BIOLOGY AND EVOLUTION OF CHROMALVEOLATE PROTISTS
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Abstract
Dinoflagellates and haptophytes are both prominent members of the
marine phytoplankton and are considered chromalveolates. The interactions of
the parasitic dinoflagellate Amoebophrya with its host dinoflagellate Akashiwo
sanguinea were studied using cell biological techniques. The free-swimming
dinospore stage of Amoebophrya has two flagella, trichocyts, striated strips,
condensed chromatin resembling heterochromatin, and electron dense bodies.
When entering the host cytoplasm and again when entering the host nucleus, the
electron dense bodies appear in a tube of microtubules close to the surface of
the host or its nucleus. Host entry is inhibited by cytochalasin D implying a role
for microfilament polymerization in the entry process. While in the host
cytoplasm, Amoebophrya appears to be separated from the host cytoplasm by
two membranes. After entering the host nucleus, the parasite grows and
undergoes mitosis forming a multinucleated trophont. The mastigocoel is an
internal cavity that contains flagella and becomes the outside of the parasite after
it leaves the host. This study indicates that the mastigocoel forms as a result of
vesicle fusion. Eventually, Amoebophrya fills the host nucleus and takes on a
beehive appearance. The beehive stage contains numerous trichocyts and
striated strips. The level of chromatin condensation in intracellular trophonts is
highly variable. It then exits its host as a multinucleated vermiform shaped
creature, which then splits up into individual infective dinospores. A
phylogenomic pipeline was designed to analyze the genome and evolutionary
history of the haptophyte Emiliania huxleyi. It appears to have genes linking it to
three lineages: heterokonts, green algae, and red algae. Genes with shared
phylogenetic affinities appear to fit into limited functional categories and be
physically localized in the genome. The phylogenetic affinities of E. huxleyi with
the green algae may be an artifact of the much greater number of sequenced
genomes from the Viridiplantae (=plants+ green algae) when compared to the
rhodophytes. The evolutionary history of E. huxleyi is still unclear although they
do seem to be similar in many ways to heterokonts and are generally believed to
have red algae derived plastids.