INTRODUCTION TO THE APUSOZOA

CORE APUSOZOA

The apusozoans are enigmatic gliding unicells that generally are biflagellated (Figures 1-3) and have an organic sheath around the base of the anteriorly-directed flagellum (Figures 1-2). The cells have a dorsal organic theca but a naked ventral area from which emerge pseudopodia. They resemble Jakobids (from the Excavata) in several ways:

• Both have heterodynamic flagella with the recurrent flagellum associated with the ventral, feeding portion of the cell.
• Both have taxa with mitochondria, which are constant for the individual taxon, but vary in the form of cristae from taxon to taxon.

The similarities likely are superficial and the result of a suite of shared primitive characters.

The apusozoans, though represented by only a few genera seem to be ubiquitous in marine and freshwater environments (Adl et al. 2012, Paps et al. 2013). They even occur in deep hydrothermal vent communities (Atkins et al. 2000a and 2000b).

BREVIATES AND MANTAMONADS

Here we include a few other basal taxa, the breviates, planomonads, and mantamonads, which are affiliated with the Amoebozoa and the Opisthokonta. Presently, they are represented by few taxa, but likely they are severely underrepresented because they are difficult to identify. Breviatea (Cavalier-Smith in Cavalier-Smith et al. 2004) is a problematic taxon that was described using a species that is in a sister clade to Entamoeba. Breviata is an amoeboid cell with a single anteriorlly-directed apical flagellum (Figure 4). Unlike most other Amoebozoa, they have a few filopods that radiate from the cell. Analyses are difficult to place them in the unikonts, but they seem to be sisters to the rest of the Amoebozoa (Minge et al. 2009, Paps et al. 2013).

The mantamonads, represented by Mantomonas plastica, have been found in coastal marine sediments (Glücksman et al. 2011). This species is biflagellated, but only the posteriorlly-directed flagellum seems to be functional. The anterior flagellum is short and thin. The overall cell shape is asymmetrical. The left side (see Figure 5) seems to be supported by a cytoskeleton into an angular shape while the right side is amoeboid.

FIGURE 1. Apusomonas, a motile cell that can produce pseudopodia (text with tooltip) A pseudopod (pseudopodium, sing; pseudopodia, pl) is an extension of a naked cell (no wall, pellicle, etc.) that is ephemeral and used for feeding or locomotion. Note the thickened retractable portion at the base of the flagellum. Image by http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/	FIGURE 2. DIC micrograph of Amastigomonas that shows both the anterior (text with tooltip) An anteriorly-directed flagellum extends in the direction of the motion of the motile cell. The interpretation is that the flagellum functions by pulling the cell. and recurrent (text with tooltip) Recurrent flagella bend to the posterior end of the cell. Typically, they are identified as recurrent when anteriorly-directed flagella are present. flagella. The thick retractable part of the anterior flagellum is visible. Image by http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/	FIGURE 3. DIC micrograph of Ancyromonas that shows both flagella. Image by http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/	FIGURE 4. Micrograph of Breviata that shows the anterior flagellum. Note the few radiating filopodia from the elongate cell. Image from Minge et al. (2009)	FIGURE 5. Micrograph of Mantomonas that shows the posterior flagellum. Note the angular lobe on the left side and the plastic right side. Image from Glucksman et al. (2011)

SYSTEMATICS OF THE APUSOZOA

The whole phylum, which likely is polyphyletic, is made up of three groups which may not be related at all because they have different kinds of mitochondrial cristae (flat and tubular; Patterson 1999). Cavalier-Smith (2002, 2003) and Nikolaev et al. (2004) indicate that the apusomonads are sisters to all other Bikonts and sisters to the Amoebozoae. Therefore, they might be among the most primitive of the biflagellated eukaryotic taxa. Indeed, Cavalier-Smith and Chao (2003) tie the apusomonads to the origin of the heliozoans and the Rhizaria (Cercozoans). Earlier, Cavalier-Smith (2002) defined the apusomonads as a phylum within the Rhizaria. Kim et al. (2006) explored the phylogenetic relationships between Apusomonas and other major eukaryotic lineages. They concluded that Apusomonas was a sister to the Opisthokonts and that the Amoebozoa were sisters to the Opisthokonts+Apusomonas, which was confirmed by larger taxon sampling by Cavalier-Smith and Chao (2010). Minge et al. (2009) and Paps et al. (2013) have shown Breviata is a sister to the Amoebozoa. Furthermore, Glucksman et al. (2013) indicate that Mantamonas + Apusomonadida is sister to Planomonadida (Ancyromonadidae + Planomonadidae), all of which is associated with the Amoebozoa and Opisthokonta. Adl et al. (2005 and 2012) place them in Eukaryotes of uncertain standing. Clearly, the apusomonads rest most uneasily anywhere, but the solution to their placement may be key to understanding the topology of the unikonts (Paps et al. 2013).

FIGURE 6. A cladogram showing the relationships between the Apusozoa (taxa in the shaded box) and the groups of the Amoebozoa. The topology is weakly supported by Kim et al. (2006), Minge et al., 2009and Paps et al. (2013).