Fungal Phylogeny

gogol — Tue, 05 Aug 2008 14:48:47 +0200

The fungi still present many unsolved phylogenetic problems. There is wide agreement that they are polyphyletic; that is, that there are as many plesiomorphs as there are independently originated groups of fungi. It also means that the evolutionary trends within this kingdom are complex. Polyphylesis of the fungi. In a phenetic approach, thefungi were classified into four divisions, i.e., Phycomycetes, Ascomycetes, Basidiomycetes, and Fungi Imperfecti. These divisions largely depended on reproductive structures, with the last being a catch-all for forms whose reproductive structures (and, therefore, life cycles) were unknown. This approach did not satisfy those who felt that classification should be more than just a way to store and retrieve information, i.e., that it should also reflect evolutionary history.

We see here that the Phycomycetes are missing, and in their place are three other divisions. These are thought to be close to monophyletic groupings. The Fungi Imperfecti, now designated Deutoromycota, are accepted as probably having no sexual reproduction; they reproduce asexually by spore-formation and elongation of hyphae. Then three unusual groups are placed here, but there is no real conviction that they are genuine fungi. The first of these are the lichens. There are 15,000 to 20,000 species of these organisms, which represent symbiotic relation between a fungus and an alga and show identifying features from the blue-green and green algae and from the Basidiomycota, Ascomycota, and Deuteromycota. Obviously, classification and evolutionary relations are difficult beyond the recognition that their status is special and that they are arbitrarily put in with the fungi. The other two groups are the cellular slime molds and the plasmodial slime molds, which seem to have independent origins. Both have a sporeforming stage that is fungal in appearance, which is now believed to represent convergent evolution. Both have an ameboid stage in their fife cycle, during which feeding is by phagocytosis. In this stage their character is clearly that ofunicellular ingestor or animal, which strongly indicates a protozoan nature. In fact, many schemes of protozoan classification include the slime molds. They are included here only because they are commonly classified among the fungi; it is an Adansonian solution to their taxonomic status, not an evolutionary one.

There remains the two major divisions of the Ascomycota and Basidiomycota. These, plus the former Phycomycophyta, are often combined into one division as the Eumycophyta or true fungi. It is within this grouping that phylogenetic trends, if they can be studied anywhere in the fungi, are most apparent.

Fungal Diversity

gogol — Tue, 05 Aug 2008 14:48:27 +0200

The second major group of multicellular organisms, the kingdom Fungi, is usefully discussed now, since it too arose in all probability from the protistan protophyta. But whereas the Metaphyta arose from photosynthetic forms, the Fungi arose from non-synthetic ancestors. The Fungi are eukaryotic and predominantly multicellular decomposers, with diverse types of organization.

The bodies of these organisms are sometimes unicellular, but more often organized into filaments. Each filament is called a hypha; collectively, they make up a mycelium. The mycelium, which may be highly branching, extends into the environment that nourishes the fungus. Typically, as decomposers, the fungi live in moist areas where there is an abundance of organic material. Hence their occurrence in dead wood or the litter on forest floors. But they also invade animal tissues--athlete's foot is caused by a fungus. They are severe problems in terms of food spoilage, since they cause molding of breads and vegetables, but they are also helpful in food production, especially in beer brewing and wine making, which depend on the fermentation of yeasts, and in certain cheeses. The colored patches in blue cheese are the result of fungal growth. In brief, fungi grow just about anywhere there is organic material that can be decomposed. Decomposition occurs through the release of enzymes that degrade substrates in the immediate vicinity of the hypha. These substrates are macromolecular constituents produced by other organisms because such molecules contain necessary building blocks for further growth of the fungi. Proteins supply amino acids, nucleic acids nucleotides, and carbohydrates and lipids sugars and other carbon compounds. These smaller molecules are assimilated into the fungal cells and used there for vegetative growth.

Reproductive functions involve sexual reproduction as well as asexual spore-formation. Although the reproductive structures of the familiar puffballs, mushrooms, toadstools, and brackets of shelf fungi are rather complex structures, they constitute the lesser part of fungal growth. The usually invisible mycelium comprises the mass of the organism and is quite simply organized. Only in the reproductive structures does complexity approach that seen in the thalli of the red or the brown algae, for example.

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Fungal Phylogeny

Fungal Diversity