MODERN BOTANY - P. RAVEN - 1990

CHAPTER IV. DIVERSITY

CHAPTER 14. UNICELLULAR PROTISTS: WATER MOLDS, SLIME MOLDS, CHYTRIDIOMYCETES, AND UNICELLULAR ALGAE

Summary

Four phyla of heterotrophic protists—oomycetes, chytridiomycetes, acrasyiomycetes, and myxomycetes—are frequently grouped with Fungi. They exhibit a more or less typical mitotic cycle; centrioles are present. The first two groups are predominantly aquatic organisms, whereas the others are terrestrial.

Oomycetes comprise forms ranging from unicellular to extensively branched, filamentous coenocytic organisms. Their sperm and zoospores bear two flagella—one whiplash and one tinsel. Cell walls are composed primarily of Cellulose and cellulose-like polymers. Sexual reproduction involves a large, non-motile egg cell and a small, motile sperm. Partially terrestrial representatives of this phylum include the genus Phytophthora, which comprises economically important plant pathogens, including Phytophthora infestans—the CAUSATIVE AGENT OF the late blight that triggered the Irish potato famine of 1846–1847—and Plasmopara viticola, the causal agent of grapevine downy mildew.

Chytridiomycetes are unicellular or coenocytic aquatic organisms. In most species studied, The Cell walls are rich in Chitin. Motile spores and Gametes possess a single posterior whiplash flagellum. Allomyces and a closely related genus are the only non-photosynthetic groups in which the Morphology/12.html">ALTERNATION OF GENERATIONS resembles that found in plants and many Algae.

Cellular slime Molds (acrasyiomycetes) are a small group of amoeboid organisms that aggregate during one stage of their life cycle into a pseudoplasmodium («slug»). Flagellated Cells are unknown. At least some representatives of this phylum undergo sexual reproduction via macrocysts.

Plasmodial slime molds (myxomycetes) can exist as a flowing multinucleate mass of protoplasm, the plasmodium, which is typically diploid. These plasmodia ultimately form sporangia in which diploid spores mature. Meiosis occurs within each spore; of the four resulting nuclei, three degenerate, while a single haploid nucleus remains within the spore. Under favorable conditions, the spores rupture to release amoebae capable of developing flagella. These amoebae or flagellated cells may function as gametes. Plasmodium formation frequently, but not always, results from the fusion of gametes.

Among the photosynthetic, predominantly unicellular protists, three phyla are examined in this chapter. Two of them—chrysophytes and dinoflagellates—possess METABOLISM/14.html">Chloroplasts that are biochemically and partially structurally similar. They are characterized by the presence of chlorophylls a and c and the accessory pigment fucoxanthin. It can be hypothesized that the ancestral forms of these phyla, as well as of brown algae (phylum Phaeophyta), acquired their chloroplasts through a series of independent symbiotic events involving a bacterium resembling the recently discovered species Heliobacterium chlorum. It is also possible that they engulfed a eukaryotic Organism containing chloroplasts of this type. A parallel example is the similarity between the chloroplasts of euglenoids and green algae (phylum Chlorophyta), which almost certainly arose independently of the chloroplasts of the other autotrophic unicellular protists discussed in this chapter.

Chrysophytes are an essential component of freshwater and marine phytoplankton. Several Representatives of the Class Chrysophyceae are organisms measuring 1 to 3 µm in size, abundantly represented in the nanoplankton. They make a substantial contribution to marine photosynthetic productivity. The second class, yellow-green algae (Xanthophyceae), includes organisms lacking the pigment fucoxanthin. The third class, diatoms, comprising approximately 5,600 species of unicellular organisms with a unique two-part silica frustule, has constituted a major portion of freshwater and marine plankton over the past 200 million years. The only flagellated cells in the diatom life cycle are male gametes, known in only a few species.

Dinoflagellates are unicellular, biflagellate, predominantly marine forms. Their two flagella beat in different planes, imparting a rotational movement to the organism. As a rule, dense cellulosic plates, often of intricate shapes, reside in vacuoles beneath The Plasma Membrane. As symbionts (zooxanthellae), these species are major contributors to the productivity of coral reefs; Symbiosis with numerous marine animals is also widespread among them.

Euglenoids are a small group of predominantly freshwater unicells. They contain chlorophylls a and b and store CARBOHYDRATES in the form of the unusual polysaccharide paramylon. Euglenoids lack a Cell wall, instead possessing rows of flexible protein strips that form a pellicle beneath the plasma membrane. The cells are highly differentiated, containing chloroplasts, a contractile vacuole, and flagella. Sexual reproduction is unknown. Euglenoids closely resemble Protozoans of the phylum Zoomastigina and may well warrant inclusion in that group.

Appendix 1. Hormonal Control of the Sexual Cycle in a Water Mold

Achyla ambisexualis is a heterothallic Water mold; male and female sex Organs develop on separate individuals. J. Raper of Harvard University investigated the Hormonal Regulation of the sexual cycle in these organisms. Female hyphae secrete a substance that stimulates the initiation of antheridium development on the hyphae of male individuals. (A. Appearance of undifferentiated hyphae prior to The addition of this substance, described and named antheridiol by A. Barksdale of the New York Botanical Garden. B. Antheridial branches formed two hours after the addition of crystalline antheridiol. C. Antheridial branches elongating toward a plastic particle containing antheridiol.)

Following the initiation of antheridium development, the male organism secretes its own hormone, termed oogiol, which induces The formation of oogonia on the female hyphae. Upon the appearance of young oogonia, the antheridial hyphae grow toward them, after which The Development of mature antheridies is completed. Raper attributed this response to the action of a substance he termed «hormone C», presumably secreted by the female organism, although subsequent research suggests rather that antheridiol alone is sufficient for this process. Following the final Formation of the antheridia, the Differentiation of the oogonia takes place (hormonally stimulated, presumably), leading to the maturation of both the oogonia themselves and the female gametes (eggs) contained within them. Thus, even among protists, the sexual process can involve a strictly coordinated sequence of hormonally regulated events.

Appendix 2. Mitosis in Dinoflagellates

Dinoflagellates exhibit a unique type of mitosis that likely retains certain features of Bacterial Cell Division. In their cells (see Fig. A), Chromosomes remain permanently visible and do not condense prior to mitosis. Each cell contains an extraordinarily large amount of DNA. The chromosomes are attached to the nuclear envelope, which persists throughout mitosis. Their protein-to-DNA ratio is much lower than that of other eukaryotes, raising the possibility that they evolved from Bacteria independently of the chromosomes of other eukaryotes.

In the center of Fig. B, cytoplasmic channels can be seen traversing the dividing nucleus during mitosis. The chromosomes remain attached to the nuclear envelope and are displaced along the walls of these channels, which contain bundles of microtubules resembling spindle microtubules. All of these microtubules are oriented in the same direction and apparently regulate the Separation of the nuclear envelope regions bearing the attached chromosomes. Figures A and B show Cryptothecodinium cohnii.

At least two dinoflagellate species possess cells with two nuclei, one of which is termed dinokaryotic and the second eukaryotic. In the second nucleus, presumably derived from an intracellular chrysophyte-like symbiont, the chromosomes do not condense at any stage of the Cell Cycle, indicating that they are not identical to «typical» eukaryotic chromosomes.