Biology Reference
In-Depth Information
¼
genes of the exosymbiont and the algal endosymbiont (
the plastid remnant
of an engulfed eukaryotic alga), possibly mitochondrial genes of both symbi-
otic participants, plastid (
¼
cyanobacterial) genes (
Oborn´k and Green, 2005
),
and quite often a high portion of other, usually bacterial genes, likely obtained
by lateral gene transfer (
Bowler et al., 2008
). It was proposed that algae within
the recently established SAR group (
Adl et al., 2012
), particularly diatoms,
passed through an ancient endosymbiotic event involving a green alga, since
they allegedly encode a high proportion of genes displaying green algal origin
(
Moustafa et al., 2009
). However, one has to keep inmind that taxon sampling
strongly influences such topologies, and so far, genome of a single rhodophyte,
Cyanidioschyzon merolae
, has been fully sequenced (
Matsuzaki et al., 2004
).
Moreover, a reevaluation of these data questioned the above-mentioned con-
clusions (
Deschamps and Moreira, 2012
).
The origin of chromeran nuclear lineage (
exosymbiont) was proposed
based on several phylogenetic analyses, initially the SSU and LSU rRNA
genes (
Moore et al., 2008
), followed by trees computed from the combined
dataset of eight nuclear genes (hsp90, hsp70,
a
-tubulin,
b
-tubulin, second
paralogue of hsp70 [biP], elongation factor 2, and SSU and LSU rRNA
genes) with a total of 7,137 characters (
Janouˇkovec et al., 2010
). Finally,
a complex phylogenetic analysis of 3000 expressed sequence tags has been
performed (
Woehle et al., 2011
). Unfortunately, the latter extensive analysis
was damaged by the presence of a substantial contamination by land plant
sequences, which in combination with low requirements for trees evaluated
as valid (for instance, bootstraps were not computed) led to doubtful results
showing a high fraction of green genes in the chromerid transcriptome.
Indeed, a careful reevaluation of these data was compatible with a more con-
servative evolutionary scenario (
Burki et al., 2012a
).
It should be mentioned that all phylogenetic analyses invariably demon-
strated the position of
C. velia
on the root of Apicomplexa (
Burki et al.,
2012a
;
Janou
ˇ
kovec et al., 2010; Moore et al., 2008; Oborn
´
k et al., 2009
).
Moreover, other characters also support the conclusion that chromerid algae
are genuine relatives of the apicomplexan parasites rather than some unusual
dinoflagellates. The
C. velia
genome contains typical histones H2A and H2B
(
Oborn´k et al., 2009
), while the related dinoflagellates have permanently con-
densed chromosomes (
Rizzo and Burghardt, 1982
), which are packed by a
recently described dinoflagellate-specific family of highly basic nuclear proteins
(
Gornik et al., 2012
). Further,
C. velia
differs from these relatives by not having
its chromosomes condensed during its entire cell cycle (
Oborn´k et al., 2009,
2011
). Last but not least, when the endosymbiont-derived genes are taken into
¼
Search WWH ::
Custom Search