Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 355

correlation with the situation in plants and some algae. However, the local-
ization of galactolipids was so far not pinpointed to a specific plastidial mem-
brane. This striking difference between chromerids and apicomplexan can
be explained by a hypothesis formulated by Bott´ et al. (2011) ; since plants
are known to relocalize galactolipids during phosphate starvation from the
plastid membranes to the extraplastidial ones, apicomplexans may have the
capacity to transfer galactolipids from their plastid in a similar way. Galact-
olipids of C. velia are enriched in very long chains of unsaturated fatty acid,
with MGDG and DGDG containing noncanonical fatty acid combination
C20:5 and C16:0, while the standard combination of plant and algal galact-
olipids is C18:3 and C16:3. In addition to that, genes coding for the MGDG
and DGDG synthases from C. velia were sequenced and phylogenetic anal-
ysis demonstrated their relation to the diatom counterparts in the frame of
the red-derived complex plastids. In particular, the chromerid DGDG
synthase branches off very early on the root of the red-derived plastids, while
the MGDG synthase forms a sister group to one of the paralogues found in
diatoms ( Bott´ et al., 2011 ). Such a phylogenetic position is compatible with
the origin of galactolipid synthesis from the red algal endosymbiont, the
ancestor of the chromerid plastid.
Sterols are cyclic lipid molecules required to build cellular membranes
and to maintain their integrity. Sterol composition is widely used in chemo-
taxonomy and in the case of dinoflagellates, sterols can be used even as a
biomarker to seek particular species ( Giner et al., 2003; Leblond and
Chapman, 2002; Leblond and Lasiter, 2012; Mooney et al., 2007 ). Sterols
can be synthesized de novo , as is the case in plants, algae, and many protists.
However, in the apicomplexan parasites, these membrane compounds are
obtained from the host ( Coppens et al., 2000; Nishikawa et al., 2005;
Sehgal et al., 2005 ). The sterol composition together with the genes
encoding sterol-synthesizing enzymes was recently investigated in
C. velia . It appears that chromerid sterol repertoire resemble that found in
the apicomplexans ( Leblond et al., 2012 ), although the latter group has been
shown to rely on import of sterols from their hosts ( Nishikawa et al., 2005 ).
Surprisingly, the dinoflagellates synthesize different spectrum of sterols than
C. velia , although these two groups are actually the closest known
phototrophic alveolates. Still, sterols of C. velia appear to be more closely
related to the sterol compounds of other phototrophs, such as glaucophytes
and chlorarachniophytes, suggesting the sterol synthesis in Chromera is
very ancient, probably ancestral

to other groups of algae ( Leblond

et al., 2012 ).