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In-Depth Information
However, checkpoint mechanisms may regulate the number of nuclei re-entering
the cell cycle at a given time point thus determining the overall number of pro-
duced progeny (Reininger et al.
2011
).
Several kinases regulate spindle dynamics and control the cell cycle in higher
eukaryotes (Nigg
2001
; Carmena and Earnshaw
2003
), including members of the
Aurora-kinase family. Pfark-1 (Plasmodium Aurora-related kinase 1) has been
observed to be associated with the spindle during blood stage schizogony, and
PfPK5 (a Cdk1-like kinase) is considered to be the main cell cycle controller in
Plasmodium (Graeser et al.
1996
; Reininger et al.
2011
). Recently, it was proposed
that the molecules that associate with and control the centrocone might be
responsible for the asynchronous division in each nucleus. This controlled asyn-
chronous division could thus have evolved in order to avoid destabilizing sudden
even numbers of nuclei within the restricted space available for the division in the
host cell cytoplasm (Arnot et al.
2011
).
Centrins associate with centrosomes, form fibrous structures, and are important
for centrosome duplication (Salisbury
1995
). In Plasmodium, four centrins appear
to be conserved from the ancestral alveolate (Mahajan et al.
2008
). These centrins
localize to spindle plaques and are differently expressed during the intraerythro-
cytic stage and thus likely perform diverse roles.
During schizogony, the endoplasmic reticulum transforms into a perinuclear
ring before it develops into an extensive branched network in later stages of cell
segmentation. Part of the endoplasmic reticulum is known to be a specialized area
of the secretory pathway. It has been shown that this site is in close proximity to
the spindle plaques, being postulated to produce precursors for organelles like
rhoptries and Golgi during division (Bannister et al.
2000
; Striepen et al.
2007
).
The apicoplast and the mitochondrion also develop into large branched structures
during schizogony (Fig.
19.3
b or Fig.
19.3
b and c). These organelles are then
synchronously cleaved-first the apicoplast, then the mitochondrion—creating
several copies of these organelles that are transferred to the individual daughter
cells. While this looks impressive during the blood stage (van Dooren et al.
2005
),
it is simply awe-inspiring during the liver stage, when they shatter into thousands
of fragments (Stanway et al.
2011
). Curiously, the apicoplast and the mitochon-
drion appear to be linked to each other in the blood stages possibly facilitating the
transfer to the budding parasites (Aikawa and Beaudoin
1968
; van Dooren et al.
2005
), while they are separated from each other in the mosquito and liver stage
(Kudryashev et al.
2010
; Stanway et al.
2011
). Why care? In Toxoplasma, the
apicoplast is connected to the centrioles during division (see below). So perhaps
the Plasmodium apicoplast could also be linked to the spindle plaques and might
thus in turn connect the mitochondrion to the nucleus-division machinery; at least
in the blood stage. This could provide the basis for targeting organelles into new
progeny parasites. If so, one wonders what the differences between the blood and
liver stages are.
To continue their life cycle, some parasites depart from their asexual replication
to differentiate into sexual parasite forms (Fig.
19.3
a). In the mosquito stomach
they can sense the new environment and start to differentiate even further and form
