Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 335

Figure 8.3 Ultrastructure of C. velia. Scanning electronmicroscopy of an autosporangium
enclosing four tightly bound autospores, and a single free autospore (left panel). Trans-
mission electron microscopy of a cross-sectioned autosporangium containing autospores
(right panel). Note prominent sausage-shaped plastids (scale bar m). The right panel

is reprinted from Oborník et al. (2011) , Copyright (2011), with permission from Elsevier 2011.

¼ 1

m

better equipped to escape from such unfavorable and harmful light conditions.
We speculate that zoospores may also represent the stage invading the
coral larvae.
Since various phylogenetic analyses supported a close relationship
between the chromerids, dinoflagellates, and apicomplexans, we have inves-
tigated the ultrastructure of C. velia , seeking structural synapomorphies
shared by these protists ( Oborn´k et al., 2011 ). The vegetative stage com-
posed of up to four coccoids ( Fig. 8.2 ) contains sutures on the surface of
its sporangium that resemble those seen on the coccidian cysts, such as
T. gondii and related coccidian genera ( Fig. 8.3 ; Kopeˇn´ et al., 2006 ). Fur-
ther, vegetative cells and autosporangia are reminiscent of the apicomplexan
cysts, particularly those belonging to the genus Cryptosporidium , a water-
borne causative agent of human cryptosporidiosis ( Hunter and Nichols,
2002 ). The alga divides asexually by binary division ( Fig. 8.4 ), forming tet-
rads or triads in an autosporangium enclosed by an additional membrane
( Fig. 8.5 ). At the ultrastructural level, C. velia displays other characters typ-
ical for alveolates, including cortical alveoli—flattened vesicles underlying
the plasma membrane ( Fig. 8.6 ) and supported by a single-layered sheet
of microtubules forming a corset surrounding the entire algal cell.
A single prominent cone-shaped plastid ( Fig. 8.2 B) is bound by four mem-
branes ( Fig. 8.7 ), a feature shared with the apicoplast, which reflects their
complex evolutionary history. In contrast to V. brassicaformis , the plastid does
not contain a conspicuous pyrenoid.