|
The Conosea
unifies amoebae which usually possess flagellate stages or are
amoeboflagellates. This clade consists of two relatively solid
groups – the Mycetozoa and Archamoebae, grouped by
Cavalier-Smith (1998) in the taxon Conosa, as well as a number
of independent lineages, including two flagellates –
Phalansterium (Cavalier-Smith et al. 2004) and Multicilia
(Nikolaev et al. 2004), and two gymnamoebae – Gephyramoeba
and Filamoeba (Amaral Zettler et al. 2000). Because of
large variations of the substitution rates in SSU rRNA genes
within this clade, its internal relationships are not resolved
yet.
The
Mycetozoa comprises two distinct groups of “slime molds” –
the Myxogastria and Protostelia (Dykstra and Keller 2000). This
is a well-defined group of protists, characterized by the
ability to form so-called “fruiting bodies”. In some lineages of
Mycetozoa the fruiting body is raised over the substratum on a
distinct stalk. Both groups possess complex life cycles
including an aggregation of cells, however the essential
difference between them is that in Protostelia, only a
pseudoplasmodium is formed (without fusion of the cells
constituting the aggregate), while in Myxogastria a true
plasmodium is formed (the cells completely fuse, forming a
single organism) (Olive 1975; Dykstra and Keller 2000). The
monophyly of Mycetozoa was proposed based on elongation factor
1-alpha gene sequences (Baldauf and Doolittle 1997) but it is
not always recovered in SSU rRNA trees (Cavalier-Smith et al.
2004; Nikolaev et al. 2004).
The
Archamoebae comprise amoeboid and amoeboflagellate protists
characterized by a secondary absence of mitochondria (mostly due
to parasitism or life in anoxic environments). This group
includes the free-living genera Mastigamoeba,
Mastigella, and Pelomyxa (the pelobionts) and the
parasitic genera Entamoeba and Endolimax (the
entamoebids). The consistent grouping of all these
amitochondriate amoeboid organisms in both SSU rRNA and actin
gene phylogenies (Fahrni et al. 2003) suggests a single loss of
the mitochondria during the evolution of Amoebozoa.
|
