BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

Third Subkingdom: Heterokontobionta

Organisms belonging to this large group are characterized by heterokont motile stages, i.e., they are equipped with one forward-directed tinsel flagellum and one backward-directed smooth flagellum that serves as a rudder (see Fig. 11.20, A, C; 11.72, F). The tinsel flagellum has two rows of stiff hairs. The latter consist of a base, a tubular shaft, and terminal hairs (one or several). Hairs occasionally found on the flagella of representatives of other evolutionary groups (e.g., Glaucobionta, Chlorobionta) are never stiff or tubular.

In this subkingdom, both heterotrophic fungus-like organisms (Labyrinthulomycota; Oomycota) and autotrophic Algae (Heterokontophyta) arose from common ancestors. First, let us consider the heterotrophic groups, which correspond to Fungi in their level of Organization (Box 11.3).

First Division: Labyrinthulomycota

Net slime Molds, grouped in the Class Labyrinthulomycetes, still possess the organization of slime molds (see Box 11.2). However, due to the heterokont type of flagellar apparatus organization, they are closely related to the Oomycota discussed below. This group includes species that infect marine plants (e.g., Zostera, Laminaria) as endoparasites¹. They are characterized by multicellular net-like plasmodia; the latter arise by division of biflagellate motile Cells within an expanding mucous sheath. Net slime molds represent a basal group among Heterokontobionta. Organisms similar to them were ancestors of the heterotrophic Oomycota discussed below, as well as of the Heterokontobionta that became autotrophic As a result of endocytobiosis².

¹ Among the labyrinthulids, parasites of some freshwater algae (for example, species of Cladophora and Vaucheria) are also known. Species from terrestrial habitats are also known, for example, wheat ROOT parasites. — Transl. note.

² Another very important group, the class Thraustochytriomycetes, is usually also assigned to this division. Externally, their thalli are convergently similar to some chytridiomycetes (for example, of the Rhizophydium type, see below), which is reflected in their name. Their cells, like those of labyrinthulids, are surrounded by a mucous sheath. This ectoplasmic network is less developed than in labyrinthulids and resembles the rhizomycelium of chytrid fungi. It serves only for nutrient exchange between the cells and the environment, as well as for attachment to the substrate. Reproduction is via zoospores (occasionally aplanospores and amoeboid cells). Zoospores are produced in large numbers (sometimes over 100) in zoosporangia. The life cycle from zoospore to a new zoosporangium can take only 2–3 hours. Therefore, thraustochytrids have become some of the most abundant marine organisms, particularly in the plankton and neuston (for example, in the hyponeuston their numbers can reach 2–3 million per 1 ml of Water), and play an important role in the Nutrition of marine filter feeders. There are indications of a Sexual process in thraustochytrids, but proof is lacking. Thraustochytrids are obligate marine and halophilic saprotrophs found worldwide. After Bacteria, they are the most abundant heterotrophic organisms in oceans and saline inland waters. Some are known as parasites of several marine invertebrates and fish. There are over 40 species. A number of species grow well on pine pollen placed in seawater, so they can be "baited" for further research. — Transl. note.

Second Division: Oomycota, or Cellulose fungi

This division, which includes about 500 species, although sharing certain similarities with fungi (Box 11.3), differs from all typical fungi in several characteristics. The thallus,

is rarely unicellular (Lagenidiales) and most commonly coenocytic, almost always having Cell walls containing cellulose. Reproduction occurs through the fusion of ♂ gametangia with oogonia (gametangiogamy); the former form Fertilization tubes that penetrate the oogonia. After fertilization, one to several zygotes (so-called "oospores") develop within the oogonia. Zoospores arising during asexual reproduction are heterokont: they bear a forward-directed tinsel flagellum and a backward-directed smooth flagellum, which is usually somewhat longer (see Fig. 11.20, B). Furthermore, oomycetes (the only class of the division) are, according to all available data, diplonts, so that Meiosis occurs before gamete formation (gametic meiosis). Hyphal aggregations and fruiting bodies are never formed.

Regarding METABOLISM, oomycetes, In addition to their Cell wall chemistry, possess the following features. The Cell wall protein contains hydroxyproline. Lysine Biosynthesis occurs via the diaminopimelic acid pathway after the Condensation of Pyruvate and aspartate into dihydrodipicolinic acid (in other fungi, via the aminoadipic acid pathway). Nicotinic acid is synthesized not from Tryptophan (as in animals and other fungi), but from C3 compounds. The Enzymes involved in tryptophan synthesis are of a unique type found nowhere else. The Molecular Weight of ribosomal RNA (25S fraction) is different from that of other fungi (except slime molds). Virtually all oomycetes are colorless; no pigments have been successfully isolated from them.

