Discosea: Keeping a Low Profile

Various views of Vannella devonica, a representative of the Vannellidae. Photos by A. Smirnov.

Having covered the Tubulinea, it's time to move on to some of the less familiar Amoebozoa. At this point, however, the competing classifications of Cavalier-Smith et al. (2004) and Smirnov et al. (2005) begin to part ways - at least superficially. Cavalier-Smith et al.'s class Discosea is roughly comparable to Smirnov et al.'s class Flabellinea, but the two are not identical. Rather, Flabellinea is a subgroup of Discosea, the latter also including a few taxa that Smirnov et al. listed as Amoebozoa incertae sedis. As such, I'll use Discosea for the larger group and Flabellinea for the smaller group. Monophyly of the Flabellinea has been recovered in a number of analyses (including both the studies just cited), monophyly of the Discosea is more doubtful. Kudryavtsev et al. (2005) found a monophyletic Discosea, but all other molecular analyses including non-flabellinean 'Discosea' show 'Discosea' as polyphyletic. The morphological characteristics of Discosea cited by Cavalier-Smith et al. (2004) ("highly flattened, often discoid amoebae that move slowly by a leading lamellipodium", and a tendency to secrete some sort of thickened protective covering or membrane) are not unique to members of the group. Nor is the other feature cited by Smirnov et al. (2005) as distinguishing Flabellinea from Tubulinea, that cytoplasmic flow in pseudopodia does not form a single central axis. Discoseans produce only a single anterior pseupodium when moving (which may or may not produce subpseudopodia), and no discosean has an adhesive uroid.

As well as the Flabellinea, Cavalier-Smith et al.'s Discosea included the Cochliopodiidae and Thecamoebida. Later analysis (Kudryavtsev et al., 2005) suggested the polyphyly of Thecamoebida, and the discosean component was restricted to the genus Dermamoeba (nevertheless, because Dermamoeba and Thecamoeba are ultrastructurally similar, I may as well cover both in this post). Subsequent studies have continued to indicate the polyphyly of 'Thecamoebida', but Pawlowski (2008) refers to unpublished data that might restore monophyly to the group. Cavalier-Smith et al. (2004) also suggested that the multiciliate Multicilia was a discosean, but Nikolaev et al. (2006) have since shown otherwise (Multicilia will feature in the next Amoebozoa post; with its removal, Discosea becomes an entirely non-ciliate group). Finally, though its authors refrained from saying so, it seems entirely possible that, if the Discosea should be monophyletic, the recently described distinctive genus Pellita (Smirnov & Kudryavtsev, 2005) may be discosean.


Thecamoeba striata. I suspect that the front of the cell is towards the lower left. Photo by Keisotyo.

A general characteristic of Amoebozoa that I have not yet had cause to mention is the presence of a glycocalyx, a protective covering of proteins and polysaccharides that lies outside but is connected to the membrane of the cell. The nature of the glycocalyx has often been used in differentiating amoebozoan taxa in the past, but recent studies suggest that it may be more variable than previously thought (e.g. Smirnov et al., 2007), so glycocalyx-based features should probably be treated with caution. Both 'Thecamoebida' and Flabellinea have particularly well-developed glycocalyces. In 'Thecamoebida', the glycocalyx is amorphous (probably the ancestral condition for Amoebozoa) but extremely thick. Thecamoebids on the move are generally oval or oblong in shape without anterior subpseudopodia. Thecamoeba has the dorsal surface of the cell shaped into longitudinal folds or wrinkles, while Dermamoeba has no dorsal folds or wrinkles (Smirnov & Goodkov, 1999).


Paramoeba aestuarina. The large dark spot is the "parasome" (or the endosymbiont Perkinsela amoebae, however you want to put it). Image from here.

In Flabellinea, the glycocalyx is usually differentiated into a covering of column-like glycostyles, though a number of flabellineans have lost the glycostyle coat (Smirnov et al., 2007). Flabellinea are divided into three families - Vanellidae, Paramoebidae and Vexilliferidae, with the latter two more closely related to each other than to Vanellidae. The basic form of Flabellinea is broad and fan-shaped; Vanellidae have a smooth leading edge without subpseudopodia, while Paramoebidae and Vexilliferidae produce subpseudopodia - short and blunt in Paramoebidae, long and slender in Vexilliferidae. Paramoebids were also distinguished in the past by their possession of a distinctive organelle known as the parasome. However, this distinction has been reworked in recent years - not because of doubts about the reality of the parasome, but because the parasome is now regarded as a separate organism in its own right, an endosymbiont rather than an organelle of paramoebids.


Diagram of Pellita, showing how the subpseudopodia extend through the thick cell coat. Figure from Smirnov & Kudryavtsev (2005).

The recently discovered Pellita (Smirnov & Kudryavtsev, 2005) resembles Flabellinea in possessing a cell covering of glycostyles. This covering is particularly thick in Pellita, and almost resembles a test rather than a coat. So thick is Pellita's covering that the normal means of amoebozoan movement and feeding via the projection of pseudopodia are not possible for it. Instead, Pellita produces short subpseudopodia with a covering of basic cell membrane only that muscle their way between the glycostyles until they project outside the coat. These subpseudopodia engulf individual bacteria if feeding, while for movement subpseudopodia produced near the leading edge of the cell adhere to the substrate and the cell rolls forward over the top of them.


Cochliopodium. Photos from here.

Finally, the Cochliopodiidae also possess an external covering, but in their case it is a rigid coat that is entirely separate from the cell membrane. In the genus Cochliopodium the coat consists of carbohydrate scales, while in the genera Gocevia and Paragocevia the coat is filamentous. The cochliopodiid coat differs from the test of Arcellinida in that it is restricted to the dorsal surface of the cell, not surrounding it as in arcellinidans. Also, the coat is divided between the daughter cells when the cochliopodiid divides; in Arcellinida, one daughter cell keeps the test while the other has to make an entire new test from scratch.

REFERENCES

Cavalier-Smith, T., E. E.-Y. Chao & B. Oates. 2004. Molecular phylogeny of Amoebozoa and the evolutionary significance of the unikont Phalansterium. European Journal of Protistology 40 (1): 21-48.

Kudryavtsev, A., D. Bernhard, M. Schlegel, E. E-Y. Chao & T. Cavalier-Smith. 2005. 18S ribosomal RNA gene sequences of Cochliopodium (Himatismenida) and the phylogeny of Amoebozoa. Protist 156: 215-224.

Nikolaev, S. I., C. Berny, N. B. Petrov, A. P. Mylnikov, J. F. Fahrni & J. Pawlowski. 2006. Phylogenetic position of Multicilia marina and the evolution of Amoebozoa. International Journal of Systematic and Evolutionary Microbiology 56: 1449-1458.

Pawlowski, J. 2008. The twilight of Sarcodina: a molecular perspective on the polyphyletic origin of amoeboid protists. Protistology 5 (4): 281-302.

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

Smirnov, A. V., & A. A. Kudryavtsev. 2005. Pellitidae n. fam. (Lobosea, Gymnamoebia) – a new family, accommodating two amoebae with an unusual cell coat and an original mode of locomotion, Pellita catalonica n.g., n.sp. and Pellita digitata comb. nov. European Journal of Protistology 41 (4): 257-267.

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.

Smirnov, A. V., E. S. Nassonova, E. Chao & T. Cavalier-Smith. 2007. Phylogeny, evolution, and taxonomy of vannellid amoebae. Protist 158 (3): 295-324.