Rhizaria is a heterogeneous
assemblage of protists, whose existence is based exclusively on
molecular evidence. This supergroup appeared first as the union
of the euglyphids and chlorarachniophytes in SSU rRNA gene
phylogenies (Bhattacharya et al. 1995). Cavalier-Smith and Chao
(1997) showed these taxa to be related to the cercomonad and
thaumatomonad flagellates and the plasmodiophorid plant
pathogens, and the new phylum Cercozoa was erected to
accommodate this assemblage (Cavalier-Smith 1998).
Over the past few years, several
other protists of previously unclear affiliation have been shown
to be part of this assemblage based on SSU rRNA data. These
notably include the testate, filose Gromia oviformis (Burki
et al. 2002), the “athalamid”, reticulose Gymnophrys cometa
(Nikolaev et al. 2003), the haplosporidian and paramyxid
parasites of bivalves (Cavalier-Smith and Chao 2003a), the
desmothoracid heliozoans (Nikolaev et al. 2004), and the
phaeodarean radiolarians, which are not closely related to
Acantharea and Polycystinea (Polet et al. 2004). Various other
amoeboid and/or flagellated organisms also belong to Cercozoa,
further increasing the morphological heterogeneity of the group
(Atkins et al. 2000; Bhattacharya and Oliveira 2000; Kühn et al.
2000; Bulman et al. 2001; Vickerman et al. 2002; Wylezich et al.
2002; Cavalier-Smith and Chao 2003b).
In the meantime, two important
lineages of amoeboid protists have been shown to be closely
related to the Cercozoa. The first one is the Foraminifera, as
demonstrated by actin (Keeling 2001; Archibald and Keeling
2004), polyubiquitin (Archibald et al. 2003), RNA polymerase II
(Longet et al. 2003), and revised SSU rRNA (Berney and Pawlowski
2003) analyses. Available data point at Gromiidae (and possibly
Haplosporidia) as the closest relatives of Foraminifera (Longet
et al. 2004).
The radiolarians are the second
important group of protists that has been shown to be related to
the Cercozoa-Foraminifera clade. This relationship was first
proposed based on SSU rRNA studies (Burki et al. 2002;
Cavalier-Smith 2002; Polet et al. 2004), and the term Rhizaria
was created by Cavalier-Smith (2002) to name this new supergroup
of eukaryotes. The existence of the Rhizaria is now further
supported by the first protein data on radiolarians (Nikolaev et
al. 2004). The Rhizaria thus includes the majority of filose and
reticulose amoebae and most actinopods, plus two parasitic
lineages and some flagellates.