All the aforementioned morphological and chemical characteristics, as well as the diploid life cycle, indicate the distinctness of Oomycota from chitinous fungi (Eumycota).

Life history. Species possessing characteristics considered ancestral live in water, most often as saprotrophs. Evolutionarily more advanced terrestrial inhabitants are parasites of higher plants. This division into groups based on lifestyle corresponds to the two most important (1, 3) orders of oomycetes.

1. Order: Saprolegniales. The aseptate, multinucleate mycelium (see Fig. 11.21, C) lives in water, most commonly fresh, but in some species also brackish, usually saprotrophically on submerged decaying plant parts and insect carcasses; less frequently, it parasitizes weakened live fish. During asexual reproduction, the tips of the hyphae expand into weakly defined club-shaped zoosporangia and are delimited by a septum from the supporting hypha. Through Cleavage of the Cytoplasm, uninucleate, pear-shaped mitozoospores arise within them, bearing two apical flagella of unequal length, one of which carries two rows of hairs (see Fig. 11.21, A; 11.20, A). After swimming for some time, the flagella are retracted. The spores, which become rounded and walled, form a germ tube on a suitable substrate and develop into a new Organism.

In some oomycetes, other zoospores are formed first. These second motile stages (Fig. 11.21, B) differ from those initially formed in their appearance and the site of flagellar attachment (Fig. 11.21, A): they are Kidney-shaped, and the flagella are attached laterally (Fig. 11.21, B). This phenomenon of diplanetism distinguishes the genus Saprolegnia from other genera¹. In other genera, primary zoospores occur only inside or in the immediate vicinity of the sporangium (Achlya) or do not occur at all (Thraustotheca, Dictyuchus); in the latter case, diplanetism is absent. Aplanes has no zoospores; the spores, encysting inside the sporangium, produce germ tubes through its wall. Geolegnia behaves similarly, but with the difference that the spores are released from the sporangium before the germ tube is formed. A characteristic feature of Saprolegnia is that the hypha bearing the sporangium, after the release of zoospores, proliferates to form a new sporangium inside the empty one.

¹ It is more correct to define diplanetism as the ability of zoospores to encyst and then emerge again as a zoospore; The phenomenon of changing zoospore Morphology is more properly called dimorphism. — Transl. note.

Box 11.3. Types of Fungal Organization

Fungi in the narrower sense, like slime molds, lack Plastids and chlorophyll; they live as saprotrophs or parasites in fresh water or on land, less frequently in the seas. They can often be cultured on suitable nutrient media—mainly saprotrophic, but also some parasitic forms; they are heterotrophic not only with respect to carbon, but also with respect to nitrogen and certain growth factors. Although traditionally studied alongside plants, these eukaryotic organisms, which form a thallus, occupy a special position in The system of living organisms. In contrast to slime molds, they do not form a plasmodium, and the thallus is usually not amoeboid but is enclosed in a cell wall made of Chitin, glucans, etc. The vegetative body is rarely in the form of a vesicle or droplet; it is most commonly filamentous. Individual filaments of a fungus are called hyphae, and the collective mass of hyphae outside of fruiting bodies is the mycelium. In the fruiting body, hyphae interweave to form a false tissue—plectenchyma.

The following Levels of organization can be distinguished in fungi:

✵ naked parasitic protoplasts (e.g., Olpidium); in all subsequent cases, the vegetative phase is characterized by the presence of a cell wall;

• rhizomycelium: a nucleated vesicle sends out anucleate filamentous growths into the substrate (e.g., Rhizophydium, see Fig. 11.24, A);

• budding mycelium: the thallus consists of droplet-shaped or somewhat elongated cells that form daughter cells by budding; as a result of their incomplete Separation, short chains of cells can arise (e.g., in Yeasts; see Fig. 2.34); from this form, one can derive

• pseudomycelium: here, cells arising from budding remain connected in branched chains (similarly, see Fig. 11.30, G);

• hyphal mycelium and plectenchyma of fruiting bodies: the thallus is formed from filamentous cells; the latter are most often branched, in some fungi not divided by septa (siphonal forms; see Fig. 11.28, D), in others they are regularly divided by septa (trichal forms; see Fig. 11.51, A); hyphae are often tightly packed together or grouped into a fruiting body (see Fig. 11.51, B).

