72
Algae
longest in the middle. Hairs on the longitudinal flagellum are shorter than those on the transverse
flagellum (0.4–0.75 μm), but similar in diameter (10 nm).
Simple, nontubular hairs are present also in some Glaucophyta and Chlorophyta.
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. (Phaeophyceae) 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 μm of the flagellum, and on the proximal 10 μm. In other Phaeophyceae such as
Himanthalia, Xiphophora, and Hormosira, spermatozoids possess only a single spine, up to 1.0- μm
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 oF The FlagellUm
Axoneme
The movements of the flagella are generated by a single functional unit, the axoneme, which consists of a long cylinder, from 10- to 100-μm long, 0.2-μm in 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
microtubules, the central pair (Figure 2.28). This arrangement is maintained by a delicate series
of linkages to give the classical 9 + 2 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
clockwise when looking from the tip of the flagellum. The former plane allows the definition of
the curvature directions during beating as left or right relatively to it. Doublets are transiently
linked by outer/inner dynein arms (ODA and IDA) that represent the flagellar motor and permanently interconnected by nexin links; the radial spokes connect the central pair to the peripheral
microtubules of the outer doublets. Divergence from the basic 9 + 2 pattern is rare, but include the
spermatozoid of some centric diatoms (9 + 0) and the chlorophyta Golenkinia minutissima (9 + 1),
as well as the haptonema of the Prymnesiophyceae (Figure 2.35). This structure develops between
the two flagella of these algae, and it is sometimes longer than the flagella themselves. It resembles
a flagellum, but contains a central shaft of 6–8 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 haptonema is
fairly straightforward. It is surrounded by the 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 microtubules, belong to the a and b families, whose sequences have been conserved during evolution (other
families, g, d, 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 polyglycylation. 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 multimolecular complexes with a pseudo-bouquet
shape and a molecular mass ranging from 1.4 MDa (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 ODA and IDA is approximately 50 nm. Among
Algae
longest in the middle. Hairs on the longitudinal flagellum are shorter than those on the transverse
flagellum (0.4–0.75 μm), but similar in diameter (10 nm).
Simple, nontubular hairs are present also in some Glaucophyta and Chlorophyta.
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. (Phaeophyceae) 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 μm of the flagellum, and on the proximal 10 μm. In other Phaeophyceae such as
Himanthalia, Xiphophora, and Hormosira, spermatozoids possess only a single spine, up to 1.0- μm
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 oF The FlagellUm
Axoneme
The movements of the flagella are generated by a single functional unit, the axoneme, which consists of a long cylinder, from 10- to 100-μm long, 0.2-μm in 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
microtubules, the central pair (Figure 2.28). This arrangement is maintained by a delicate series
of linkages to give the classical 9 + 2 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
clockwise when looking from the tip of the flagellum. The former plane allows the definition of
the curvature directions during beating as left or right relatively to it. Doublets are transiently
linked by outer/inner dynein arms (ODA and IDA) that represent the flagellar motor and permanently interconnected by nexin links; the radial spokes connect the central pair to the peripheral
microtubules of the outer doublets. Divergence from the basic 9 + 2 pattern is rare, but include the
spermatozoid of some centric diatoms (9 + 0) and the chlorophyta Golenkinia minutissima (9 + 1),
as well as the haptonema of the Prymnesiophyceae (Figure 2.35). This structure develops between
the two flagella of these algae, and it is sometimes longer than the flagella themselves. It resembles
a flagellum, but contains a central shaft of 6–8 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 haptonema is
fairly straightforward. It is surrounded by the 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 microtubules, belong to the a and b families, whose sequences have been conserved during evolution (other
families, g, d, 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 polyglycylation. 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 multimolecular complexes with a pseudo-bouquet
shape and a molecular mass ranging from 1.4 MDa (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 ODA and IDA is approximately 50 nm. Among