Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 332

shown that such an arrangement is present already in ciliates and had obviously
evolved multiple times during evolution ( Waller and Keeling, 2006; Waller
et al., 2003 ). The ambiguous phylogeny of the apicoplast reflects the extreme
divergence of its fast-evolving genes. Indeed, the AT content of the
P. falciparum apicoplast genes can reach up to 97% and phylogenetic analyses
of such biased sequences are heavily affected by various phylogenetic artifacts,
including the long-branch attraction phenomenon (Dacks et al., 2002).
However, the discovery of a new group of photosynthetic alveolates
called Chromerida represented a true breakthrough in this respect
( Moore et al., 2008; Oborn ´ k et al., 2012 ). These algae contain relatively
conserved plastid genomes with the gene repertoire overlapping with those
of the apicomplexan and dinoflagellate plastids. Moreover, it was unambig-
uously shown that the chromerid plastid is the closest known phototrophic
relative to the apicoplast. Accumulating evidence derived from nucleus-
encoded genes further showed that chromerids share a common ancestry
with the Apicomplexa ( Janouˇkovec et al., 2010; Koˇen´ et al., 2011;
Moore et al., 2008; Oborn´k et al., 2009 ).
2. CHROMERIDA: A NEW GROUP OF ALGAE ISOLATED
FROM AUSTRALIAN CORALS
Dinoflagellates are photosynthetic alveolates with complex plastids
well known as important symbionts of corals ( Freudental, 1962; Trench,
1993 ). During an investigation of the coral-associated flora in Australia,
new algae were isolated from stony corals by a procedure usually used to
isolate intracellular symbionts. When shape and morphology are considered,
one of these algae to some extent resembled symbiotic dinoflagellates of the
genus Symbiodinium , but molecular phylogeny complemented with ultra-
structural and metabolic analysis revealed its unexpectedly close relationship
with the apicomplexan parasites ( Janouˇkovec et al., 2010; Koˇen´ et al.,
2011; Moore et al., 2008; Oborn´k et al., 2009, 2011 ).
Along with the formal taxonomic description of Chromera velia , the new
phototrophic phylum Chromerida was established in the frame of the alve-
olates ( Moore et al., 2008 ). Recently, the life cycle and morphology of
Vitrella brassicaformis heralded the description of another chromerid lineage
( Oborn´k et al., 2012 ). Topology of phylogenetic trees based on the SSU
rRNA genes suggested the affiliation of chromerids to colpodellids, free-
living heterotrophic predators of algae closely related to the apicomplexan
parasites ( Fig. 8.1 ; Janouˇkovec et al., 2012a,b; Moore et al., 2008 ).