BIOLOGY AND EVOLUTION OF CHROMALVEOLATE PROTISTS

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.