Amoebozoan Classification: Putting the Formless in Formation

Chaos carolinense, the species generally regarded today as the main exemplar of the genus Chaos (see the note below). In case you were wondering, this individual is moving towards the right. Photo by David Patterson.

In my last post, I described some of the oddities of the well-known micro-organism Amoeba. In this post, I'll expand the field of view to look more generally at the clade of Amoebozoa. Study of amoeboids has certainly been going on for a long time - an amoeboid was among the few micro-organisms listed by Linnaeus (1758), under the name of Volvox chaos*. But how, you may be wondering, does one go about characterising a shapeless blob? And how does one distinguish one type of shapeless blob from another?

*Later raised by Linnaeus to the status of a separate genus, Chaos. The name Chaos is still in use for a genus very closely related to Amoeba (the main difference is that Chaos is multinucleate, while Amoeba has a single nucleus) but it pays not to look to carefully at the taxonomy. Debate about the identity of the original Volvox chaos raged down the years - whether it was the modern Chaos, the modern Amoeba, or something else entirely - but the debate was largely futile, because the original description cited by Linnaeus illustrates little more than a shapeless blob with a few dots over it. Many authors included the modern 'Chaos' in the genus Pelomyxa, another large multinucleate amoeboid, but Pelomyxa is an entirely different beast. King & Jahn's (1948) argument for recognition of the three genera Amoeba, Pelomyxa and Chaos with those names is in accord with the modern usage, but has the air more of arbitrary pragmatism rather than adherence to priority - the genus that is now Chaos needed a name, and the name Chaos was going begging. The modern usage is now well-established, and it wouldn't really benefit anyone to go stirring things up now.

If you are wondering exactly that, then I'm sorry to point out to you that you're under something of a misunderstanding about the nature of cellular structure. Not that I blame you, because it's an easy enough misunderstanding to develop. Most basic descriptions of intra-cellular structure might suggest that the interior of a cell is basically liquid (or at most jelly-like) with the nucleus and other organelles freely floating about like so many chunks of carrots and peas in a vegetable soup. But while the cytoplasm is fluid, it's not water. It's a complex mixture of all sorts of molecules - actin filaments, microtubules, etc. - almost more like an enormous bowl of noodles than a broth. It is the interactions between these molecules that give a cell its shape, and also that make it move. The movement and shape-changes of an amoeboid are not random, but follow a pattern. And different types of amoeboid will move according to a different pattern. The form and manner that the amoeboid adopts while moving is generally one of the first things to observe in its identification.



Jahn et al. (1974) divided amoeboid micro-organisms into two classes based on the mode of pseudopodium formation. In one class (as shown in the figure from Jahn et al., 1974, above), the cytoplasm was liquified and pushed forward by contraction, re-coagulating at the front of the resulting broad, lobose pseudopodium. In the second class (shown in the figure below), long filamentous pseudopodia were extended with each side of the pseudopodium moving against the other in an opposite direction.



The distinction between lobose and filose amoeboids has been reinforced with further study, though as it turns out both modes have evolved multiple times (filose pseudopodia more often than lobose pseudopodia). Filose pseudopodia are found among such organisms as the Rhizaria (including foraminifers and radiolarians), while Amoebozoa are characterised by lobose pseudopodia (in those amoebozoans that don't produce distinct pseudopodia, the entire cell moves in this way). Lobose pseudopodia are also found among the Heterolobosea, another group of micro-organisms not closely related to Amoebozoa (heteroloboseans include Naegleria, the causative agent for amoebic meningitis, and acrasid slime moulds). Nevertheless, pseudopodium formation differs between the two in that in amoebozoans, movement is smooth and continuous, while heteroloboseans produce pseudopodia eruptively, cycling between periods of extension and periods of "resting" (heteroloboseans also have a distinct mitochondrial structure from amoebozoans).


Acanthamoeba, a common amoebozoan in soil and fresh water, occasionally causing eye infections in humans. Acanthamoeba produce distinctive short, narrow subpseudopodia from the single flattened cell-wide pseudopodium, as seen in this photo by David Patterson.

Among amoebozoans except testate forms, archamoebae and slime moulds, Smirnov & Goodkov (1999) recognised nineteen "morphotypes" distinguished by their mode of movement - whether the amoeboid extends multiple pseudopodia, or moves as a single unit; the form of the uroid (the posterior end of the cell while the amoeboid is moving); whether the surface of the cell is ridged or smooth; and other such details. Though Smirnov & Goodkov (1999) explicitly established their morphotype distinctions as identification characters only, without necessarily indicating higher classification, molecular phylogenetic studies have indicated a rough (but not exact) correlation between locomotive mode and phylogeny (Smirnov et al., 2005). For instance, Tubulinea, the class of amoebozoans including Amoeba and Chaos, generally produce tubular or subcylindrical pseudopodia with cytoplasm streaming down a distinct single central axis. Members of another class, Flabellinea, have flattened pseudopodia without a single central axis.


Vannella simplex, a member of the Flabellinea. Note the single broad flat fan-shaped pseudopodium. Photo from here.

Not everything is movement, of course. Other features distinguishing amoebozoans include the texture and ornamentation (if any) of the outer cell surface; the shape, distribution and number of the nucleus/nuclei and other organelles; and the presence and nature of a protective test (interestingly, while the testate filose amoeboids do not appear to form a monophyletic group among the Rhizaria, the testate lobose amoebae do seem to be monophyletic among the Amoebozoa - Nikolaev et al., 2005). There is a good detailed online guide at Alexey Smirnov's website (and a hat-tip to Psi Wavefunction for pointing the site out to me). A number of the higher taxa among the Amoebozoa have become reasonably robust in the last few years - if you're not too completely sick of amoeboids, I may introduce you to a few over the next few posts.

REFERENCES

Jahn, T. L., E. C. Bovee & D. L. Griffith. 1974. Taxonomy and evolution of the Sarcodina: a reclassification. Taxon 23 (4): 483-496.

King, R. L., & T. L. Jahn. 1948. Concerning the genera of amebas. Science 107: 293-294.

Nikolaev, S. I., E. A. D. Mitchell, N. B. Petrov, C. Berney, J. Fahrni & J. Pawlowski. 2005. The testate lobose amoebae (order Arcellinida Kent, 1880) finally find their home within Amoebozoa. Protist 156: 191-202.

Smirnov, A. V., & A. V. Goodkov. 1999. An illustrated list of basic morphotypes of Gymnamoebia (Rhizopoda, Lobosea). Protistology 1: 20-29.

Smirnov, A., E. Nassonova, C. Berney, J. Fahrni, I. Bolivar & J. Pawlowski. 2005. Molecular phylogeny and classification of the lobose amoebae. Protist 156: 129-142.