Novel Clade 1: Pseudopirsonia, Auranticordis

Introduction

The cercozoan Novel Clade 1 was first established by Bass & Cavalier-Smith (2004) on the basis of molecular phylogenetic study inferred from SSU rDNA sequences. This clade originally contained only one described taxon, Pseudopirsonia mucosa, and a few other sequences derived from environmental studies of marine habitats. Subsequent study by Chantangsi et al. (2008) discovered another new member of this clade, namely Auranticordis quadriverberis (Figure 1). Other gliding zooflagellates also probably belong here.

Pseudopirsonia was first discovered in a study of parasitoid nanoflagellates–that is, very small eukaryotes that parasitize other protists, often phytoplankton (Kühn et al. 1996). This genus was originally described as a member of Pirsonia, which are heterokont parasitoids of planktonic diatoms, most closely related to Hyphochytrium (Kühn et al. 1996; Schnepf et al. 1990). However, phylogenetic analyses show that Pseudopirsonia is cercozoan and differs from true Pirsonia in morphology, movement, and feeding behaviour. However, the life cycles of the two genera show many similarities, and both include formation of conspicuous trophosomes and auxosomes (Kühn et al. 2004). Auranticordis is an unusual orange coloured marine free-living gliding zooflagellate with four posterior flagella (Chantangsi et al. 2008).

Characteristics

The group is too little studied for their key unifying structural characters to be determined, but known species have either two or four flagella inserted ventrally (Chantangsi et al. 2008; Kühn et al. 1996).

Habitat

Cercozoan members of the Novel Clade 1 inhabitat aquatic marine environments ranging from a cold (6°C) to temperate (15°C) areas (Kühn et al. 1996). Pseudopirsonia and Auranticordis were originally isolated from North Sea, Wadden Sea near List (Sylt, Germany) and Spanish Banks, Vancouver (BC, Canada), respectively (Chantangsi et al. 2008; Kühn et al. 1996).

Life Cycle and Development

Life cycle and development were studied only in Pseudopirsonia. Briefly, Pseudopirsonia parasitizes diatoms by intruding its pseudopods into the diatoms’ frustules and phagocytoses portions of the diatom protoplast. The pseudopodial part of the organism located inside the diatom cell differentiates into the trophosome. Digested materials and nutrients are then transported into the body of the flagellate, now called auxosome, attaching outside of the diatom frustule. The auxosome develops and divides longitudinally, forming offspring as long as trophosomes continue to phagocytose. This developing cell forms a globular, morula-like group in various arrangements usually coated with mucilage. (Kühn et al. 1996, 2004).

Discussion of Phylogenetic Relationships

The phylogeny in Kühn et al. (2004) shows Pseudopirsonia as a close relative of a Cercomonas strain (GenBank Accession No. AF277495). However, this sequence is mis-labelled and is not a cercomonad. The cercozoan trees in Bass & Cavalier-Smith (2004), Bass et al. (2005), and Cavalier-Smith et al. (2008) show that Pseudopirsonia belongs to cercozoan Novel Clade 1 (Bass & Cavalier-Smith 2004), which groups in an indeterminate position near spongomonads and/or imbricates. Other members of novel clade 1 are only known from environmental sequences, all from marine (coastal) water columns.

A study by Chantangsi et al. (2008) shows close phylogenetic relationships among Auranticordis, Pseudopirsonia, and two other sequences derived from unidentified taxa (GenBank Accession No. AB252755 and AB275058), forming the Auranticordida clade. Members of this clade are all from saline habitats and also share a derived molecular character within the context of 160 cercozoan sequences covering representatives from all known cercozoan subclades: namely, the substitution of cytosine (C) for thymine (T) at position 324 (with reference to the complete SSU rDNA sequence of Cercomonas sp.; GenBank accession No. AF411266, culture ATCC PRA-21) in Helix 12, based on the predicted secondary structure of the SSU rRNA gene in Palmaria palmata (Wuyts et al. 2000).

References

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