Invertebrate Pathology, 76 (3), 208-215 DOI: 10.1006/jipa.2000.4967

Glockling, S., & Beakes, G. (2000). Video Microscopy of Spore Development in Haptoglossa heteromorpha, a New Species from Cow Dung Mycologia, 92 (4) DOI: 10.2307/3761431

Glockling, S. (2002). Ultrastructural morphogenesis of dimorphic arcuate infection (gun) cells of Haptoglossa erumpens an obligate parasite of Bunonema nematodes Fungal Genetics and Biology, 37 (3), 250-262 DOI: 10.1016/S1087-1845(02)00532-7

ROBB, E., & BARRON, G. (1982). Nature's Ballistic Missile Science, 218 (4578), 1221-1222 DOI: 10.1126/science.218.4578.1221

Appendix:
For those curious about oomycete evolution:

See Beakes & Sekimoto 2009 in Oomycete Genetics and Genomics (Eds. Lamour & Kamoun). I'm seriously annoyed by the whole arrows coming out of extant lineages/organisms thing...although "Crown Oomycetes" is rather hilarious!