MODERN BOTANY - P. RAVEN - 1990

CHAPTER IV. DIVERSITY

CHAPTER 13. FUNGI

Evolution of Fungi

The earliest Fungi were likely Unicellular Eukaryotes, with no surviving modern counterparts. These organisms gave rise to coenocytic fungi with multinucleate Cytoplasm (the term "coenocytic" meaning "contained in a common vessel," i.e., not internally divided by Cell walls). Today, such fungi—commonly referred to as zygomycetes—are classified by taxonomists within the phylum Zygomycota.

In members of the phyla Ascomycota (ascomycetes) and Basidiomycota (Basidiomycetes), the mycelium is divided by transverse cell walls—septa—featuring openings, or pores (Fig. 13-7). In certain fungi, the cytoplasm and its contents flow relatively freely throughout the entire hypha. Evolutionary relationships among the three fungal phyla remain difficult to establish, with no concrete evidence linking zygomycetes to the other two groups. It was once believed that basidiomycetes evolved from ascomycetes, though the reverse is now hypothesized; one thing is certain—these two groups are closely related.

Class="center">Fig. 13-7. Transmission electron micrograph of a septum from the ascomycete Fusarium acuminatum. The large structures are Mitochondria, and the small dark ones are Ribosomes. The section passes through the central pore of the septum at The final stage of its formation.

The oldest fungus-like fossils date back 900 million years, but definitive membership in this kingdom can only be confirmed starting from Ordovician finds (450–500 million years ago) (see Appendix B at the end of the book). Coenocytic fungi, apparently zygomycetes, were already associated with the underground Organs of the earliest vascular plants by Silurian times (approximately 400 million years ago). It is entirely possible that fungi represent some of the earliest eukaryotes, yet their early history remains practically unknown. All three modern phyla were already in existence by the end of the Carboniferous period (roughly 300 million years ago). Apparently, the earliest fossil basidiomycetes are much older than ascomycetes, providing an additional argument in favor of the latter originating from them. As previously noted, the vast majority of fungi are terrestrial; the colonization of land by plants and animals beginning 410 million years ago served as the pivotal event driving their primary evolutionary diversification. Older fungi were evidently freshwater or marine organisms, and their fossil forms remain difficult to identify based on current knowledge.

Reproduction in Fungi

The reproductive structures of fungi are separated from the hyphae by complete septa and are termed gametangia if they produce Gametes, and sporangia (in zygomycetes) or conidiophores (in ascomycetes and basidiomycetes) if they produce asexual spores. Gametes do not differ in size, meaning fungi are isogamous. Meiosis in all fungi occurs immediately following zygote formation, in other words, it is zygotic (see Fig. 10-11).

Fungi are characterized by reproduction via non-motile spores. Some of these spores are extremely small and can be carried to great heights and vast distances, which likely accounts for the wide dispersal of many fungi. Spores of certain species are disseminated by adhering to the bodies of insects or other animals. They form striking powdery coatings frequently observed on various types of mold (see Fig. 13-17). In a few species, spores are forcefully "discharged" into the air (see Appendix "Phototaxis in Fungi").

Mitosis and Meiosis

Fungi possess numerous biological features whose understanding is still in its infancy. One of the most fascinating processes is nuclear division. In fungi, mitosis and meiosis proceed differently than in plants, animals, and most protists. The nuclear envelope does not break down and reform; instead, it simply constricts between the two daughter nuclei, and the spindle apparatus forms entirely within it. Centrioles are absent in fungi. This unique combination of traits indicates a lack of direct phylogenetic affinity between fungi and other extant eukaryotes, justifying their Classification into a separate kingdom.

Heterokaryosis and Parasexuality

Heterokaryosis

Among the genetic distinctions separating fungi from other groups of organisms, heterokaryosis—discovered in 1912 by the mycologist H. Burgeff—occupies a special place. A fungal strain is heterokaryotic if the nuclei sharing a common cytoplasm are genetically distinct As a result of mutation or the fusion of genetically different hyphae, a phenomenon widespread in nature. If the nuclei are genetically similar, the strain is homokaryotic.

Heterokaryosis plays a crucial role in fungal genetics and evolution. If genetically distinct nuclei from heterokaryotic forms find their way into the cytoplasm of different hyphae, these hyphae may diverge phenotypically. Thus, even in the presence of merely Two Types of nuclei, a single mycelium can yield three different phenotypes (one original with both nuclear types) depending on the type or types of nuclei inherited by the resulting hyphae.

Heterokaryosis is partially analogous to diploidy in other organisms, since the morphological and PHYSIOLOGICAL CHARACTERISTICS OF heterokaryotic organisms are governed by the interaction of genetically distinct nuclei. Recessive Mutations can accumulate within certain nuclear lineages, masked by the action of allelic or other genes from cooperating nuclei. Because many nuclear lines are incapable of surviving or successfully competing in the homokaryotic state, heterokaryotic strains are favored by natural Selection (Fig. 13-8).

Fig. 13-8. Arg-1 and Arg-10 are mutant strains of Neurospora, each lacking a specific enzyme involved in Arginine Biosynthesis. Neither of these strains can grow on minimal medium, whereas the heterokaryon formed by their combination is fully capable of doing so. The pooling of Genetic information from the parental mutant nuclei ensures the Synthesis of the missing amino acid.

Parasexuality

The parasexual cycle in fungi was discovered in 1952 by two researchers at the University of Glasgow. Working with Aspergillus nidulans, G. Pontecorvo and J. Roper discovered that haploid nuclei can fuse within a heterokaryotic mycelium to form diploids, some of which are heterozygous in the process (i.e., arising from genetically dissimilar nuclei). It is estimated that in A. nidulans, there is one diploid heterozygous Nucleus per 1,000 haploid nuclei.

Within the diploid nucleus, Chromosomes may associate with one another accompanied by Crossing Over. Occasionally, this is followed by the regeneration of haploid nuclei that are genetically distinct from the originals. These novel haploid nuclei can subsequently participate once more in heterokaryotic combinations.

It has recently been demonstrated that such parasexual cycles—whose genetic outcome parallels that of a true sexual cycle, yet operates via a different pathway—exist in several groups of fungi. Their role in nature is not yet fully understood. Most likely, the parasexual cycle represents a flexible and widespread system for genetic variation in fungi that reproduce rarely or not at all via sexual means.