BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

III. Kingdom (Domain): Eukaryotes (Eucarya, Eukaryoten)

In terms of species number and biomass, eukaryotes make up the majority of living organisms. The Introduction/5.html">Eukaryotic Cell (eucyte) is distinguished by the presence of a true Cell Nucleus, which is separated from the Cytoplasm by a double-membrane envelope containing pores (see Fig. 2.26). In addition, an important feature of eukaryotes is the presence of various cell Organelles which, like The Nucleus, are clearly demarcated from the ground cytoplasm. These include The Endoplasmic reticulum, dictyosomes (Golgi apparatus), Mitochondria, and Microbodies (see 2.2.8, 2.2.6.6). Photoautotrophic eukaryotic plants are characterized by METABOLISM/14.html">Chloroplasts surrounded by two or more membranes (see 2.2.9.1). The flagella of Eukaryotic Cells are constructed according to a single plan; they have two central single microtubules and 9 peripheral doublets (2 + 9! Structure). If a Cell wall is present, it is formed by a meshwork of macromolecules linked only by adjacent valencies (Cellulose, Chitin, etc.). In general, eukaryotes are clearly distinguished from prokaryotes by the greater complexity of their cells. This difference accounts for the deep gulf between all fossil and extant prokaryotes on the one hand, and eukaryotes on the other (cf. dinophytes, see Appendix 2 to Rhodobionta). The Amino acid sequences of enzymatic Proteins with similar Functions in PROKARYOTES AND EUKARYOTES match to a much lesser extent than those among representatives of either of these two groups. For instance, the elements of the cytochrome c chain of Bacteria and eukaryotes have up to 60 % different Amino Acids, whereas the corresponding value when comparing humans and wheat plants is 45 %, mammals and birds — 12 %, and humans and chimpanzees — 0%.

The origin of eukaryotes and the accompanying formation of membrane-bound organelles in the eucyte are currently most often viewed in the light of the endosymbiotic theory (see 2.4; Fig. 11.9). It is based on the recognition of multiple, independent engulfments of symbionts by primitive eucytes.

Fossil finds allow us to approximately determine the time of the evolutionary divergence of eukaryotes. The oldest unicellular organisms from the Archean (over

3.4 billion years ago) have an average size of 5 µm and thus correspond to modern prokaryotes. Later, about 1.4 billion years ago, larger cells began to predominate, averaging 13 µm, which is characteristic of eukaryotes. Therefore, there is reason to believe that the divergence of prokaryotes and eukaryotes could have occurred about 2 billion years ago, whereas the divergence of plants and animals occurred 'only' 1.1 billion years ago.

Reproduction, alternation of nuclear phases and generations

Typically, The Cell nucleus divides by mitosis. During asexual (vegetative) reproduction, mitotic division of nuclei occurs. In sexual reproduction, the cytoplasm and nuclei of two cells, often specialized as Gametes, fuse (plasmogamy and karyogamy = syngamy); the subsequent Meiosis naturally leads from the diplophase to the haplophase, thus providing the alternation of nuclear phases characteristic of eukaryotes.

Syngamy. The recombination of parental hereditary material associated with sexual reproduction (genetic recombination) decisively contributed to the evolution of eukaryotes. Accordingly, sexual reproduction—with the exception of only a few groups where it seems to be completely absent (e.g., in Euglenophyta)—became firmly established and developed at the Cytology/cytology/16.html">Early stages of evolution. At the same time, the reliability of syngamy was increasingly reinforced by The formation of A large number of Germ Cells or polar nuclei, which ensured a large number of diploid nuclei arising from syngamy. Thanks to this, genetic recombination began to occur much more intensively. In general, syngamy is carried out by the fusion of two germ cells (gametes; gametogamy). They can be of the same type, in some cases flagellated (isogamy), or of different types: smaller and flagellated male ones, larger and also flagellated female ones (anisogamy¹), or only male cells have flagella, while female ones lack them (oogamy) (see, for example, Chlamydomonadaceae). If gametangia (cells that differ from the rest and contain first sexual nuclei and then sometimes gametes) fuse directly with each other without releasing gametes, then gametangiogamy takes place (see, for example, Oomycota). If vegetative cells fuse with each other without special differentiation, this process is called somatogamy (see, for example, Hymenomycetidae). In this case, the sexual nuclei, rather than gametes, pair up and fuse with each other, as occurs in gametogamy.

¹ In Russian literature, the term heterogamy is more commonly used. — Translator's Note.