Fungi with thalli in the form of bladder-like single cells or non-septate hyphae were formerly treated as phycomycetes ("algal fungi") and contrasted with fungi having septate mycelium. The transverse walls of the latter have a central, simple or complex pore. The pore is usually open and can facilitate the migration of cytoplasm and nuclei. The cytoplasm inside the hyphae is in intensive motion.

The term "phycomycetes" ("algal fungi") should be reserved for fungi for which a relationship with algae has been established (i.e., for Oomycota; see Fig. 11.9).

Glycogen and fat are widely found as reserve substances, less commonly mannitol and other substances; starch does not occur as a reserve substance in fungi.

Reproduction is carried out by various kinds of specialized cells, which, when arising endogenously, are called spores. Conidia are always formed exogenously and serve for asexual reproduction, or as an exception, for The transfer of ♂ nuclei during sexual reproduction. In aquatic fungi, spores are often naked and flagellated (zoospores, planospores); in terrestrial ones, they are enclosed in a cell wall and lack flagella (aplanospores). Spores can arise, in the presence of a sexual process, after meiosis (meiospores) or be formed after mitotic nuclear divisions (mitospores). Some fungi can also reproduce by fragmentation of the mycelium into individual cells (oidia). Resting structures in the form of hard, tuberous hyphal aggregations (sclerotia) often arise. Notable is the grouping of mycelium into cord-like strands several meters long (rhizomorphs), which serve for dispersal (for example, in Armillaria mellea).

Fig. 11.19. Sexual interaction in fungi. Rectangles indicate mycelia with ♂ (donor) and ♀ (recipient) Organs. Dioecy: ♂ and ♀ organs on different mycelia. Monoecy: ♂ and ♀ organs on the same mycelium. Heterothallism: a single isolated mycelium cannot form zygotes. In monoecious fungi, heterothallism can be due to homogeneous incompatibility, i.e., only those ♂ nuclei are compatible with ♀ nuclei that differ from them in mating type: - ♂ x +♀ and +♂ x - ♀, but not, for example, -♂ x - ♀

During sexual reproduction, copulation occurs between Gametes (isogamy, anisogamy, or oogamy), entire gametangia (gametangiogamy), gametes or conidia with gametangia (gameto- or conidio-gametangiogamy), or two thallus cells not differentiated as specialized sex cells (somatogamy). Gametangia (if present) are never surrounded by a multicellular jacket; therefore, by analogy with algae, they are not called antheridia and oogonia (cf. Bryophytina), but depending on differentiation, mode of formation, and further development, simply ♂ or ♀ gametangia, such as spermogonia (♂), spermatangia (♂), oogonia (♀), and ascogonia (♀); other designations also exist.

Spore-producing organs in fungi (as in algae) are also never surrounded by jackets of sterile cells. Therefore, they are currently called sporocysts rather than sporangia, in contrast to organs with the same Functions in higher land plants (e.g., mosses, ferns).

A special name for structures lacking cellular jackets that produce gametes or gamete nuclei (i.e., the term "gametocysts" instead of "gametangia") is superfluous. It would be better to retain the established concept of "gametangium" and its derivatives, such as "gametangiogamy," etc. Furthermore, THE PRINCIPLE OF not introducing new concepts without extreme necessity should be followed. Gametangia protected by sterile cells, on the other hand, should be called antheridia and archegonia.

Asexual reproduction often predominates; in some cases, sexual reproduction is unknown or has been lost during evolutionary development. Those PARTS OF THE thallus that produce reproductive structures without a change of nuclear phases (mitospores, conidia, etc.) are called "asexual states" (anamorphs) in fungi. In contrast, the perfect state (teleomorph) consists of those parts of the thallus in which nuclear fusion (karyogamy) and change of nuclear phases (meiosis) occur.

Sexual differentiation and distinction into ♂ and ♀ organs or reproductive cells is often difficult to detect. Nevertheless, the donor Nucleus can be treated as ♂, and the recipient as ♀. Based on this premise, one can distinguish between monoecious and dioecious sexual systems (Fig. 11.19). Dioecy occurs when a single mycelium is either a nuclear donor or a nuclear recipient (Fig. 11.19, left). In monoecy, each mycelium can act as both a donor and a recipient of nuclei.

The next pair of concepts, often used in describing Sexual reproduction in fungi, does not fully correspond to the designations discussed above and is based on other genetic characteristics. Homothallic fungi in culture form zygotes and consequently fruiting bodies from single spores, whereas in heterothallic species, two mycelia of different mating types (e.g., "+" and "-") are required for this.

