Ultrastructure of the flagellar apparatus in Rhinomonas reticulata var. atrorosea (Cryptophyceae, Cryptophyta)

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  • ABSTRACT

    Rhinomonas reticulata var. atrorosea G. Novarino is a photosynthetic marine flagellate that is known to have typical characteristics of cryptomonads. We examined the flagellar apparatus of R. reticulata var. atrorosea by transmission electron microscopy. The major components of the flagellar apparatus of R. reticulata var. atrorosea consisted of four types of microtubular roots (1r, 2r, 3r, and mr), a non-keeled rhizostyle (Rhs), mitochondrion-associated lamella (ML), two connections between basal bodies, a striated fibrous root (SR) and a striated fiber-associated microtubular root (SRm). Four types of microtubular roots originated near the ventral basal body and extended toward the left side of the basal bodies. The non-keeled Rhs originated at the Rhs-associated striated fiber, which was located between two basal bodies and extended into the middle of the cell. The ML was a plate-like fibrous structure associated with mitochondria and originating from a Rhs-associated fiber. It split into two parts and extended toward the dorsal-posterior of the cell to a mitochondrion. The SR and SRm extended parallel to the anterior lobe of the cell. The overall configuration of the flagellar apparatus in R. reticulata var. atrorosea was similar to the previously reported descriptions of those of Cryptomonas paramecium, C. pyrenoidifera, C. ovata, Hanusia phi, Guillardia theta, and Proteomonas sulcata. However, the flagellar apparatus system of R. reticulata var. atrorosea was more complex than those of other cryptomonad species due to the presence of an additional microtubular root and other distinctive features, such as a rhizostyle-associated striated fiber and large ML.


  • KEYWORD

    cryptomonad , diagrammatic reconstruction , flagellar apparatus , Rhinomonas , rhizostyle , TEM , ultrastructure

