BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

Second Subkingdom: Myxobionta

The plasmodia characterizing this taxon arise through the fusion of myxoflagellates or myxamoebae, or develop from individual Cells without a preceding sexual process. Motile flagellated stages occur in The life cycle. Cell walls, where discernible at certain developmental stages, are composed of galactosamine and Cellulose.

First Division: Myxomycota

Class I: Myxomycetes (Myxomycetes). The vegetative (= somatic) phase is a diploid, multinucleate, acellular true plasmodium (see Fig. 2.9; 11.16) that feeds phagotrophically. Fruiting bodies develop from the plasmodia, with part of the protoplasm hardening to form characteristic structures, while another part containing the nuclei is transformed into meiospores. The latter have a Cell wall consisting of at least two layers which, according to recent studies, contains neither cellulose nor Chitin, but predominantly polymeric galactosamine. Glycogen is formed as a storage nutrient. Plasmodia and especially fruiting bodies are often brightly colored. The Chemical Structure of the pigments is only partially understood; it differs from that of Fungi.

Fig. 11.16. Myxomycota. Plasmodium margin in Badhamia utricularis (2x)

Life cycle. Spores germinate in Water or on a damp substrate. Their viability is often preserved for a very long time. For instance, spores from a herbarium specimen stored for over 70 years were successfully germinated. Upon germination, the spores release either uninucleate, naked amoeboid motile myxamoebae or flagellated myxoflagellates (Fig. 11.17, A). Myxoflagellates are most commonly biflagellate, with one flagellum often highly reduced or completely absent. In the latter case, at least a second, non-functional blepharoplast is present. Motile cells can transform into myxamoebae by losing their flagella. The latter reproduce, like motile cells, by division. Myxamoebae or myxoflagellates fuse in pairs to form an amoebozygote (or a flagellated planozygote), in which the nuclei then fuse (plasmogamy followed by karyogamy). The resulting diploid structure undergoes numerous mitotic nuclear divisions and develops into a large multinucleate plasmodium (see Fig. 11.16). Such plasmodia can, in turn, fuse with one another. Mitoses are closed and occur synchronously in all nuclei of a single plasmodium. Within the plasmodia, the protoplasm is in rapid streaming motion. Plasmodia develop under high humidity in forest soil, straw, among grass, mosses, or in decaying wood. Later, they change shape and slowly crawl onto The surface of the substrate. Cell walls are absent both On the surface and inside the plasmodium. Its leading edge (see Fig. 11.16) consists of denser protoplasm; towards the rear, it often appears as a network of individual strands. The diameter of the plasmodium in some species can exceed 20 cm (e.g., Fuligo, Brefeldia).

The formation of fruiting bodies begins under certain, still poorly understood exogenous conditions (substrate depletion, light, Temperature, pH); endogenous factors may also play a decisive role. Before this, The behavior of the plasmodium changes: it crawls out of the damp substrate toward the light and, with significant water loss, transforms into numerous sporocarps (sporangia, see Fig. 11.17, D). These fruiting bodies have an outer, often calcareous wall—the peridium—and frequently a stalk, which may extend into the sporocarp as a columella, as well as a system of threads—the capillitium. These structures are formed from hardened, non-nucleate protoplasm that is not used for spore formation. The Development of the capillitium apparently begins with the deposition of structural material in special vesicles. The nucleated protoplasm divides to form uninucleate, initially diploid spores. Following Meiosis, four haploid nuclei are formed in each spore, three of which degenerate, leaving only one. As the fruiting body matures, the peridium ruptures, and the spores are released from the meshwork of the capillitium. In some species, the capillitium, like the elaters of liverworts, AIDS in the release of spores from the sporocarp through hygroscopic shape changes. In the life cycle, the myxoflagellates and non-copulated myxamoebae are haploid, while the plasmodia, fruiting bodies, and young spores are diploid, and mature spores are haploid again. Thus, the diploid phase dominates during development.

Fig. 11.17. Myxomycota: A – C – myxoflagellates: A (and B) – short flagellum not shown, B – copulation (1,500x), C – long and short flagella shown; D – Leocarpus fragilis, numerous fruiting bodies on moss (natural size); E – Comatricha typhoides, part of the capillitium (180x); F – Stemonitis fusca, fruiting body (5x); G – Cribraria rufa, fruiting body (30x); H – Trichia varia; capillitium thread and spores (300x)

Box 11.2. Types of Slime Mold Organization

Organisms of both subkingdoms—Acrasiobionta and Myxobionta—are called slime Molds. Slime molds are characterized by a vegetative body that is a cell-wall-less, multinucleate, amoeboid motile protoplasmic mass—the plasmodium. It arises:

✵ as an aggregation plasmodium; myxamoebae crawl together into protoplasmic aggregates without losing their individuality;

✵ as a true plasmodium: myxamoebae or myxoflagellates must fuse with each other, forming a diploid multinucleate plasmodium.

