I was at a bit of a loss as to what to post on next, so I asked my partner to "name an organism, any organism". He suggested "amoeba", so I'm following his instructions. Besides, the subject of today's post was bound to raise its pseudopodia somewhere along the line. I've decided to write on what I'll informally dub TAFKAMI - The Amoeba Formally Known As Mastigamoeba invertens. TAFKAMI is an anaerobic amoeboflagellate with a cilium shorter than the body.
A little backgroud, first. Protist phylogeny has always been a contentious, uncertain world - compared to multicellular organisms, unicells have relatively few obvious characters to unite various groups of taxa. However, the availability of better and better electron microscopy and the continued improvements in molecular phylogenies mean that in recent years, a growing consensus has developed that the majority of eukaryotes fall into a few large "supergroups" (Simpson & Roger, 2004) - the opisthokonts (including fungi and animals), amoebozoans, excavates (mostly flagellates), rhizarians (including radiolarians and foraminiferans), chromalveolates (including ciliates, brown algae and dinoflagellates) and plants (including green and red algae). There are still a few random taxa that don't necessarily fall into any of these groups.
The supergroup Amoebozoa includes the majority of amoebae with lobose pseudopodia, as well as most slime moulds and the Archamoebae, a group of amitochondriate anaerobic amoebae (take a moment to appreciate the assonance). The Archaemoebae include Mastigamoeba proper (as well as Entamoeba, the causative organism of amoebic dysentery).
According to Walker et al. (2006), TAFKAMI was first isolated back in 1992. Since then, it has been what is technically referred to as a right pain in the khyber. Comparisons using both microscope and molecular techniques between TAFKAMI and other supposedly related organisms increasingly indicated that it was not Mastigamoeba, or any other known amoeba. In molecular studies (such as Cavalier-Smith & Chao, 2003) TAFKAMI leapt about madly, sometimes with amoebozoans, sometimes with apusomonads (another small group that doesn't fit into any of the supergroups), sometimes entirely elsewhere.
Walker et al. (2006) recently established TAFKAMI as a new taxon, Breviata anathema (the original description of Mastigamoeba invertens from 1892 does not allow reliable identification of what that species is). They also presented a detailed comparison of Breviata with the main contenders for close relationship.
Cavalier-Smith et al. (2004) felt that TAFKAMI was the basalmost member of the Amoebozoa, placing it in its own class Breviatea. The main reason for doing so was that TAFKAMI was supposed to possess a single ciliary basal body (the organelle that the flagellum emerges from). However, Walker et al. found that Breviata actually had double basal bodies. There is reasonably good evidence (Cavalier-Smith, 2002; Simpson & Rogers, 2004) that eukaryotes with double basal bodies form a single über-clade, the bikonts (including the excavates, rhizarians, chromalveolates and plants). It seems quite believable that Breviata is a member of this clade. Breviata also lacks the molecular markers of Amoebozoa proper (Cavalier-Smith et al., 2004).
Under certain parameters, molecular phylogenies supported an association of Breviata with the afore-mentioned apusomonads, which are also bikonts. Breviata also has similar pseudopodia to apusomonads. However, I would be just as sceptical of a direct apusomonad-Breviata connection. At the present, I don't feel that Breviata can be placed as anything more that "basal bikont". (It is worth noting, too, that one amoebozoan group, the Myxogastrea, has independently evolved double basal bodies.)
Perhaps most interestingly, Walker et al. identified a large organelle overlying the nucleus that they suggested as a possible hydrogenosome. Hydrogenosomes are respiratory organelles that are generally regarded as having been independently derived from mitochondria in a number of anaerobic groups. As such, Breviata potentially joins the growing list of supposedly amitochondriate taxa retaining mitochondrial remnants.
Cavalier-Smith, T., & E. E.-Y. Chao. 2003. Molecular phylogeny of centrohelid Heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss. Journal of Molecular Evolution 56 (4): 387-396.
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: 21-48.
Simpson, A. G. B., & A. J. Roger. 2004. The real ‘kingdoms’ of eukaryotes. Current Biology 14 (17): R693-R696.
Walker, G., J. B. Dacks & T. M. Embley. 2006. Ultrastructural description of Breviata anathema, n. gen., n. sp., the organism previously studied as ‘‘Mastigamoeba invertens’’. Journal of Eukaryotic Microbiology 53 (2): 65-78.
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