Soon after the gametes fuse (syngamy), meiosis is known to occur in the thick-walled zygotes of the Volvocales, Ulotrichales, Oedogoniales, Chlorellales, and Zygnematales.

Classification

The four important classes in the Chlorophyta are the Prasinophyceae (Micromonadophyceae), Charophyceae, Ulvophyceae, and the Chlorophyceae (Table 5.1). The classes were originally formulated in the 1970s by Karl Mattox and Kenneth Stewart (Fig. 5.4) working at Miami University of Ohio, and by Jeremy Pickett-Heaps (photograph in Preface) working at the University of Colorado. Their classification scheme was based largely on ultrastructural characteristics. Later investigations utilizing molecular genetics have verified their work.

The Charophyceae are in the line that evolved into land plants (embryophytes). As such, there have been a number of proposals made over the years to include the Charophyceae and land plants into a supergroup (Sluiman, 1985). The latest fashion is the term Viridiplantae for this line of evolution (Cavalier-Smith, 1981). A splinter of this is to include the Chara and its relatives with

the land plants into the “Steptophyta,” casually ignoring the close relationship between Chara and the remainder of the lower green algae.

Class 1 Prasinophyceae: scaly or naked flagellates with interzonal spindles that are persistent during cytokinesis; primitive green algae, some of which gave rise to the other classes in the Chlorophyta.

Class 2 Charophyceae: motile cells asymmetrical; two flagella attached in a lateral position in the cell; flagellar root consisting of a broad band of microtubules and a second smaller microtubular root; multilayered structure (MLS) may be present; no rhizoplast; scales common outside of motile cells; persistent interzonal mitotic spindle in telophase; phragmoplast produces new cross walls after cell division; eyespots usually not present; glycolate broken down by glycolate oxidase; urea broken down by urease; predominantly freshwater; sexual reproduction involves the formation of a dormant zygote; meiosis occurs when the zygote germinates.

Fig. 5.4 Kenneth Stewart (left) and Karl Mattox (right).

Kenneth Stewart Born September 4, 1932 in Moberly, Missouri. Dr. Stewart received his B.Sc. from Southern Illinois University at Carbondale, Illinois, in 1954. He served in the Army and worked in non-academic jobs until enrolling at the University of California, Davis, where he received his Ph.D. in 1968. In 1968 he joined the Department of Botany at Miami University, Oxford, Ohio.

Karl Mattox Born August 22, 1936 in Cincinnati, Ohio. He received his B.Sc. (1958) and M.A. (1960) from Miami University, Ohio, and his Ph.D. (1962) from the University of Texas. From 1962 to 1966, he was assistant professor in the Department of Botany at the University of Toronto. In 1966, he moved to the Department of Botany at Miami University.

Class 3 Ulvophyceae: flagella attached at anterior end of cell; motile cells have near-radial symmetry externally; flagella roots consist of four cruciately arranged microtubular roots and sometimes a rhizoplast; no multilayered structure (MLS); scales may be present on motile cells; persistent interzonal spindle in telophase; cleavage furrow produces the new cross wall in cell division; eyespots common; glycolate broken down by glycolate dehydrogenase; urea broken down by urease; predominantly marine; no dormant zygotes; alternation of generations common.

Class 4 Chlorophyceae: motile cells with radial or near-radial external symmetry; flagella attached at anterior end of cell; flagella roots consist of four cruciately arranged microtubular roots and sometimes a rhizoplast; no multilayered structure (MLS); theca common in motile cells; in telophase, the interzonal spindle collapses; phycoplast produces the new cross wall in cell division; eyespots common; glycolate breakdown by glycolate dehydrogenase; urea broken down by urea amidolyase; predominantly

freshwater; zygote undergoes a dormant period; meiosis occurs when the zygote germinates.

Position of flagella in cells

In the Charophyceae, the flagella are attached in a lateral position in the cell (Fig. 5.5). In the

Ulvophyceae and Chlorophyceae, the flagella are attached at the anterior end.

Flagellar roots

Flagellar basal bodies are anchored in the protoplast by microtubular roots and/or rhizoplasts

(Fig. 5.5) (Melkonian et al., 1988).