In monoecious-heterothallic fungi, nuclear fusion within a single mycelium is impossible (as in the case of dioecy). Genetically, this incompatibility is based on the presence of at least two alleles of a single mating factor, designated as "+" and "-" (or by other symbols). Nuclei with identical signs (e.g., "+" and "+") are incompatible and do not fuse with each other—this is referred to as homogenic incompatibility (Fig. 11.19, right). Heterogenic incompatibility has been established in crosses between geographic races of the same species; it is based on the incompatibility of different signs.

Motile cells (zoospores, gametes) of fungi can have Different types of flagellar apparatus: opisthokont—with a single smooth posterior flagellum; acro-

kont—with a single tinsel anterior flagellum; heterokont—with two flagella, one of which is smooth and the other is tinsel.

Gametangia are delimited by a transverse septum from their supporting hyphae. Spherical oogonia initially contain many nuclei, most of which, however, degenerate, after which cytoplasm (oosphere) groups around each of the remaining nuclei, contracting to form a spherical naked egg cell. These egg cells, from one to several, lie freely in the oogonium, i.e., they are not surrounded by periplasm. Multinucleate ♂ gametangia arise as a single Structure that does not form gametes. It is chemotropically attracted to the oogonium (via antheridiol, oogoniol) and produces simple or branched fertilization tubes into the oogonium, which reach the egg cells (gametangiogamy: Fig. 11.21, E, F), analogous to fertilization by a pollen tube in seed plants. Only one nucleus enters each egg cell and fuses with the egg nucleus. Thereafter, the egg cell becomes a

zygote with a rigid wall resistant to microbial infection.

After a period of dormancy, the zygotes germinate without reduction division, producing a multinucleate germ tube, which usually soon forms a germ sporocyst (see Fig. 11.21,1). There are monoecious (♂ gametangia and oogonia on the same thallus) and dioecious species.

Fig. 11.20. Oomycota: A—Oomycetes, tinsel flagellum (left) and smooth flagellum (right) of a Phytophthora infestans zoospore (8,000x); B—Achlya (Oomycetes, heterokont zoospore with a tinsel anterior and smooth posterior flagellum); C—Rhizidiomyces (Hyphochytridiomycetes, zoospore with a single anterior tinsel flagellum); D—Saprolegniales, life cycle. Light lines—haplophase, dark lines—diplophase; R!—reduction division. Diplogenotypic Sex Determination (o = ♂ nucleus; • = ♂ nucleus)

2. Order: Leptomitales. These are aquatic saprotrophs with regularly constricted but non-septate hyphae and bladder-like sporocysts. As in members of the next order, an oospore surrounded by periplasm develops in the oogonium. Leptomitus is an inhabitant of heavily polluted (waste) waters.

3. Order: Peronosporales. This includes parasites that mainly infect terrestrial higher plants, causing a disease known as downy mildew (especially Peronosporaceae). The fungal hyphae growing intercellularly in the host tissue send short outgrowths—haustoria—into the living cells (Fig. 11.22, D). Most often, the mycelium emerges from the host's Stomata and forms branched sporocystophores here (Fig. 11.22, A), which are collectively visible to the naked eye as moldy patches. Sporocysts differ from those of Saprolegniales in that they detach from their supporting hyphae, usually as spherical or ellipsoidal structures. Most commonly (for example, in Plasmopara), entire (!) sporocysts are carried by the wind to the leaves of other plants, where, in the Presence of water droplets (rain, dew), they release their contents, which have divided in the meantime, as several kidney-shaped zoospores (corresponding to the secondary zoospores of Saprolegnia, Fig. 11.22, C; thus, there is no diplanetism here).

Fig. 11.21. Oomycota, Saprolegniales: A—sporangium yielding zoospores s with two flagella at the anterior end (200x); B—second type of spores with laterally attached flagella (approx. 350x); C—section of a non-septate hypha with numerous nuclei (500x); D—hypha with sex organs: a—♂ gametangium that produced fertilization tubes into the oogonium, o—fertilized egg cells (600x); E—fertilization tube with ♂ nuclei; F—♂ nucleus entering the egg cell; G—zygote with fused nuclei (E–G—600x); H—germ tube; J—germ sporangium with still non-motile zoospores (H, J—1,400x). A, B, D—Saprolegnia mixta; C—Thraustotheca; E–G—Achlya flagellata; H—Isoachlya intermedia; J—Thraustotheca primoachlya

Due to the progressive ADAPTATION TO A terrestrial lifestyle (cf. Trentepohliophyceae among algae; see Fig. 11.93), the zoosporocysts of Peronosporales increasingly transform into conidia.

