Page 82 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 65

paraflagellar rod (PFR), which is known only in the order of Pedinellales (Chrysophyceae, Hetero-
kontophyta), and in members of the Euglenophyta and Dinophyta. PFRs are complex and highly
organized lattice-like structures that run parallel to the axoneme. Among the different groups,
PFRs are very similar structurally and biochemically.
In the Pedinellales, the membrane of the single emergent flagellum is expanded into a sheath or
fin supported along the edge by the rod, which is cross-banded. Owing to the presence of the fins,
these algae are excellent swimmers. Their axoneme beats in a distinct planar wave. Paraflagellar
structures of characteristic appearance are present in some dinoflagellates. A hollow cylinder
with the wall composed of helically arranged filaments is present in the single emergent (longitudi-
nal) flagellum of Noctiluca gametes, and in the longitudinal flagellum of both Gyrodinium lebour-
iae and Oxyrrhis marina. This rod is about as big as the axoneme in diameter and runs along the
axoneme to which it is attached for almost its entire length. The thin filaments (nanofilaments
with a 2–4 nm diameter) of its highly geometrically organized network are periodically attached
to some of the outer doublet microtubules of the axoneme, which in O. marina is doublet no.
4. A different type of paraxial rod is present in the complex-shaped transverse flagellum of dino-
flagellates. This rod takes a nearly straight path along the inner wall of the sulcus and is regularly
banded in the transverse direction. In contrast, the axoneme itself is distinctly helical.
A rod is present in most members of Euglenophyta. In genera with two emergent flagella, such
as Eutreptia and Eutreptiella, both flagella carry a paraxial rod. In Eutreptiella, where the two
flagella differ in length, the rods differ from each other in thickness and fine structure, the longer
flagellum carrying a more complex rod than the shorter flagellum. In genera with a flagellar appar-
atus reduced to one long emergent flagellum and one short flagellum not extending beyond the
reservoir region, only the emergent flagellum retains the rod, which extends its entire length. An
example is Euglena gracilis; in this alga, the rod arises just above the flagellar transition zone
and is located latero-ventrally with respect to the axoneme and the cell body. In cross-thin sections
of isolated and demembranated flagella, the rod appears hollow with an outer diameter of 90 nm.
Images obtained from negative staining preparations show that the rod is made up of several coiled
filaments, with a diameter of 22 nm, forming a seven-start left-handed helix with a pitch of 458 and
a periodicity of 54 nm. Extending from the surface of the rod a series of goblet-like projections can
be observed, which form the point of attachment between the rod and one of the axonemal doublet
microtubules (Figure 2.37a and 2.37b). The PFR does not assume any consistent orientation with
respect to the central-pair microtubules of the emergent flagellum.
Two major protein components of the PFR have been identified in euglenoids, and dinoflagel-
lates with a number of possible minor protein constituents. These major proteins (referred from now
as PFR 1 and PFR 2 ) migrate in the SDS–PAGE as a doublet of similar abundance. Depending on the
organism, the mobility for PFR 1 ranges from 70 to 80 kDa, and for PFR 2 from 62 to 70 kDa. Coiled-
coils are a common structural motif in the filament formed using the PFR1 and PFR2 proteins.
Database searches reveal a 41-residue conserved region of the PFR 1 /PFR 2 family that bears a sig-
nificant relationship to a conserved motif within the central coiled-coil rod of tropomyosin.

Other Intraflagellar Accessory Structures
Other intraflagellar accessory structures are present in dinoflagellates besides the PFR, the so-called
R fiber (Rf), and the so-called striated fiber (Sf). These structures do not show any kind of lattice or
precisely organized structure. However, they deserve mentioning because they run for long dis-
tances along the axonemal structure and therefore are also candidate for modulating the axonemal
2þ
beating, possibly by Ca -dependent contraction or through dipole–dipole forces.
The Rf is made of thin filaments 2–4 nm in diameter. It may be as large as the axoneme or the
PFR in diameter (300–500 nm), runs along the major part of the axoneme and is attached to it via
the PFR, the PFR linking the Rf to the axoneme. The Rf may contract and shows transversal stria-
tions of variable periodicities and thickness only during its contraction, but not in its relaxed or fully