Another possibility is the asexual origin of a plasmodium from a single cell through nuclear division (but without Cell Division).

The plasmodia present in the life cycle are thus different in nature and analogous to the prokaryotic plasmodia of myxobacteria (Myxobacteriales). Reproduction is by spores, which arise in specialized fruiting bodies (see Fig. 11.17, G), unless they are parasitic forms living inside a host. Motile stages have two smooth flagella, usually of unequal length, or less frequently, one flagellum is reduced. There is no close evolutionary relationship between the different divisions of slime molds. Highly peculiar in their form and lifestyle, slime molds share many features with members of the protozoan group defined by zoologists, to which they are closely related, but differ in the formation of fruiting bodies and spores. The similarity consists, in particular, of the following:

✵ heterotrophy—most forms feed phagotrophically, like animals, ingesting whole solid particles;

✵ the presence of amoeboid stages in the life cycle;

✵ the absence of cell walls, at least during the vegetative phases of development.

This similarity to Protozoans has led many researchers to classify slime molds under the name Mycetozoa within the animal kingdom.

Nutrition of plasmodia or their preceding unicellular stages in nature is probably always achieved by engulfing various microorganisms, such as Bacteria, Protozoa, spores, Yeast cells, fungal hyphae, etc. Food particles are enclosed in digestive vacuoles and enzymatically digested; undigested residues are expelled after some time. In culture, most species can be maintained only when fed living microorganisms (e.g., bacteria). Some species can also be maintained under purely saprotrophic conditions on nutrient media of specific composition.

Under culture conditions, the plasmodia of some species can ingest and accumulate unicellular green Algae inside themselves without digesting them. Becoming green in this way, the plasmodia acquire photosynthetic activity. Although the green algae do enter the fruiting bodies, under natural conditions such a Symbiosis cannot be maintained from generation to generation.

Systematics. The Classification of myxomycetes is primarily based on differences in the appearance of their fruiting bodies. In more primitive groups, the capillitium is absent. Evolution must have also proceeded from sessile to stalked fruiting bodies, and from solitary to complex ones. Approximately 500 known species are divided into the following orders.

1. Order: Ceratiomyxales. Spore formation occurs exogenously. Spores (presumably single-spored sporocarps) are borne on stalks on the surface of a Column-like fruiting body. Upon germination, each spore produces a plasmatic outgrowth containing 4 haploid nuclei, which, after mitosis, form 8 haploid motile cells. The order includes only one genus, Ceratiomyxa, with a single species represented by numerous forms and found worldwide on decaying wood.

In all subsequent orders (2–6), spores are formed endogenously within the fruiting bodies.

Fruiting bodies in the following 4 orders (2–5) are formed from semicircular swellings of the plasmodium, which are initially connected to it and to each other by plasmatic strands, but later become isolated. The basal layer adjacent to the substrate,

of the plasmodium ('hypothallus') either does not persist at all as the fruiting bodies mature, or remains only as a slimy membrane.

2. Order: Liceales. In contrast to the subsequent orders (3–6), a capillitium and columella are absent here (e.g., Lycogala, Cribraria; Fig. 11.17, G).

3. Order: Echinosteliales. A columella is present.

4. Order: Trichiales. A columella is absent. The capillitium consists of threads with free ends, as in Trichia (Fig. 11.17, H). In all three preceding orders (2–4), the spore mass is light-colored.

5. Order: Physarales. The spore mass is black or of various shades—from dark violet to rusty. White lime deposits are visible on the peridium and often also on the capillitium (e.g., Didymium). This order also includes Leocarpus (Fig. 11.17, D); Badhamia (Fig. 11.16); Fuligo septica with a complex fruiting body (aethalium).

6. Order: Stemonitales. Fruiting bodies, ranging in size from 0.5 to 1–2 cm, develop on a 'hypothallus'. Internally, they are differentiated into a columella, capillitium, and spores. In Stemonitis, the capillitium forms a closed network at the periphery, while in Comatricha, the threads have free ends (Fig. 11.7, E). Lamproderma is distinguished by a peridium with a metallic luster, and in Brefeldia, a large, flat fruiting body is composed of many sporocarps (an aethalium).