Alternation of nuclear phases and generations. The original eukaryotes were (if they were already capable of sexual reproduction) pure haplonts with a life cycle completely spent in the haplophase: after the fusion of germ cells (gametes) or sexual nuclei into a single diploid cell (zygote), the latter directly undergoes meiosis. The resulting haploid cells, through mitotic nuclear and cell divisions, form the haploid final stage in ontogeny. Only the zygote is diploid. The alternation of nuclear phases (the transition from diplophase to haplophase and vice versa) is referred to here as zygotic, as it is associated with the zygote (e.g., Ulothrix, see Fig. 11.101, A¹). In haplodiplonts (see Fig. 11.101, B), the zygote nucleus resulting from syngamy reproduces mitotically during its own diploid phase of The life cycle (sporophyte) until the numerous diploid nuclei thus formed undergo meiosis and, consequently, give

rise to numerous recombinant haploid reproductive structures (meiospores). These initiate the haploid phase of the life cycle: a haploid Organism (gametophyte) grows and produces gametes. Here, the alternation of nuclear phases occupies an intermediate position between zygote formation and gamete formation.

¹ The example is unfortunate, since in Ulothrix the zygote does not directly undergo meiosis, but first germinates into a small pear-shaped plantlet; the diploid nucleus migrates into it, and meiosis occurs there. This plantlet is often interpreted as the diploid stage of the alga (Codiolum stage), in which case one can no longer speak of a zygotic alternation of nuclear phases. — Translator's note.

Haplodiplonts are characterized by an Morphology/12.html">ALTERNATION OF GENERATIONS, i.e., a regular sequence of generations in ontogeny; they reproduce differently and are accordingly restricted to different reproductive cells during development. In plants, the haploid generation is called the gametophyte. The latter arises from meiospores and completes its development with the formation of haploid gametes. The diploid sporophyte arises from a zygote (previously formed from fused gametes) and completes its development with the formation of meiospores, which are haploid due to the immediately preceding meiosis of diploid nuclei. Strictly speaking, it is incorrect to designate the diploid sporophytic generation as a phase of the life cycle with asexual reproduction, since meiosis and the associated formation of meiospores are always inseparable from sexual reproduction. Unless, of course, the sporophytic generation can additionally reproduce asexually to form diploid mitospores. If the gametophyte and sporophyte have largely the same appearance, it is referred to as an isomorphic (e.g., Cladophora, see Fig. 11.101, B), and if they differ in appearance, as an anisomorphic (or heteromorphic) alternation of generations (Cutleria, Laminaria, see Fig. 11.87). In general, the alternation of generations is biphasic, and the gametophyte and sporophyte represent independently living organisms.

In the rarer triphasic alternation of generations, the gametophyte is followed by two differently reproducing sporophytic generations (Rhodophyta, see Fig. 11.65). In this special case, only the transitions from the gametophyte to the first sporophytic generation and from the second sporophytic generation to the gametophyte are associated with the alternation of nuclear phases. The transition from the first to the second sporophytic generation occurs, however, within the same nuclear phase. This shows that the alternation of generations does not always have to be accompanied by an alternation of nuclear phases. Two generations can also be linked to each other in such a way that they form a single individual. If one generation is nourished at the expense of the other, this is referred to as gonotrophy (Bryophytina, see Fig. 11.108, D). Haplodicaryonts (e.g., Hymenomycetidae, see Fig. 11.51, B; Derbesia, Fig. 11.101, C) largely correspond to haplodiplonts, but instead of a diplophase they have a dikaryophase, in which the sexual nuclei, although entering the same cell (zygote), divide separately over a long period of the life cycle, resulting in the formation and maintenance of a binucleate phase (dikaryon). Only immediately before meiosis do the sexual nuclei fuse to become diploid. In diplonts (e.g., Oomycota, see Fig. 11.20, D), the entire life cycle takes place in the diplophase. The gametes resulting from meiosis are haploid; the alternation of nuclear phases is thus gametic. Among plants, diplonts arose through the progressive reduction of the gametophyte and the Integration of the few cells remaining from the gametophyte into the sporophyte (Fucus, see Fig. 11.87). Thus, diplonts often have only a hidden alternation of generations (seed plants; see Box 11.10).

The evolution of eukaryotes is characterized by progressive complexity, differentiation, and functional specialization of Organs, as well as adaptation to various nutritional strategies and habitats. This gave rise to levels (types) of Organization, which should most often be viewed not as evolutionarily related groups, but namely (excluding animals, which are not considered here) slime Molds (Box 11.2), Fungi (see Box 11.3), Lichens (Mycobionta, Appendix 2), Algae (see Box 11.5), Embryophyta (see Box 11.8), and vascular plants (see Box 11.9).

Particularly noteworthy is the multiple occurrence of the same type of organization in lichens through the independent formation of symbioses between various algae and fungi. Similarly, fungi represent parallel lineages that evolved towards a heterotrophic lifestyle rather than a phylogenetically uniform group. The same applies to the early-diverging evolutionary lineages of eukaryotic algae, at least when considering photoautotrophic endosymbionts.