Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 349

in linear form and contains inverted repeats of AtpH , psbA , and ORF1 at
both ends of the linear DNA molecule ( Janouˇkovec et al., 2010 ;
Janouˇkovec et al., submitted for publication). Moreover, it has been shown
recently that psaA is uniquely split into two genes and consequently two
independently formed transcripts and even two proteins (psaA1 and psaA2)
are formed in C. velia ( Janouˇkovec et al., submitted for publication ). In con-
trast, the second chromerid alga, V. brassicaformis , possesses a highly con-
served and compact 85 kb-long circular plastid genome, yet it still
contains almost the same number of genes (81) as the C. velia plastid. Inter-
estingly, many above-described molecular characters, including the non-
canonical code, the split psaA gene, and the linear form of the plastid
genome with inverted repeats at both ends likely resembling telomere-like
sequences, are absent from the Vitrella organelle. Reassuringly, the
sequences of both plastid genomes provide evidence for a common origin
of the heterokont, chromerid, and apicomplexan plastids through the
homologous structure of their plastid ribosomal operon ( Janouˇkovec

et al., 2010 ).

5.2. Reduced mitochondrial genomes of chromerids
Apicomplexan parasites as well as dinoflagellates are known to possess a tiny
mitochondrial genome. If the highly reduced mitochondria-derived organ-
elles such as the mitosomes and hydrogenosomes lacking any genome
( Tovar et al., 1999 ) are not considered, the apicomplexan mitochondrion
carries the smallest known genome, encoding only three protein-coding
genes ( cox1 , cox3 , and cytb ), supplemented by extensively fragmented rRNA
genes ( Feagin, 1992; Vaidya et al., 1989; Waller and Jackson, 2009 ). How-
ever, based on available data, it appears that C. velia has an even smaller
mitochondrial genome than Plasmodium , with just two protein-coding
genes, namely, cox1 and cox3 being found so far (P. Flegontov, D.-H.
Lai, J. Janouˇkovec, P. Keeling, A. Pain, M. Oborn´k, and J. Lukeˇ,
unpublished results).

5.3. Chromerosome: as a possible mixotroph

C. velia also contains a rather mysterious structure in its cell ( Fig. 8.15 ). Orig-
inally incorrectly identified as a mitochondrion ( Moore et al., 2008 ), this
prominent structure now labeled chromerosome ( Oborn´k et al., 2011 )
contains solid reinforcement of unknown nature and is terminated by a kind
of flexible extension resembling a flagellum. However, electron microscopy

C. velia