Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 329

1. INTRODUCTION
The phylumApicomplexa is a group of single-celled eukaryotes living
as obligatory parasites of animals. These protists infect metazoan hosts rang-
ing from invertebrates, such as polychaetes ( Rueckert et al., 2010 ),
sipunculids ( Leander, 2006 ), cephalous molluscs ( Kopeˇn´ et al., 2006 ),
or various insects ( Hecker et al., 2002 ), to reptiles, amphibians, and mam-
mals, including humans ( Duszynski et al., 1999; Smith, 1996 ). Apicomplexa
also contains Plasmodium , the causative agents of malaria, the most devastat-
ing parasitic disease of humans, and the coccidium Toxoplasma gondii , likely
the most prevalent parasite of humans with proposed influence on their
behavior ( Flegr, 2007 ). As one of the most speciose eukaryotic groups with
an estimate of over a million species ( Pawlowski et al., 2012 ), not surpris-
ingly, the apicomplexans include many parasites of wild and domestic ani-
mals ( Chartier and Paraud, 2012 ). Apicomplexan parasites taxonomically
belong to alveolates, a group of protists, which also includes ciliates
(Ciliophora), usually free-living heterotrophic protists possessing numerous
cilia on their cell surface and a unique genetic system of macro- and micro-
nuclei, and mostly phototrophic dinoflagellates (Dinophyta), algae with
various complex plastids that are of high ecological importance in aquatic
environments ( Adl et al., 2012 ).
Most apicomplexans are known to contain a set of characteristic struc-
tures, in particular, the apical complex and the apicoplast. While the apical
complex, a sophisticated apparatus usually composed of the conoid rink,
rhoptries, and micronemes, is used to penetrate the host cell, the apicoplast
represents a secondary nonphotosynthetic plastid derived from a putatively
photosynthetic organelle. This remnant plastid is surrounded by four mem-
branes reflecting its complex origin in secondary or tertiary endosymbiotic
event (reviewed in Foth and McFadden, 2003 ; Lim and McFadden, 2010;
Oborn´k et al., 2009; Roos et al., 1999 ). Its genome is highly reduced to the
35 kb-long DNA circle ( Gardner et al., 1991; Kilejian, 1975 ) and lacks any
traces of genes involved in photosynthesis. Apicoplast genome structure,
gene content, and gene synteny are quite conserved among the
apicomplexans, supporting a single origin of this formerly photosynthetic
organelle ( Denny et al., 1998; Lang-Unnasch et al., 1998 ). However, not
all apicomplexans carry a plastid. It has been proved that members of the
genus Cryptosporidium , parasitizing the intestine of vertebrates including
humans, lack the apicoplast ( Abrahamsen et al., 2004; Xu et al., 2004;