Page 80 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 63

Flagellar Spines

Flagellar spines are a peculiarity of unknown function confined to male gametes of a few oogamous
brown algae. The spermatozoids of Dictyota sp. are unique in possessing a longitudinal row of 12
very short spines on their single hairy flagella (these spermatozoids are basically biflagellate, but
the second flagellum is reduced to its basal body only). Spines are absent on the distal 2.5–
3 mm of the flagellum, and on the proximal 10 mm. In some Fucales (Himanthalia, Xiphophora,
and Hormosira), spermatozoids possess only a single spine, up to 1.0 mm long. In all these
algae, each spine is made up of electron-dense material, located between the flagellar membrane
and the peripheral axonemal doublets.

INTERNAL FEATURES
Axoneme

The movements of the flagella are generated by a single functional unit, the axoneme, which con-
sists of a long cylinder, from 10 to 100 mm long, with a 0.2 mm diameter. Its structure, as seen in
cross-sections by electron microscopy, is almost ubiquitous: it is made of nine equally spaced outer
microtubule doublets (A and B) approximately 40 nm in diameter surrounding two central micro-
tubules, the central pair (Figure 2.29). This arrangement is maintained by a delicate series of lin-
kages to give the classical 9þ2 pattern. The nine outer doublets are numbered starting from
number 1 located in the plane orthogonal to the plane including the central pair and counting clock-
wise when looking from the tip of the flagellum. The former plane allows the definition of the cur-
vature directions during beating as left or right relative to it. Doublets are transiently linked by
outer/inner Dynein arms (ODA and IDA) that represent the flagellar motor, and permanently inter-
connected by nexin links; the radial spokes connect the central pair to the peripheral microtubules
of the outer doublets. Divergence from the basic 9þ2 pattern are rare, but include the spermatozoid
of some centric diatoms (9þ0) and the chlorophyta Golenkinia minutissima (9þ1), as well as the
haptonema of the Prymnesiophyceae (Figure 2.36). This structure develops between the two fla-
gella of these algae, and it is sometimes longer than the flagella themselves. It resembles a flagel-
lum, but contains a central shaft of six to eight microtubules arranged in a cylinder, with no
doublets. In transverse section, the microtubules are disposed in an arc of a circle or in a ring
and are surrounded by a limb of the smooth endoplasmic reticulum. The distal part of the hapto-
nema is fairly straightforward. It is surrounded by plasma membrane, which is continuous over
the tip of the haptonema and may be smooth, drawn into a tip or form a spathulate projection.
The bulk of axonemal proteins (70%) is made of tubulins, the building blocks (heterodimers)
that polymerize linearly to form microtubules. Those tubulins, which constitute the wall of micro-
tubules belong to the a and b families, whose sequences have been conserved during evolution
(other families, g, d, and e, are responsible for microtubule nucleation at the level of the basal
bodies/centrosomes). A large molecular diversity among tubulins is generated by a series of
post-translational modifications such as acetylation, detyrosylation, polyglutamylation, or poly-
glycylation. Tektin filaments are present at the junction between the A and B microtubules of
each doublet. The internal and external arms that graft to the peripheral doublets represent
10–15% of the global protein mass of axonemes and are essentially formed by the “dynein-
ATPases” motor (the Greek word “dyne” means force). Microtubular dyneins are large multi-
molecular complexes with a pseudo-bouquet shape, and a molecular mass ranging from 1.4
(bouquets with two heads) to 1.9 MDa (bouquets with three heads) for the whole molecule, and
approximately 500 kDa for the largest subunits containing the ATP hydrolysis site. The size of
both outer and inner dynein arms is approximately 50 nm. Among the 250 different polypeptides
present in the axoneme, as estimated by bidimensional electrophoresis, only a few have been
associated with a function.