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.
Copyright © 1998 Elsevier GmbH. All rights reserved.