In Pythium, the zoosporocysts, which remain attached to their supporting hyphae, always release zoospores. The sporocysts are very similar to vegetative hyphae. In Phytophthora, Plasmopara, and Pseudoperonospora, the sporocysts detach and are wind-dispersed; they usually germinate by producing zoospores, but under specific environmental conditions (low humidity!), they can also form germ tubes. In Phytophthora, the sporocystophores are already clearly distinct from the other hyphae; after the sporocysts detach, they can continue to grow as vegetative hyphae. In Plasmopara, Pseudoperonospora, and Peronospora, the sporocystophores (conidiophores) are morphologically differentiated in a genus-specific manner; they do not continue to grow after sporulation. The sporocysts of Peronospora germinate only by infection hyphae; the organs homologous to sporocysts have thus become entirely conidia, which can be actively discharged due to Changes in the shape of their supporting structure as humidity decreases.

Sexual reproduction. The sex organs develop inside the host plant: oogonia as rounded swellings at the hyphal tips, and ♂ gametangia as tube-like outgrowths (Fig. 11.22, E). Both organs are delimited by a transverse septum and contain many nuclei. There is no distinct differentiation of ♂ gametes; the egg cell of each oogonium (usually a single one) is surrounded by periplasm. Fertilization and zygote formation within the oogonium are shown in Fig. 11.22, E–G, using the genera Peronospora and Albugo as Examples.

Fig. 11.22. Oomycota, Peronosporales. Plasmopara, P. viticola (A–D); A – sporangiophores emerging from stomata; B – oogonia (with ♂ gametangium) and zygotes (100x); C – formation and release of zoospores (600x); D – germination of zoospores (z) through stomata into intercellular spaces (250x); E – Peronospora, P. parasitica, young multinucleate oogonium (o) and ♂ gametangium (g); F, G – Albugo, A. candida: F – oogonium with fertilization tube (b) of ♂ gametangium introducing the ♂ nucleus, G – zygote in the oogonium, surrounded by a young wall and periplasm (p) (E–G – 600x); H – Pythium, P. ultimum, zygote germinating with zoospores (800x) oz – central uninucleate part of the oogonium; 1, 2, 3 – developmental stages

The number of functional nuclei in the ♂ gametangium and oogonium varies from many to one, depending on the genus. In fertilized oogonia, a single oospore is formed, which in some species may contain many diploid nuclei (coenozygote). In some Peronosporaceae (e.g., Basidiophora entospora), functional ♂ gametangia are completely absent; pairwise fusion of nuclei occurs within a single oogonium (autogamy).

Some species reproduce exclusively by asexual means over a significant part of their range. The heterothallic pathogen of potato late blight (Phytophthora infestans) is represented by both mating types in its native range in South and Central America, so sexual reproduction occurs there. However, apparently only one of the two mating types was introduced to Europe, North America, etc.; here, reproduction occurs exclusively by asexual means¹.

¹ Currently, both mating types are present in Eurasia, making sexual reproduction possible. — Translator's Note.

Zygotes germinate either directly by producing zoospores or, more commonly, by forming a germ tube that terminates in a sporocyst containing zoospores (Fig. 11.22, H). In more evolutionarily advanced forms, the germ tube penetrates the host tissue directly without delimiting a sporocyst (analogous to the transformation of zoosporocysts into conidia).

Within genera (especially Peronospora), species are often restricted to one or a few hosts. Speciation is associated with different host Selection and initially more or less continuous, but with further evolution increasingly discrete, changes in morphological characters (e.g., conidium size). At the same time, these characters are influenced by environmental Variability (for example, the Effect of Temperature, humidity, and substrate, in addition to age, on conidium size); therefore, the genetically determined process of speciation and the resulting species divergence can be masked by phenotypic modifications.

Lifestyle and pathogenicity of Peronosporales. Only a few Representatives of the order inhabit fresh water or soil (some members of Pythiaceae). Being mostly parasites of terrestrial plants (e.g., Peronosporaceae, Pythiaceae), they can cause numerous diseases in cultivated plants. These organisms can be distributed worldwide, but their development requires high humidity.

