168 EVOLUTION OF THE CHLOROPLAST

fructification in a left-hand spiral. An extinct family, the Trochiliscaceae, also had a spirally twisted envelope, but it was twisted to the right.

Ulvophyceae

The motile cells of the Ulvophyceae have apically attached flagella, near-radial symmetry externally, and a cruciate microtubular-root system that is not associated with a multilayered structure. These characteristics differ from those of the Charophyceae but are similar to those of the Chlorophyceae. The Ulvophyceae, however, differ from the Chlorophyceae in having (1) a persistent interzonal spindle that does not collapse at telophase, and (2) motile cells without a cell wall.

The algae in the Ulvophyceae are predominantly marine although there are a number of freshwater species. All filamentous marine green algae or larger green seaweeds studied ultrastructurally have been shown to belong to the Ulvophyceae. The life cycle usually involves the alternation of a haploid thallus with a diploid thallus. The wide occurrence of alternation of generations in the Ulvophyceae might be due to the more stable marine environment fostering the evolution of a longer life cycle. Dormant zygotes are not known in the class. A number of genera in the Ulvophyceae produce swarmers with scales (Mattox and Stewart, 1973), indicating that the class had a scaly ancestor.

Classification

The following orders are placed in the Ulvophyceae. Cladistic analysis of nuclear-encoded rRNA sequence data have shown that the Ulotrichales and Ulvales form one group in the class, while the Caulerpales, Siphonocladales, and Dasycladales form a second group (Watanabe et al., 2001).

Order 1 Ulotrichales: uninculeate filamentous algae with a parietal chloroplast.

Order 2 Ulvales: uninucleate cells with a parietal chloroplast; thallus is a hollow cylinder or a sheet, one or two cells thick.

Order 3 Cladophorales: multinucleate filamentous algae with a parietal perforate or reticulate chloroplast.

Order 4 Dasycladales: thallus has radial symmetry composed of an erect axis bearing branches; thallus uninucleate but multinucleate just before reproduction; gametes formed in operculate cysts.

Order 5 Caulerpales: coenocytic algae lacking cellulose in the walls; siphonoxanthin and siphonein usually present.

Order 6 Siphonocladales: algae with segregative cell division; siphonoxanthin present.

Ulotrichales

Uninucleate filamentous green algae with a parietal chloroplast constitute this order.

Ulothrix (Fig. 5.31) is found in quiet or running freshwater and occasionally on wet rocks or soil. The thallus consists of unbranched filaments of indefinite length that are adfixed to the substratum by a special basal cell. All of the cells except the basal one are capable of cell division and forming zoospores or gametes. Species with narrow cells form 1, 2, or 4 quadriflagellate zoospores per cell, whereas those with broad cells form 2, 4, 8, 16, or 32 zoospores per cell. The zoospores have a conspicuous eyespot and are liberated through a pore in the side of the parent wall. Zoospores from species with narrow filaments are the same size, whereas those from broad-celled species form macroand microzoospores that differ from each other in size, position of the eyespot, and length of the swarming period. Zoospores that are not discharged from the parent may secrete a wall and become thin-walled aplanospores. These later germinate to form a new filament.

Gametes of Ulothrix are formed in the same way as zoospores but are biflagellate. The gametes are of the same size, with fusion occurring only between gametes from different filaments; and there is never any parthenogenetic development of unfused gametes. The zygote remains for a while, settles, secretes a thick wall, and undergoes a resting period during which it accumulates a large amount of storage material. The first division of the zygote is meiotic, with the zygote

CHLOROPHYTA 169

Fig. 5.31 The life cycle of Ulothrix zonata. (Adapted from

Smith, 1955.)

forming 4 to 16 zoospores or aplanospores (BergerPerrot et al., 1993).

In many northern lakes in the United States and Canada, Ulothrix zonata grows abundantly in early spring in shallow waters along rocky shorelines. Ulothrix zonata is dominant until the water temperature reaches 10 °C, when it disappears owing to massive conversion of the thallus to zoospores. At this time, Cladophora glomerata becomes the dominant attached alga. In culture, formation of the quadriflagellate zoospores of Ulothrix zonata occurs around 20 °C at relatively

high light levels (520 E m 2 s 1) and photoperiods of either short-day (8 hours light : 16 hours dark) or long-day cycles (16 hours light : 8 hours dark). Zoospore formation is minimal at 5 °C, low irradiance (32.5 E m 2 s 1), and neutral day length (12 hours light : 12 hours dark) (Graham and Krantzfelder, 1986).

Ulvales

In nature, these plants have a thallus that is either an expanded sheet one (Monostroma) or two cells (Ulva; Fig. 5.33) thick. Ulva can also occur as a hollow cylinder. The genus Enteromorpha was previously erected for those Ulva in a hollow cylinder. These thalli composed of hollow cylinders are

170 EVOLUTION OF THE CHLOROPLAST

Fig. 5.32 The structure of thallusin, a morphogenetic inducer in Ulva. Thallusin is produced by bacteria living on the surface of Ulva.

now recognized as species of Ulva (Hayden et al., 2003).

The normal morphology of these algae is lost if the plants are grown without bacteria. Under these conditions, Ulva develops into a pincushionlike colony, whereas Monostroma grows as a group of round cells with rhizoids (Provasoli and Pinter, 1980). Bacteria of the Cytophaga-Flavobacterium- Bacteroides group grow on the surface of the algae and produce a morphogenetic factor called thallusin (Fig. 5.32) (Matsuo et al., 2005). The alga absorbs thallusin, resulting in the familiar morphology of the thallus.

