Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 346

acquisition of secondary plastid of a red lineage ( Burki et al., 2012b ). We
tend to believe in a scenario involving two independent secondary endo-
symbioses with red algae, one having occurred on the root of the SAR
group, with the second one being responsible for the emergence of an inde-
pendent lineage of cryptophytes ( Burki et al., 2012b ). Such an evolutionary
pattern is further supported by the presence of the nucleomorph, a remnant
nucleus of the endosymbiotic alga, which was found in cryptophytes ( Curtis
et al., 2012 ).
The number of membranes surrounding the chromerid plastid betrays
their origin via a secondary or even other complex endosymbiotic event
( Janouˇkovec et al., 2010; Moore et al., 2008 ). A substantial evidence sup-
ports its common ancestry with the apicoplast, the nonphotosynthetic relic
plastid of Apicomplexa, in particular, molecular phylogeny inferred from the
plastid SSU rRNA genes ( Moore et al., 2008 ), followed by a comprehensive
analysis of 30 conserved plastid protein-coding genes ( Janouˇkovec et al.,
2010 ). However, it should be noted that in most constructed phylogenetic
trees inferred from the plastid genes, chromerid, apicomplexan, and dino-
flagellate plastids constitute very long branches. Unfortunately, phylogenetic
analyses containing such genes tend to be affected by various artifacts
( Mindell and Thacker, 1996; Oborn´k et al., 2002; Philippe and Germot,
2000; Stiller and Hall, 1999 ). Therefore, nonphylogenetic evidence
supporting a relationship between chromerid plastid and the apicoplast is
of particular importance, with the potential to counterbalance conclusions
based on artifactual tree topology. Such is the case of the alternative genetic
code for tryptophan in the plastid-encoded genes in C. velia (also see below)
( Janouˇkovec et al., 2010; Moore et al., 2008 ), which is a conspicuous syn-
apomorphy with the apicoplast of Coccidia, a group of advanced
apicomplexan parasites, and the same applies for the noncanonical pattern
of heme biosynthesis (also see Chapter 6.1.) which is homologous to the
Apicomplexa ( Ko ˇ en´ et al., 2011 ). Further, analysis of gene synteny in
the plastid ribosomal operon suggests that plastids of these enigmatic
coral-associated algae share a common origin with plastids

from the

apicompexans, the heterokont algae, and dinoflagellates ( Janouˇkovec

et al., 2010 ).
One of the obstacles preventing determination of the origin of the
apicoplast has been the absence of any pigmentation of this non-
photosynthetic organelle. Since the chromerids apparently share origin with
the apicomplexans, and their plastid is closely related to the apicoplast, we
have investigated their pigments, the analysis of which showed surprising