The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes

Abstract

Recent molecular phylogenetic studies revealed the extraordinary diversity of single-celled eukaryotes. However, the proper assessment of this diversity and accurate reconstruction of the eukaryote phylogeny are still impeded by the lack of molecular data for some major groups of easily identifiable and cultivable protists. Among them, amoeboid eukaryotes have been notably absent from molecular phylogenies, despite their diversity, complexity, and abundance. To partly fill this phylogenetic gap, we present here combined small-subunit ribosomal RNA and actin sequence data for the three main groups of "Heliozoa" (Actinophryida, Centrohelida, and Desmothoracida), the heliozoan-like Sticholonche, and the radiolarian group Polycystinea. Phylogenetic analyses of our sequences demonstrate the polyphyly of heliozoans, which branch either as an independent eukaryotic lineage (Centrohelida), within stramenopiles (Actinophryida), or among cercozoans (Desmothoracida), in broad agreement with previous ultrastructure-based studies. Our data also provide solid evidence for the existence of the Rhizaria, an emerging supergroup of mainly amoeboid eukaryotes that includes desmothoracid heliozoans, all radiolarians, Sticholonche, and foraminiferans, as well as various filose and reticulose amoebae and some flagellates.

Fig. 1.

Phylogenetic relationships among 121 eukaryotes, emphasizing the polyphyly of heliozoans and radiolarians (gray and black boxes, respectively) and the emergence of the Rhizaria, a previously undescribed supergroup of mainly amoeboid protists (light gray box). The topology shown corresponds to a supertree reconstructed from the consensus trees of Bayesian analyses of 111 SSU rRNA gene sequences and 68 actin gene sequences (see text). Numbers at nodes represent PP of the SSU rRNA (upper) and actin (lower) Bayesian analyses. Dashes indicate support values <0.5, and an X indicates that the considered node is absent in one of the trees due to the differences in available taxa sampling between the two genes. Support values within the lineages were omitted for clarity. The separate ML trees of SSU rRNA and actin analyses are in Figs. 2 and 3.