Phylogenetic Relationships among the Cryptophyta: Analyses of Nuclear-Encoded SSU rRNA Sequences Support the...

The Cryptophyta comprise photoautotrophic protists with complex plastids which harbor a remnant eukaryotic nucleus (nucleomorph) and a few heterotrophic taxa which either lack a plastid (Goniomonas) or contain a complex plastid devoid of pigments (Ieucoplast; Chilomonas). To resolve the phylogenetic relationships between photosynthetic, leucoplast-containing and aplastidial taxa, we determined complete nuclear-encoded SSU rRNA-sequences from 12 cryptophyte taxa representing the genera Cryptomonas, Chilomonas, Rhodomonas, Chroomonas, Hemiselmis, Proteomonas and Teleaulax and, as an outgroup taxon, Cyanoptyche gloeocystis (Glaucocystophyta). Phylogenetic analyses of SSU rRNA sequences from a total of 24 cryptophyte taxa rooted with 4 glaucocystophyte taxa using distance, parsimony and likelihood methods as well as LogDet transformations invariably position the aplastidial genus Goniomonas as a sister taxon to a monophyletic lineage consisting of all plastid containing cryptophytes including Chilomonas. Among the plastid-containing taxa, we identify six major clades each supported by high bootstrap values: clade I (Cryptomonas and Chilomonas), clade II (Rhodomonas, Pyrenomonas, Rhinomonas and Storeatula), clade III (Guillardia and the 'unidentified cryptophyte' strain CCMP 325), clade IV (Teleaulax and Geminigera), clade V (Proteomonas) and clade VI (Hemiselmis, Chroomonas and Komma). Clade I (Cryptomonas and Chilomonas) represents a sister group to clades II-VI which together form a monophyletic lineage; the phylogenetic relationships between clades II-VI remain largely unresolved. Chilomonas is positioned within the Cryptomonas clade and thus presumably evolved from a photosynthetic taxon of this genus. In our analysis the characters blue and red pigmentation do not correspond with a basal subdivision of the phylum, thus rejecting this character for higher-level classification of cryptophytes. However, different spectroscopic subtypes of phycoerythrin (PE I-III) and phycocyanin (PC II-IV) represent informative characters at a lower taxonomic level. Phycocyanin types are confined to the later diverging clade VI and within Hemiselmis, a species with phycocyanin is monophyletic with two species containing phycoerythrin. This supports previous molecular studies which demonstrated that the β subunit of all cryptophyte biliproteins, regardless of spectroscopic type, is phylogenetically derived from the red algal β-phycoerythrin gene family, therefore the cryptophyte phycocyanins presumably originated by chromophore replacement from phycoerythrin. Our phylogenetic analysis does not support a previous suggestion that the aplastidial cryptophyte Goniomonas evolved from an ancestor containing a complex cryptomonadtype plastid by nucleomorph and plastid loss.