272 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE

Fig. 7.19 Scanning electron micrographs of (a) Karenia

(Gymnodinium) brevis and (b) Karenia (Gymnodinium) mikimoto.

(From Haywood et al., 2004.)

and red algae in that it is on the surface of thylakoids, is water-soluble, and acts as a lightharvesting pigment. If Glenodinium (Fig. 7.22) is cultured under decreasing light intensity (to 1⁄12 of

the original light intensity) the amount of PCP increases about seven fold, whereas the amount of chlorophyll a and peridinin not associated with PCP increases only 1.5 times. Little change occurs in chlorophyll c (Prézelin, 1976). This is a type of chromatic adaptation; as the cells receive less light (i.e., grow in deeper water), they produce more PCP, with the peridinin capturing light and passing it to chlorophyll a.

Phototaxis and eyespots

The action spectra for phototaxis is the same in all dinoflagellates that have been studied, with maximum phototaxis obtained at a wavelength of 450 nm (Fig. 7.20) (Horiguchi et al., 1999). An eyespot is not necessary for a phototactic response, indicating that the phototactic machinery was carried in the host organisms in the endosymbiosis leading to photosynthetic dinoflagellates (e.g., the phototactic receptor is not in the eyespot but is probably associated with the plasma membrane).

Less than 5% of the Dinophyceae contain eyespots, and those that do are mostly freshwater species; yet the eyespots are among the most complex in the algae.

The simplest type of eyespot consists of a collection of lipid globules in the cytoplasm not surrounded by a membrane (e.g., Woloszynskia coronata) (Dodge, 1971) (Fig. 7.21). A second type of eyespot consists of a row of lipid globules in a plastid-like structure at the cell periphery (e.g.,

Peridinium westii, W. tenuissima) (Messer and BenShaul, 1969; Crawford et al., 1970).

Fig. 7.20 The effect of different wavelengths of light on phototaxis in dinoflagellates. Maximum photosynthesis occurs at a wavelength around 450 nm. (Adapted from Horiguchi et al., 1999.)

DINOPHYTA 273

The eyespot of Glenodinium foliaceum (Dodge and Crawford, 1969) is immediately under the anterior portion of the sulcus and is about 6 m long and 3m wide (Fig. 7.22). It is more or less rectangular in outline with a hook-shaped projection at the anterior end. This flattened sac-like structure contains

Fig. 7.21 Woloszynskia tenuissima. (a) Ventral view showing the two flagella, the girdle (g), and the eyespot (e). (b) Side view showing the thecal plates, which were not drawn in (a). (After Crawford et al., 1970.)

two rows of large lipid globules separated by a granular space. Surrounding the eyespot is a triplemembrane envelope identical in appearance to that surrounding the chloroplasts. Adjacent to the eyespot is a non-membrane-bounded lamellar body consisting of a stack of flattened vesicles arranged more or less parallel to one another. The lamellar body is about 2 m long and 0.75 m wide, and contains up to 50 vesicles. The vesicles are connected at their edges, and also at the ends of the stack, with rough endoplasmic reticulum.

The most complex type of eyespot is found in the Warnowiaceae of the Peridiniales. The eyespot in Nematodinium armatum (Moronin and Francis, 1967) and that in Erythropsis cornuta (Gruet, 1965) have been studied at the finestructural level and found to be essentially similar in construction (Figs. 7.23, 7.24). In N. armatum, the eyespot is located toward the rear of the cell alongside the girdle, and consists of a lens mounted in front of a pigment cup, oriented so that the axis through the center of the lens and pigment cup is nearly perpendicular to the longitudinal axis of the cell body. The lens lies just below the plasmalemma. The pigment cup is made up of three main parts. Most of its wall consists of

Fig. 7.22 (a) A ventral view of a cell of Glenodinium foliaceum showing the location of the eyespot (e). (b) Threedimensional diagram of the eyespot area. The two flagella arise just above the lamellar body (l). The eyespot (e) lies beneath the sulcus. (mt) Microtubular roots; (b) banded root;

(t) thecal plate. (After Dodge and Crawford, 1969.)

274 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE

Fig. 7.23 (a) Nematodinium armatum. (b) Erythropsis cornuta. (l) Lens; (n) nucleus; (o) ocellus (eyespot); (p) pigment cup; (s) longitudinal sulcus; (te) tentacle.

a single layer of large oblong pigment granules 0.3m in diameter. Toward the lip of the cup, the pigment granules are smaller and loosely arranged in several layers. Within the base of the cup is a dense layer of fibrils 33 nm in diameter, parallel to the axis of the eyespot; this layer is covered by a mat of transverse fibrils and has been named the retinoid because of its supposed light-receptive function. Above the retinoid is a canal that opens into a furrow of the transverse flagellum; the outermost membrane on the cell surface is continuous with the membrane of the canal. The lens unit is a complex structure completely surrounded by mitochondria. Inside the mitochondria is a granular layer separated by a membrane from the lens. The central core of the lens consists of membranebounded domed or concentric layers of dense material; the major bulk of the lens surrounds this core and consists of several large, nearly empty lobes. Between the core and the lobes is a network of vesicles of intermediate size.

Fig. 7.24 (a) A cell of Nematodinium armatum, showing the lens (l), pigment cup (p), cnidocysts (cn), and a chloroplast (c). (b) A complete eyespot (ocellus) with retinoid (r), canal leading to outside (can), and mitochondria (m) surrounding the lens (l). (After Moronin and Francis, 1967.)

