Field of Science

Sunday Protist - Nucleariids

While exploring the various corners of the protistan world, I've been neglecting our close relatives - the Opisthokonts. Let's quickly remedy the situation.

A couple weekends ago I had some pond water on hand, and it turned out to be quite productive. I was on a bit of a heliozoan and amoeba spree when I encountered these things:

At first it seemed like a 'heliozoan'*, but wasn't quite round enough. Then I noticed filopodia. Heliozoa with filopodia? Nah. But it didn't quite qualify for your typical amoebozoan either, so I was rather confused. To make it even more fun, some of them had spicules sticking out (see the optical section through the top in the rightmost image above). Then I started seeing similar things without spicules, and they seemed to be related:

So I spent an hour or so trying to figure this one out**. They turned out to be Nucleariids, a group of filose amoebae basal to fungi. The top one appears to be Rabdiophrys, which has been, in fact, confused with or considred as heliozoa; the bottom one may well be Nuclearia itself - some mixed images of Nuclearia and Rabdiophrys can be found here.

Nucleariids are filose amoebae, meaning they produce long thin thread-like pseudopodia without internal microtubule bundles (which would be axopodia, like those of 'heliozoa'). They are on the fungal side of the great cauldron of Miscellaneous Opisthokonts sometimes called 'Choanozoa' by TC-S. Other times, he seems to reserve Choanozoa for those on the animal side of opisthokonts. Yet other times, he seems to fail to piss off cladists and actually use monophyletic terms. Which one of those is 'in season' likely depends on the monsoon patterns in Bangladesh. Or the temperature fluctuations on one of Jupiter's moons. Further research needs to be done. Materials include ethanol and acid, if I recall. Anyway, here's a damn tree already:

(Ruiz-Trillo et al. 2007 Trends Genet; Opisthokonta - our fellow ass-tailed relatives)

The Choanozoa/Misc Opisthokonts actually tend to be insignificant-looking amoeboid things most of the time, except for Choanoflagellates, which are these really cute lorica-building flagellates with a cone of microvili surrounding the flagellum on their asses. Speaking of which, opisthokont means 'posterior flagellum', or, less pretentiously, ass-tail. Some fungi (chytrids) have flagellated motile spores, and their flagellum happens to be on the posterior relative to the cell's swimming direction. As mentioned earlier, in most eukaryotes the flagellum performs a pulling action, whereas the opisthokont flagellum pushes the cell. This actually poses some problems for filter-feeding organisms, which use flagella for propelling food particles towards their 'mouth', and may be part of the reason some Choanoflagellates started aggregating into colonies - to stop themselves from moving away from their prey when using flagella.

Here's another specimen of putative Rabdiophrys:

And yet another:

I can keep going:

Seriously, I've got A LOT of those guys!

Switching to Nuclearia now:

Note the absense of spicules:

Hey, it could be worse: I also have a freaking pile of non-descript random amoeboid things. SMALL non-descript random amoebae.

There's a 'cellular' slime mould that turns out to be among the nucleariids: Fonticula (Brown et al. 2009 MBE; advance publication). The poor thing has been lumped with everything from Acrasids to cellular slime moulds, and subsequently neglected for a couple decades. This is the first documented case of slime mould aggregation in opisthokonts, which may contribute a thing or two to the evolution of fungal and metazoan multicellularity. (off topic note: ciliates can aggregate too!)

So now we've finally covered an opisthokont. Phew. That was bugging me.

* Just FYI, heliozoa are not a real group - they're united by their sun-like morphology, axopodia and nothing else...
** Being too impatient to use dichotomous keys (which also simply fail to exist for some organisms), I use a combination of papers, websites and Google image search to find stuff. Basically, you find something that lists a bunch of organisms in the vicinity of what you think it might be, and then look to see if any of the pictures might match. If it's something so obscure that even Google is unaware of its existence, you have to sift through ancient forlorn journal articles sometimes, but usually it doesn't take that long to cross another name off the list.

If completely stumped, I'll just start googling random morphological descriptions in both scholar and image search, until hitting something familiar. As random and haphazard and unprofessional as this method is, it's actually much more effective than figuring out dichotomous keys, in my opinion. Especially when you're unfamiliar with the structural terminology. And
especially when such a thing doesn't even exist for your group of organisms, as is too often the case for protists... [/hint for Mystery Micrographs]

Brown, M., Spiegel, F., & Silberman, J. (2009). Phylogeny of the "forgotten" cellular slime mold, Fonticula alba, reveals a key evolutionary branch within Opisthokonta Molecular Biology and Evolution DOI: 10.1093/molbev/msp185

RUIZTRILLO, I., BURGER, G., HOLLAND, P., KING, N., LANG, B., ROGER, A., & GRAY, M. (2007). The origins of multicellularity: a multi-taxon genome initiative Trends in Genetics, 23 (3), 113-118 DOI: 10.1016/j.tig.2007.01.005

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