Eukaryotic Tree

After much flux, we seem to be converging on a stable phylogeny for eukaryotic organisms (Fig. 1; Baldauf, 2003; Simpson and Roger, 2002; Keeling, 2004; Nikolaev et al. 2004; Simpson and Roger, 2004; although see, e.g., Philip et al., 2005). Most eukaryotes fall into one of six major clades: 1) the opisthokonts, containing the animals and fungi and a few unicellular groups; 2) the amoebozoans, containing the lobose amoebae (both naked and testate) and the slime molds; 3) the plants, containing the red and green algae (and the land plants) and a minor group known as the glaucophytes; 4) the chromalveolates, a clade that itself unites two major groups, the alveolates (containing the dinoflagellates, ciliates, and apicomplexans), and the chromists (including the diatoms, the oomycetes, the xanthophyte algae, and the brown algae); 5) the rhizarians, a group

* Many members of the Bacteria (=Eubacteria) and Archaea (=Archaebacteria) are also heterotrophic, but I restrict my discussion here to eukaryotic heterotrophs. Thus, when I use the term 'heterotroph', I am referring only to eukaryotic heterotrophs.

characterized by the possession of filose pseudopods, that includes the foraminifera, the (polyphyletic) radiolarians, and the cercozoans; and 6) the excavates, a controversial grouping (Simpson and Roger, 2004) that includes the euglenids and several parasitic taxa such as Giardia. Recent gene fusion data suggest that these six clades are divided into two groups: the 'unikonts' (opisthokonts and amoebozoans), and the 'bikonts' (plants, chromalveolates, rhizarians, and excavates), with the root of the eukaryotic tree falling between these two groups (Stechmann and Cavalier-Smith, 2002, 2003).

Heterotrophic taxa are highlighted in Fig. 1. Although many eukaryotes are capable of mixotrophy—acquiring nutrition via photosynthesis and phagotrophy, I will restrict my discussion below to those taxa most or all of whose members are strictly heterotrophic. Thus, I will focus on the early fossil record of only five eukaryotic clades: the opisthokonts, the amoebozoans, the chromalveolates, the rhizarians, and the excavates. With few exceptions, all plants are photosynthetic.

Eukaryotic Tree

Figure 1. A current view of eukaryote relationships, based on molecular and ultrastructural data (modified from Baldauf 2003; Simpson and Roger, 2002; Keeling, 2004; Nikolaev et ad, 2004; Simpson and Roger, 2004). Clades composed primarily of heterotrophs shown in boxes with solid lines; clades with both heterotrophs and autotrophs shown in boxes with dashed lines. A question mark indicates clades that are not strongly supported (Keeling, 2004). Rooting of the tree is based on gene fusion data (Stechmann and Cavalier-Smith 2002, 2003).

Figure 1. A current view of eukaryote relationships, based on molecular and ultrastructural data (modified from Baldauf 2003; Simpson and Roger, 2002; Keeling, 2004; Nikolaev et ad, 2004; Simpson and Roger, 2004). Clades composed primarily of heterotrophs shown in boxes with solid lines; clades with both heterotrophs and autotrophs shown in boxes with dashed lines. A question mark indicates clades that are not strongly supported (Keeling, 2004). Rooting of the tree is based on gene fusion data (Stechmann and Cavalier-Smith 2002, 2003).

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