DINOPHYTA 275

Nucleus

The first dinoflagellates to evolve had nuclei similar to other algae with a closed mitosis and histones associated with the nucleic acids (Taylor, 1999). Oxyrrhis (Fig. 7.56(a)) is representative of such a dinoflagellate having microtubules formed inside the nuclear membrane during mitosis (Triemer, 1982; Gao and Li, 1986). Histones were gradually lost as dinoflagellate evolution proceeded and a type of mitosis evolved with the mitotic spindle outside the nuclear membrane.

The nuclei of the more advanced dinoflagellates are striking cytologically in that they have their chromatin condensed into 2.5 nm fibrils in the interphase nuclei (Figs. 7.2, 7.4). The organization of the chromatin is different from either prokarytoic or eukaryotic cells and has been referred to as mesokaryotic or dinokaryotic

(Rizzo, 1991).

Unicellular eukaryotic organisms usually have between 0.046 and 3 picograms (pg) of DNA per nucleus. Dinoflagellates, however, have much more DNA in their nuclei, with values ranging from 3.8 pg per nucleus in Cryptothecodinium cohnii (Fig. 7.3(e)–(g)) to 200 pg per nucleus in Lingulodinium polyedrum (Fig. 7.40(b)) corresponding to about 200 000 Mb (in comparison, hexploid Triticum wheat is 16 000 Mb and the haploid human genome is 3 180 Mb) (Sigee, 1984). This implies that a large amount of the DNA is genetically inactive (structural DNA) in dinoflagellates.

Nuclear division in the more advanced dinoflagellates has the nuclear envelope remaining intact during division, the nucleolus persisting and dividing by pinching in two, and the chromosomes attached to the nuclear envelope (Barlow and Triemer, 1988).

Mitosis in Syndinium (Fig. 7.25(a)) can be used as an example of the more advanced type in the dinoflagellates. The onset of division is usually marked by the duplication of the flagellar bases from two to four. Throughout this stage the nucleus enlarges, and many Y- and V-shaped chromosomes are found. The nucleus becomes invaginated, resulting in the formation of 1 to 15 channels traversing the dividing nucleus (Fig. 7.26). The channels contain tunnels of cytoplasm passing through the nucleus outside of the

(a)

(b)

Fig. 7.25 (a) Syndinium turbo. (b) Gymnodinium neglectum.

((a) after Chatton, 1952; (b) after Schiller, 1933.)

nuclear membrane. There are bundles of microtubules in these channels, which are not connected to the intact nuclear envelope. The chromosomes are probably attached to the nuclear membrane or a specialized kinetochore in the nuclear membrane. At metaphase there is no formation of a metaphase plate as is common with eukaryotes, and the chromosomes are still scattered. The nucleolus persists throughout the whole nuclear cycle and divides by constricting in the middle. In anaphase the cell and nucleus expand laterally, and the chromosomes move to opposite ends of the nucleus. With continued lateral expansion, the central isthmus eventually severs, and the two daughter nuclei become independent (Leadbeater and Dodge, 1967b; Kubai and Ris, 1969; Ris and Kubai, 1974).

Projectiles

The Dinophyceae have a number of different projectiles, which are fired out of the cell when it is irritated, resulting in a sudden movement of the cell in the opposite direction from the discharge.

A trichocyst has a membrane-bounded, rodshaped crystalline core (Bouck and Sweeney, 1966) (Fig. 7.27). Along the anterior one-third of the core are short, fine, tubular elements that project slightly downward. At the extreme outer end of the core, a group of 20 to 22 fibers extend from the outer part of the core to the enclosing membrane, and still finer fibrils then connect

276 CHLOROPLAST E.R.: EVOLUTION OF ONE MEMBRANE

Fig. 7.26 Diagrammatic representation of nuclear division in Syndinium sp. (a) Interphase. (b) Early division; kinetochores

and chromosomes have duplicated. (c) Early stage of chromosome segregation. Central spindle between separating basal bodies of flagella. (d) Late stage of chromosome separation. Central spindle in cytoplasmic channel through nucleus. (e) Division of nucleus. (B) Basal body of flagellum; (K) kinetochore;

(C) chromosome; (Mt) microtubules; (NM) nuclear membrane. (After Ris and Kubai, 1974.)

the larger fibrils to the apical portion of the trichocyst membrane. Just within the enclosing membrane are fine, thread-like, opaque hoops. The outer part of the trichocyst membrane is attached to the plasma membrane between the thecal vesicles or to the thecal vesicles beneath round, thin areas of thecal plates that form trichocyst pores. Trichocysts originate in areas rich in Golgi bodies and are probably derived from them initially as spherical vesicles that eventually become spindle-shaped and develop into the trichocysts. On irritation a “charged” trichocyst is converted to a “discharged” trichocyst in a few milliseconds, possibly by the rapid uptake of water. The discharged trichocysts are straight,

tapering rods many times longer than the charged trichocyst (up to 200 m long in Prorocentrum) (Figs. 7.30(c), 7.55, 7.56(c)). The discharged trichocyst has transverse banding, with a major period of 50 to 80 nm. Although most dinoflagellates have trichocysts, ultrastructural investigations have shown that some do not (i.e., Gymnodinium neglectum (Fig. 7.25(b)), Aureodinium pigmentosum, Woloszynskia tylota, and the symbiotic Symbiodinium microadriaticum). The actual benefit of trichocysts to the cell (if any) is still obscure. They could be a mechanism for quick escape as the cells move sharply in the opposite direction of discharge, or they could be able to directly “spear” a naked intruder.