Cell Biology of Chromerids: Autotrophic Relatives to Apicomplexan Parasites - International Review of Cell and Molecular Biology - page 343

sporangia, and flagellated zoospores are supposed to emerge from the
released immotile autospores ( Oborn´k et al., 2011 ). However, by light
microscopy, we have recently observed extremely large cells

( zoosporangia) in the culture of C. velia , which contain four or eight flag-

ellated zoospores. Excystation of zoospores from these zoosporangia occurs
within about 2 min (Kruˇinsk´ et al., unpublished results). Although the
function of zoospores remains unknown, we have observed their very fast
encystation, during which the cells under unfavorable conditions discarded
their flagella, and within 10 min, the thin zoospores secreted a thick wall.
Vegetative cell can be formed either from released autospore or encystated
zoospore and commences a new life cycle ( Oborn´k et al., 2011 ).
In contrast to C. velia , V. brassicaformis forms sporangia containing dozens
to hundreds of autospores or zoospores ( Figs. 8.2 and 8.13 ). After reaching
about 10-30 m in diameter, the content of vegetative cells becomes fine
grained and the forming sporangium becomes colored according to the type
of spores it will produce. Inside of the green autosporangium, autospores
formed gradually from the periphery of the cell are eventually released
and start a new life cycle. An alternative pathway results in the production
of lightly brown zoosporangia containing a conspicuous orange structure
similar to the reddish-like globule found in the Eustigmatophyte algae.
Interestingly, newly formed zoospores equipped with two heterodynamic
flagella move very quickly within the still intact zoosporangium and swarm
out of it after the rupture of its wall. The fate of the released zoospores is yet
to be established. The high number of spores and the size of its sporangia may
imply that V. brassicaformis should grow faster than C. velia , yet the opposite is
the case. The number of spores in both types of sporangia represents the
main difference between the life cycles of

¼

these chromerid algae

( Oborn´k et al., 2011, 2012 ).

4. EVOLUTION OF EXOSYMBIONT
Origin of an alga with a secondary plastid is generally complex. Partic-
ularly in the case of the alveolates, which also include dinoflagellates known to
host plastids of various origins

in the frame of the same nuclear

( ¼ exosymbiont) lineage, phylogenetic positions of both secondary host

( endosymbiont) should be carefully

specified. In addition to that, the nuclear genomes of secondary algae (

¼ exosymbiont) and complex plastid ( ¼

algae
harboring secondary plastid) are hybrids, as they bring together eukaryotic

¼