The Practical significance of myxomycetes is minor. Physarum (Physarales) has gained great popularity as a model Organism in PHYSIOLOGICAL AND BIOCHEMICAL research.

In the next class, plasmodia arise without a preceding sexual process, directly from solitary cells (and also without the aggregation of plasmodia).

II. Class: Protosteliomycetes. Multinucleate, reticulate plasmodia are formed from cells with or without flagella. One to four spores are borne exogenously on slender stalks.

Second Division: Plasmodiophoromycota

This division differs from all previously discussed slime molds by the presence of chitinous cell walls, as well as by a unique feature of nuclear division: in metaphase, the Chromatin masses are arranged perpendicularly on both sides of a large, somewhat elongated nucleolus, resulting in a cruciform figure within the nuclear membrane. It remains questionable whether it is justified to consider Plasmodiophoromycota as derivatives of myxomycetes that have transitioned to endoparasitism. Admittedly, these two groups share similar zoospores with two flagella of unequal length. Both Haploid and Diploid plasmodia participate in the life cycle; in myxomycetes, they are always diploid, while in protosteliomycetes, they are haploid.

A well-known representative of the single class Plasmodiophoromycetes is Plasmodiophora brassicae, the CAUSATIVE AGENT OF clubroot of cabbage¹ (Fig. 11.18).

¹ It is more correct to call this disease 'clubroot of crucifers', since it affects not only cabbage. — Translator's Note.

Life cycle. Overwintered resting spores (hypnospores) of the parasite germinate in the soil in spring as biflagellate haploid zoospores, which, after losing their flagella, penetrate the ROOT hairs of young cabbage plants as amoeboids. Here, each invading amoeba forms a multinucleate haploid plasmodium (Fig. 11.18, B). It can fragment into multinucleate particles, which in turn (after The breakdown of cell walls in the host Tissues) move further from Cell to Cell, thereby rapidly expanding the focus of infection. Later, after the plasmodium cleaves into initially uninucleate and then multinucleate fragments, multinucleate gametangia are formed. The latter divide into biflagellate Gametes, the number of which corresponds to the number of nuclei. After the destruction of the host tissue, they are released and copulate with each other in the soil.

After shedding their flagella, the diploid planozygotes again penetrate the roots of the cabbage plant, which has already developed by that time (now not exclusively through the root hairs), where they grow into multinucleate protoplasts without cell walls—diploid plasmodia. The host plant responds to this by forming galls (Fig. 11.18, A). Then, after meiosis, thick-walled haploid resting meiospores (resting spores, hypnospores) are formed within the Cells of the host plant. These overwinter together with the infected plant and, in spring, after the decay of the gall tissue, enter the soil once again.

Fig. 11.18. Plasmodiophoromycota, Plasmodiophora brassicae: A — clubroot on the roots of a kohlrabi plant (1/3x); B — plasmodia in a root Hair (300x); C — root cortex cells with spores (520x); D — spore germination (1,240x)

Zoospores and gametes bear at their anterior end two flagella of very different lengths, lacking mastigonemes. The alternation of haploid and diploid plasmodia corresponds to an Morphology/12.html">ALTERNATION OF GENERATIONS, although the exact points of karyogamy and meiosis in the life cycle have not yet been fully clarified.

Species of some related genera (e.g., Polymyxa) are parasites of various terrestrial and aquatic plants, causing similar organ enlargements (60 obligate endoparasitic species affecting vascular plants, algae, and fungi).

Overview of Slime Molds. Slime molds (Acrasiobionta and Myxobionta; about 600 species) lie at the Base of the evolutionary development of heterotrophic eukaryotes. Protosteliomycetes and myxomycetes most likely arose from colorless flagellates, whereas acrasiomycetes originated from some amoebae. Due to their unique morphology and life cycles, these groups occupy a highly isolated position ('Mycetozoa'). Regarding the plasmodiophoromycetes, it remains unclear whether their plasmodium arose ancestrally or only secondarily in connection with an endosymbiotic lifestyle. In the latter case, this class should be grouped with the fungi discussed below, aligning them with groups characterized by similar traits. However, motile stages with two smooth flagella of unequal length, such as those in myxomycetes, protosteliomycetes, and plasmodiophoromycetes, are not found anywhere among the fungi.