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Flagella can be defined as motile cylindrical appendages found in widely divergent cell types
throughout the plant and animal kingdom, which either move the cell through its environment or
move the environment relative to the cell.
Motile algal cell are typically biflagellate, although quadriflagellate types are commonly found
in green algae; it is generally believed that the latter have been derived from the former, and a
convincing example of this derivation is Polytomella agilis from Chlamydomonas. A triflagellate
type of zoospore such as that of Acrochaete wittrockii (Chlorophyceae, Chlorophyta) may have
originated from a quadriflagellate ancestor by reduction, whereas the few uniflagellate forms
are most likely descendant of biflagellated cells. Intermediate cases exist, which carry a short second flagellum, as in Mantoniella squamata (Prasinophyceae, Chlorophyta) or Euglena gracilis,
where one flagellum is reduced to a stub (Figure 2.27); in some species, one flagellum of the pair is
reduced to a nonfunctional basal body attached to the functional one, as in the uniflagellate swarmer
of Dictyota dichotoma (Phaeophyceae, Heterokontophyta). A special case of multiflagellate alga is the naked zoospore of Oedogonium, where the numerous flagella form a ring or crown around the apical portion of the cell (stephanokont zoospore). |
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FIGURE 2.27 Scanning electron microscopy image of the reservoir of E. gracilis in longitudinal section
showing the locomotory emerging flagellum bearing the photoreceptor and the nonemerging flagellum
reduced to a stub. (Bar: 0.50 µm.) (Courtesy of Dr. Franco Verni.) |
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FIGURE 2.28 Scanning electron microscopy image of an isokont cell. (Dunaliella sp.). (Bar: 3 µm.) |
The characteristics of the flagella in a pair, that is, relative length and surface features, have led
to a specific nomenclature. When the two flagella differ in length and surface features, one being
hairy and the other smooth, they are termed “heterokont.” This term applies to all the members of
the division Heterokontophyta. When the two flagella are equal in length and appearance, the term
“isokont” is used (Figure 2.28), which applies to the algae of the division Haptophyta and to green algae such as Chlorophyceae and Charopyceae. Within this group, there are few genera whose flagella
differ in length, which are termed “anisokont.”
Description of flagella anatomy will proceed from outside to the inside, from the surface
features and components to the axoneme and additional inclusions to the structures anchoring
the flagella to the cell.
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