Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 345

account, an ordinary plastid-encoded tufA gene was, in contrast to dinoflagel-
lates, amplified from C. velia ( Janouˇkovec et al., 2010; Oborn´k et al., 2009 )
and maximum-likelihood phylogenetic analysis placed it on the root of its
apicomplexan homologues ( Oborn´k et al., 2009 ). It should be mentioned that
chromerids also share some molecular characters with dinoflagellates, such as
the use of bacterial type II Rubisco and oligoU tails in their plastid transcripts.
However, this is not surprising, since dinoflagellates represent the closest photo-
trophic relatives of the chromerid algae, with both phototrophic alveolates and
their plastids sharing a common ancestry ( Janouˇkovecetal.,2010 ).

5. EVOLUTION OF CHROMERID ORGANELLES
Like the related algae, chromerids contain two semiautonomous
organelles: the mitochondrion and the plastid. The complex plastid is,
according to the number of surrounding membranes, of secondary origin,
presumably from an as yet unspecified engulfed rhodophyte. Unexpectedly,
the plastids of both described chromerid species substantially differ, and this
difference is based on available data, attributed to different speed of evolu-
tion ( Janouˇkovec et al., 2010; Oborn´k et al., 2012 ). The level of mutual
divergence between C. velia and V. brassicaformis , as well as between their
plastids, is mainly reflected by the structure of plastid genomes. In addition
to the mitochondrion and the plastid, a unique structure called
chromerosome is found in almost each C. velia vegetative cell. The function
of this large and conspicuous organelle-like structure remains unknown
( Oborn´k et al., 2012 ).

5.1. Evolution of chromerid plastids
Tom Cavalier-Smith proposed that the entire eukaryotic supergroup
Chromalveolata appeared, evolved, and expanded thanks to the secondary
endosymbiotic event between a heterotrophic eukaryote ( ¼ exosymbiont)

and a rhodophyte ( endosymbiont), giving rise to the complex plastids sur-

rounded by more than two membranes ( Cavalier-Smith, 1999 ). Although
the taxon Chromalveolata has been already virtually replaced by the SAR
( ¼ Stramenopila, Alveolata, Rhizaria) group ( Adl et al., 2012 ), secondary
endosymbiosis remains the unquestionable major player in the evolution
of these complex ubiquitous eukaryotes. However, the exact number of
endosymbiotic events that occurred in the evolutionary history of algae
with secondary red plastids remains controversial. Current extensive
phylogenomic analyses are compatible with two rather than a single

¼