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  • [Fig. 1.] Transmission electron micrographs of general structure. (A) Longitudinal section of Rhinomonas reticulata var. atrorosea showing the chloroplast (Cp), Golgi body (G), gullet (Gu), nucleus (N), starch (S), pyrenoid (Py), mitochondria (Mt), and ejectosome (Ej). (B) Cross section at the gullet level showing the Cp, Ej, Py, and nucleomorph (Nm). (C) Cross section at the nuclear level showing the Cp and N. L, lipid. Scale bars represent: A, 1 μm; B & C, 0.5 μm.
    Transmission electron micrographs of general structure. (A) Longitudinal section of Rhinomonas reticulata var. atrorosea showing the
chloroplast (Cp), Golgi body (G), gullet (Gu), nucleus (N), starch (S), pyrenoid (Py), mitochondria (Mt), and ejectosome (Ej). (B) Cross section at
the gullet level showing the Cp, Ej, Py, and nucleomorph (Nm). (C) Cross section at the nuclear level showing the Cp and N. L, lipid. Scale bars
represent: A, 1 μm; B & C, 0.5 μm.
  • [Fig. 2.] Transmission electron micrographs of the connecting structures and microtubular roots at the basal body level. Serial cross sections of the two basal bodies from the posterior to the anterior direction showing the spatial relationships among the microtubular roots of the flagellar apparatus. (A) Cross section of the proximal ventral basal body (VB) showing that the electron dense layer (EDL) is connected to a triplet of ventral basal bodies. (B) Cross section of the two basal bodies showing the connecting structures. C1 and C2 connect with the two basal bodies. (C & D) Serial cross sections of the two basal bodies showing the origin points of the one-stranded microtubular root (1r), two-stranded microtubular root (2r), three-stranded microtubular root (3r), and another microtubular root (mr). (E-H) Serial cross sections of the two basal bodies showing the four root types. Immediately after genesis, the 1r, 2r, and 3r extended in parallel with the dorsal basal body, but the mr extended toward the ventral side of the cell. DB, dorsal basal body; DF, dorsal flagellum; VF, ventral flagellum. Scale bars represent: A-H, 0.2 μm.
    Transmission electron micrographs of the connecting structures and microtubular roots at the basal body level. Serial cross sections of the two basal bodies from the posterior to the anterior direction showing the spatial relationships among the microtubular roots of the flagellar apparatus. (A) Cross section of the proximal ventral basal body (VB) showing that the electron dense layer (EDL) is connected to a triplet of ventral basal bodies. (B) Cross section of the two basal bodies showing the connecting structures. C1 and C2 connect with the two basal bodies. (C & D) Serial cross sections of the two basal bodies showing the origin points of the one-stranded microtubular root (1r), two-stranded microtubular root (2r), three-stranded microtubular root (3r), and another microtubular root (mr). (E-H) Serial cross sections of the two basal bodies showing the four root types. Immediately after genesis, the 1r, 2r, and 3r extended in parallel with the dorsal basal body, but the mr extended toward the ventral side of the cell. DB, dorsal basal body; DF, dorsal flagellum; VF, ventral flagellum. Scale bars represent: A-H, 0.2 μm.
  • [Fig. 3.] Transmission electron micrographs of microtubular roots. (A-D) Serial oblique sections showing the spatial relationship among the onestranded microtubular root (1r), two-stranded microtubular root (2r), and another microtubular root (mr) extending from the left side of the two basal bodies in the anterior direction. The mr extended at a different angle. The three-stranded microtubular root (3r) extended toward the left side of the two basal bodies. DB, dorsal basal body; VB, ventral basal body; VF, ventral flagellum. Scale bars represent: A-D, 0.2 μm.
    Transmission electron micrographs of microtubular roots. (A-D) Serial oblique sections showing the spatial relationship among the onestranded
microtubular root (1r), two-stranded microtubular root (2r), and another microtubular root (mr) extending from the left side of the two
basal bodies in the anterior direction. The mr extended at a different angle. The three-stranded microtubular root (3r) extended toward the left
side of the two basal bodies. DB, dorsal basal body; VB, ventral basal body; VF, ventral flagellum. Scale bars represent: A-D, 0.2 μm.
  • [Fig. 4.] Transmission electron micrographs of the mitochondrion-associated lamella (ML) and rhizostyle (Rhs). (A) Oblique section of the two basal bodies showing the ML and Rhs, which originated at the rhizostyle-associated striated fiber (RS). The Rhs consisted of three microtubules. (B) Oblique section of the proximal ventral basal body (VB) showing that the ML divides into two fibrous bands. (C) Longitudinal section showing the ML extending toward the posterior and dorsal side of the cell. (D) Longitudinal section showing the spatial relationship between the Rhs and ML. (E) Longitudinal section showing the short Rhs extending to the middle of the cell. DB, dorsal basal body; Mt, mitochondria. Scale bars represent: A-E, 0.2 μm.
    Transmission electron micrographs of the mitochondrion-associated lamella (ML) and rhizostyle (Rhs). (A) Oblique section of the two basal bodies showing the ML and Rhs, which originated at the rhizostyle-associated striated fiber (RS). The Rhs consisted of three microtubules. (B) Oblique section of the proximal ventral basal body (VB) showing that the ML divides into two fibrous bands. (C) Longitudinal section showing the ML extending toward the posterior and dorsal side of the cell. (D) Longitudinal section showing the spatial relationship between the Rhs and ML. (E) Longitudinal section showing the short Rhs extending to the middle of the cell. DB, dorsal basal body; Mt, mitochondria. Scale bars represent: A-E, 0.2 μm.
  • [Fig. 5.] Transmission electron micrographs of the striated fibrous root (SR) and striated fiber-associated microtubular root (SRm). (A) Cross section of the two basal bodies showing the SR originating at the ventral basal body. (B) Longitudinal section of the ventral flagellum (VF) showing that that the SR and SRm extended parallel to the dorsal anterior lobe of the cell. (C) Longitudinal section of the two basal bodies showing that the SRm originated between the two basal bodies and consisted of three microtubules. (D-F) Serial cross sections of the SRm showing that the number of microtubules gradually increased to five and that the SRm consisted of four microtubules associated with a wing-like structure. DB, dorsal basal body; DF, dorsal flagellum; VB, ventral basal body. Scale bars represent: A-F, 0.2 μm.
    Transmission electron micrographs of the striated fibrous root (SR) and striated fiber-associated microtubular root (SRm). (A) Cross
section of the two basal bodies showing the SR originating at the ventral basal body. (B) Longitudinal section of the ventral flagellum (VF)
showing that that the SR and SRm extended parallel to the dorsal anterior lobe of the cell. (C) Longitudinal section of the two basal bodies
showing that the SRm originated between the two basal bodies and consisted of three microtubules. (D-F) Serial cross sections of the SRm
showing that the number of microtubules gradually increased to five and that the SRm consisted of four microtubules associated with a wing-like
structure. DB, dorsal basal body; DF, dorsal flagellum; VB, ventral basal body. Scale bars represent: A-F, 0.2 μm.
  • [Fig. 6.] Diagrammatic reconstructions of the flagellar apparatus in Rhinomonas reticulata var. atrorosea. Not to scale. (A) Diagram showing the overall flagellar apparatus. (B) Diagram showing a plane view from above. (C) Diagram showing a magnified view from the oblique left side. C1, connecting fiber 1; C2, connecting fiber 2; DB, dorsal basal body; DF, dorsal flagellum; EDL, electron dense layer; mr, microtubule root; ML, mitochondrion-associated lamella; Rhs, rhizostyle; RS, rhizostyle-associated striated fiber; SR, striated fibrous root; SRm, striated fiber-associated microtubular root; VB, ventral basal body; VF, ventral flagellum; 1r, one-stranded microtubular root; 2r, two-stranded microtubular root; 3r, three-stranded microtubular root.
    Diagrammatic reconstructions of the flagellar apparatus in Rhinomonas reticulata var. atrorosea. Not to scale. (A) Diagram showing the overall flagellar apparatus. (B) Diagram showing a plane view from above. (C) Diagram showing a magnified view from the oblique left side. C1, connecting fiber 1; C2, connecting fiber 2; DB, dorsal basal body; DF, dorsal flagellum; EDL, electron dense layer; mr, microtubule root; ML, mitochondrion-associated lamella; Rhs, rhizostyle; RS, rhizostyle-associated striated fiber; SR, striated fibrous root; SRm, striated fiber-associated microtubular root; VB, ventral basal body; VF, ventral flagellum; 1r, one-stranded microtubular root; 2r, two-stranded microtubular root; 3r, three-stranded microtubular root.