Protists And The Dawn Of The Eukarya 561

While Volvox is colonial and spherical, Oedogonium is mul-ticellular and filamentous, and each of its cells has only one nucleus. Cladophora is multicellular, but each cell is multinu-cleate. Bryopsis is tubular and coenocytic, forming cross-walls only when reproductive structures form. Acetabularia is a single, giant uninucleate cell a few centimeters long that becomes multinucleate only at the end of its reproductive stage. Ulva lactuca is a thin, membranous sheet a few centimeters across; its unusual appearance justifies its common name: sea lettuce (Figure 28.25b).

Chlorophyte life cycles are diverse

The life cycles of chlorophytes show great diversity. Let's examine two chlorophyte life cycles in detail, beginning with that of the sea lettuce Ulva lactuca (Figure 28.26). Like many chlorophytes, sea lettuce exhibits alternation of generations. The diploid sporophyte of this common seashore organism is a broad sheet only two cells thick. Some of its cells (sporo-cytes) differentiate and undergo meiosis and cytokinesis, producing motile haploid spores (zoospores). These swim away, each propelled by four flagella, and some eventually find a suitable place to settle. The spores then lose their flagella and begin to divide mitotically, producing a thin filament that de-

Haploid gametophytes jC

I Mitosis jC

I Mitosis

Haploid spores

Life Cycle Ulva

HAPLOID (n) DIPLOID (2n)

Haploid gametophytes

Cycle Ulva Lactuca

Haploid (n) gametes

Asexual ' reproduction

Haploid (n) gametes h

Asexual ' reproduction

Cy x

Male and female gametes look the same-Ulva is isogamous.

HAPLOID (n) DIPLOID (2n)

The diploid sporophytes and haploid gametophytes look alike—the life cycle is isomorphic.

Fusing gametes

Fusing gametes

Fertilization

The diploid sporophytes and haploid gametophytes look alike—the life cycle is isomorphic.

Biocycle Ulva Lactuca

Diploid (2n)

zygote

28.26 An Isomorphic Life Cycle The life cycle of Ulva lactuca is an example of isomorphic alternation of generations.

Diploid (2n)

zygote

28.26 An Isomorphic Life Cycle The life cycle of Ulva lactuca is an example of isomorphic alternation of generations.

velops into a broad sheet only two cells thick. The gameto-phyte thus produced looks just like the sporophyte—in other words, Ulva lactuca has an isomorphic life cycle.

In Ulva lactuca, an individual gametophyte can produce only male or female gametes—never both. The gametes arise mitotically within single cells (called gametangia), rather than within a specialized multicellular structure, as in plants. Both types of gametes bear two flagella (in contrast to the four flagella of a haploid spore) and hence are motile.

In most species of Ulva the female and male gametes are indistinguishable structurally, making those species isoga-mous—having gametes of identical appearance. Other chlorophytes, including some other species of Ulva, are anisogamous—having female gametes that are distinctly larger than the male gametes.

Female and male gametes come together and unite, losing their flagella as the zygote forms and settles. After resting briefly, the zygote begins mitotic division, producing a mul-ticellular sporophyte. Any gametes that fail to find partners can settle down on a favorable substratum, lose their flagella, undergo mitosis, and produce a new gametophyte directly; in other words, the gametes can also function as zoospores. Few chlorophytes other than Ulva have motile gametes that can also function as zoospores.

In contrast to the isomorphic life cycle of Ulva, many other chlorophytes have a heteromorphic life cycle, in which sporo-phyte and gametophyte generations differ in structure. In one variation of the heteromorphic life cycle—the haplontic life cycle (Figure 28.27)—a multicellular haploid individual produces gametes that fuse to form a zygote. The zygote functions directly as a sporocyte, undergoing meiosis to produce spores, which in turn produce a new haploid individual. In the entire haplontic life cycle, only one cell—the zygote—is diploid. The filamentous organisms of the genus Ulothrix are examples of haplontic chlorophytes.

Some other chlorophytes have a diplontic life cycle like that of many animals. In a diplontic life cycle, meiosis of diploid sporocytes produces haploid gametes directly; the gametes fuse, and the resulting diploid zygote divides mi-totically to form a new multicellular sporophyte. In such organisms, every cell except the gametes is diploid. Between these two extremes are chlorophytes in which the gameto-phyte and sporophyte generations are both multicellular, but one generation (usually the sporophyte) is much larger and more prominent than the other.

There are green algae other than chlorophytes

As we mentioned above, the chlorophytes are the largest clade of green algae, but there are other green algal clades as well. Those clades are branches of a clade that also includes the plant kingdom. The green algal clade that is sister to the

Haploid i gametes (n)

Zygote (2n)

Some haploid cells can divide mitotically to form zoospores,

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