Ulva fronds are composed of two layers of cells, with each cell having a large cup-shaped chloroplast toward the exterior of the cell (Fig. 5.33). The holdfast is formed by the cells of the thallus, sending down long slender filaments that coalesce to form the holdfast. The holdfast portion is perennial and proliferates new blades each spring. Cell division may occur anywhere in the thallus, but all divisions are in a plane perpendicular to the thallus surface.

The vegetative state of Ulva mutabilis is maintained by the blade cells excreting regulatory factors into the cell walls and into the environment. The production of one of these regulatory factors, a glycoprotein, decreases as the thallus matures. Eventually, the level of this regulatory factor decreases to the point where the regulatory factor is too low to maintain the vegetative

state, and gametogenesis begins (Stratmann et al., 1996).

Ulva (Fig. 5.33) has an isomorphic alternation of generations, with the gametophyte forming biflagellate gametes and the sporophyte producing quadriflagellate zoospores. There is a periodic fruiting pattern in the genus, which is controlled by the lunar cycle. Gametes are released a few days before zoospores, with fruiting occurring during a series of neap tides (tide of minimum range occurring at the first and third quarters of the moon) in U. pertusa in Japan (Sawada, 1972) and during a series of spring tides (tide of maximum range during the new and full moon) in U. lobata on the Pacific Coast of North America (Smith, 1947). Ulva (Enteromorpha) intestinalis also shows a similar fortnightly periodicity at the beginning of a series of spring tides in England, except that here both gametes and zoospores are released at the same time (Fig. 5.34) (Christie and Evans, 1962). Although it is possible to correlate fruiting in the genus with tidal patterns, it is not the tidal movements that induce fruiting and release of swarmers because the plants will exhibit a regular periodicity even when submerged. It is probably the amount of moonlight that the plants receive that gives the initial impetus to fruiting, although this may be timed by the tide as is the case with Dictyota.

Reproductive areas form near the margins of Ulva fronds (Fig. 5.33), with the fertile portions changing from green to olive-green to brownish green due to the accumulation of -carotene (Hiraoka et al., 1998). Gametogenesis and sporogenesis (Melkonian, 1980a,b) are similar and marked by a sharp reduction in photosynthetic capacity (Gulliksen et al., 1982). A cell divides to form 8 to 32 motile cells, with meiosis occurring in the formation of zoospores but not in the formation of gametes. Prior to the development of the motile cells, the mother cell forms a beak-like outgrowth extending to the thallus surface, through which the swarmers escape. The swarmers are released when the thallus is wet by the water of the incoming tide. The positively phototactic biflagellate gametes have a parietal chloroplast with a pyrenoid and eyespot. Haploid fronds are unisexual, so gametes from the same frond will not fuse. Gametes can be the same size, or one can be

CHLOROPHYTA 171

Fig. 5.33 The life cycle of Ulva arasaki. (Adapted from

Chihara, 1969.)

172 EVOLUTION OF THE CHLOROPLAST

Fig. 5.34 Relationship between liberation of swarmers (gametes and zoospores) by Ulva (Enteromorpha) intestinalis and tidal amplitude. Maximum liberation of swarmers occurs 3 to 5 days before the highest tide of each lunar period. (After Christie and Evans, 1962.)

slightly larger than the other. The mixing of gametes of different strains results in the formation of cell clusters, joined by the tips of their flagella (Miyamura et al., 2003; Miyamura, 2004). Clusters separate almost immediately into mating pairs held together by their flagella (Bråten, 1971). The anterior ends of the gametes fuse within a few seconds, and the flagella separate, with the cells becoming negatively phototactic and swimming away from the light source. Within 3 minutes the cells jackknife and fuse laterally, with the quadriflagellate zygote remaining motile for a couple of minutes. The zygote settles, attaches to the substrate by its anterior flagellated end, and absorbs the flagella into the protoplasm. The zygote secretes a wall as soon as it settles, and nuclear fusion has occurred 30 minutes after the onset of copulation. The chloroplast from the plus gamete disintegrates (Bråten, 1973). In many ways the above process is similar to that in Chlamydomonas except that it occurs much more quickly.

Within a few days the zygote germinates, with mitotic division of the nucleus. After the first cell division one cell develops into a rhizoid, whereas the other eventually forms the blade. In some species it is possible to get a parthenogenetic development of gametes into a new plant.

Zoospores of Ulva are usually negatively phototactic, whereas gametes are positively phototactic. Upon fusion of gametes, phototaxis is reversed and the partially fused gamete pairs swim away from the light source (Miyamura et al., 2003). The total number of particles in the outer chloroplast envelope membrane over the large eyespot of Ulva zoospores is 11 300, whereas over the smaller eyespots of the female and male gametes there are only 5500 and 4900 particles, respectively. The lower number of particles in the gametes may be related to their positive phototaxis, especially because on fusion of the gametes, the total number of particles becomes 10 400, close to that of zoospores, with the fused gametes becoming negatively phototactic (Melkonian, 1980b).

Ulva is normally a marine genus although it can be found in brackish waters, particularly in estuaries. It normally grows on rocks in the middle to low intertidal zone, although the fronds are not situated at the same level throughout the year. During the colder months the plants grow mainly in the middle intertidal zone, covering wide vertical areas. In the warmer months the Ulva is lower in the intertidal zone and in a narrower band. Here the fronds are less exposed and subjected to less desiccation, which is more damaging to the plants in the high summer temperatures.

Ulva is an opportunistic alga, capable of rapid colonization and growth when conditions are favorable. This occurs primarily because of a rapid growth rate and the ability to take up and store nutrients available in pulsed supply. Because of