Microtubular roots

Microtubular roots consist of groups of 24-nm diameter microtubules that can have one of two basic configurations: (1) There can be a microtubular root consisting of a large broad band of microtubules with a smaller second microtubular root

(Charophyceae), or (2) there can be four groups of cruciately arranged microtubular roots running from the basal bodies (Ulvophyceae and Chlorophyceae). The cruciately arranged microtubular roots have what is called an X-2-X-2 arrangement. This notation refers to the fact that two of the microtubular roots are usually composed of two microtubules, whereas the two other roots can have different numbers of microtubules in different organisms. Thus Chlamydomonas moewusii has a 4-2-4-2 arrangement, whereas motile cells of Ulothrix sp. have a 5-2-5-2 arrangement (Moestrup, 1978). One of the roots containing two microtubules is often linked to the outer membrane of

Fig. 5.5 Schematic drawings of the side view of swarmers produced by three of the classes of green algae. (a)

Charophyceae: scaly cell with one large root (the MLS), one smaller root, and flagella extending at an angle from the point of insertion. (b) Ulvophyceae: four microtubular roots (two of each kind) in a cruciate arrangement, a pair of fibrous roots, and a scaly covering over the cell. (c)

Chlorophyceae: cruciate roots and the cell covered with a theca. (F) Fibrous roots; (M) microtubular root; (S) scales;

(T) theca. (After Mattox and Stewart, 1984.)

the chloroplast envelope and is probably involved in phototaxis.

Rhizoplasts (fibrous roots)

A rhizoplast is usually a cylinder containing 5- to 10-nm-diameter filaments interrupted at approximately 80-nm intervals by bands of electron-dense material (in the electron microscope). A rhizoplast runs from the basal bodies posteriorly toward the nucleus. Rhizoplasts are contractile (Salisbury and Floyd, 1978), and the distance between the bands in the rhizoplast varies depending on the state of contraction of the rhizoplast. The size of the filaments in the rhizoplast is similar to that of actin–myosin filaments in animal muscle cells. The method of contraction of the rhizoplast may be similar to that of muscle. Rhizoplasts may be present in the Prasinophyceae, Chlorophyceae, and Ulvophyceae, but are absent in the Charophyceae.

Multilayered structure

A multilayered structure (MLS) (Fig. 5.6) consists of a more or less rectangular body attached to the anterior end of the single broad band of microtubules in the Charophyceae and in the spermatozoids of lower land plants. The MLS lies directly beneath the basal bodies of the flagella. The MLS consists of four layers. The layer closest to the plasma membrane contains the microtubules of the root. Under this are two electron-dense layers. The bottom-most layer is composed of small microtubules. An MLS may be present in the

Prasinophyceae and Charophyceae, but is absent in the Chlorophyceae and Ulvophyceae.

Occurrence of scales or a wall on the motile cells

Motile cells covered with scales may occur in the

Prasinophyceae, Charophyceae, and Ulvophyceae. The presence of scales on the motile cells is probably the primitive condition. As evolution progressed, the scales became interweaved along their edges so a coherent cell covering was formed, as in the genus Tetraselmis (Fig. 5.12) (Domozych et al., 1981; Mattox and Stewart, 1984). The end result of this evolution was the theca that covers the motile cells in the Chlorophyceae. This theca has a crystalline substructure and is composed of hydroxyproline-rich glycoproteins associated with various polysaccharides (Roberts, 1974;

Miller, 1978; Deason, 1983). The theca in motile cells of the Chlorophyceae is thus not to be confused with the cell walls of non-motile stages of the more advanced Chlorophyta, which have cellulose as the main skeletal molecule.

Cellulose is produced by the enzyme cellulose synthetase that occurs as proteins embedded in the plasma membrane of the cell. Six to ten cellulose synthetase molecules are grouped into a single subunit. The subunits are, in turn, aggre-

gated into terminal complexes. In the Chlorophyta, there are two different types of terminal complexes (Fig. 5.7) (Okuda et al., 1994; Tsekos, 1999):

1In the Charophyceae, terminal complexes have subunits aggregated into rosettes.

2In the Chlorophyceae and Ulvophyceae, terminal complexes consist of linear rows of subunits.