A dangerous parasite of potato is Phytophthora infestans (Pythiaceae); the fungus initially causes foliage blight and then spreads to the tubers, as sporocysts are washed into the soil by rain and infect the tubers through lenticels. Zoospores are chemotactically attracted to roots; in fungal species that infect specific host plant species, this is triggered only by the corresponding host plant species. In rainy years, more than 20% of the potato crop can be destroyed. In the 19th century, late blight epidemics left the populations of entire large regions without food. For instance, in 1845–1846, potato late blight led to the Great Famine in Ireland, severely reducing its population, which was followed by a wave of emigration to the USA. Even today, Ireland has not reached its former population of 8 million people.

Downy mildew of grapevine (Fig. 11.22, A), caused by Plasmopara viticola (Peronosporaceae), is also of economic importance; in wet weather, the infection can become epidemic. The berries turn into leathery, rotten structures. Approximately 20% of the grape harvest is destroyed annually by this and other, less important, fungal diseases (another 20% by pests). Downy mildew infections also occur on sugar beet, onion, hops, and other cultivated plants. In 1959, the pathogen of tobacco blue mold, Peronospora tabacina, appeared in Europe for the first time (having previously occurred in America and Australia; the disease is named blue mold due to the whitish-blue color of the conidial mass). In the rainy summer of 1960, the fungus destroyed a significant portion of tobacco plantations in Central Europe. Pythium debaryanum, a fungus widespread in soil, causes damping-off ('black leg') in seedlings of various plants, leading to their death. To control downy mildew fungi (for powdery mildew, see Erysiphales), leaves are sprayed with copper-containing fungicides (originally Bordeaux mixture), which inhibits sporocyst germination.

Overview of Oomycetes. The orders grouped in this class demonstrate a transition from an aquatic to a terrestrial lifestyle, the gradual replacement of zoospores by conidia, and a shift from hydrochorous to anemochorous dispersal. Associated with these evolutionary phenomena, which culminate in Peronosporaceae, are increased biological demands and an increasing expression of parasitic traits. This is reflected in the transition from saprotrophy to parasitism, specialization toward specific hosts and their organs, and, finally, the host being only partially affected. In nitrogen nutrition, There is a progressive restriction (unrelated to the aforementioned trends) to organic nitrogen compounds. While some Peronosporales can utilize nitrate nitrogen in addition to ammonium nitrogen, Saprolegniales and Leptomitales are unable to do so, and the latter, moreover, do not even assimilate ammonium nitrogen, utilizing only its Organic compounds. Only specific parts of the thallus participate in reproduction, while the rest continues to grow ('eucarpic'); only in the most primitive forms (e.g., Lagenisma of Lagenidiales; Thraustochytridiales¹) does the entire thallus function as a gametangium ('holocarpic').

¹ Thraustochytriales are currently classified not under oomycetes, but in the phylum Labyrinthulomycota. — Translator's note.

Phylogeny. For oomycetes, which are related to autotrophic algae of the phylum Heterokontophyta (see Appendix 4 to Rhodobionta; Fig. 11.9), shared characteristics include heterokont flagellated stages, a siphonous thallus structure, and cellulose in the cell walls. This distinguishes them from all other fungi. However, the mode of reproduction in oomycetes—gametangiogamy and the associated loss of flagellated gametes—is highly modified. The only motile stages are zoospores, which serve for asexual reproduction. The latter, except in highly evolutionarily advanced forms (e.g., Peronospora), constitute a significant part of the division.

The small class Hyphochytriomycetes (15 species) is characterized by features partially similar to those of oomycetes. Although their motile stages possess only a single, anteriorly directed flagellum, it is tinsel-type, just as in oomycetes (Fig. 11.20, C). The Cell walls contain cellulose in addition to chitin. However, these features are insufficient to unequivocally determine the phylogenetic relationships of Hyphochytriomycetes. They inhabit fresh and marine waters as parasites of algae and fungi, or as saprotrophs on plant and insect remains. Species of this class, for example, those with simple flask-shaped cells inside the Cells of the host (the brown alga Ectocarpus), are mostly 'holocarpic'.

Third Phylum: Heterokontophyta

The groups of Heterokontobionta assigned here are represented by photoautotrophs. Photoautotrophy arose early in the evolution of originally heterotrophic cells as a result of their engulfing photosynthetic organisms—in this case, already eukaryotic ones—as endosymbionts. The new phototrophic organizational form that arose in this way still has a primitive and usually poorly differentiated structure of an algal level of organization (see Box 11.5). This phylum is discussed below, along with other photoautotrophic eukaryotes (see Appendix 4 to Rhodobionta).