KINGDOM FUNGI
The fungi are as
distinct from the mosses and higher vascular plants as they are from the
animals and are a distinct kingdom. There
are about 100,000 species of fungi that have been described, and it is
estimated that as many as 200,000 more may await discovery. There may
actually be as many species of fungi as there are species of plants, although
far fewer have been described thus far. The fungi have no direct
evolutionary connection with the plants and apparently were derived independently
from a different group of single-celled eukaryotes. The oldest fossils
that resemble fungi occur in the strata about 900 million years ago, but the
oldest that have been identified with certainty as fungi are from the
Ordovician period, 450 to 500 million years ago. Representatives of
Division Zygomycota were associated with the underground portions of the
earliest vascular plants in the Silurian period, some 400 million years
ago. Fungi may be among the oldest eukaryotes; the four major groups were
in existence by the close of the Carboniferous period, some 300 million years
ago
The
fungi, together with the heterotrophic bacteria and a few other groups of
organisms are the decomposers of the biosphere.
Their activities are as essential to the continued functioning of the
biosphere as are those of the primary producers. Decomposition releases carbon dioxide into
the atmosphere and returns nitrogenous compounds and other minerals to the soil
where they can be used again by the green plants and eventually the animals. It is estimated that, on the average, the top
20 centimeters of fertile soil may contain nearly 5 metric tons of fungi and
bacteria per hectare (2.47 acres). As
decomposers, fungi often come into direct conflict with human interests. A fungus that feeds on wood makes no
distinction between a fallen tree in the forest or a railway tie. Equipped with a powerful arsenal of enzymes
that break down organic products, fungi are often nuisances and are sometimes
highly destructive. This is especially
true in the tropics, where the warmth and dampness promote fungal growth. Fungi attack cloth, paint, leather, jet fuel,
insulation on cables and wires, photographic film, and even the coating of the
lenses of optical equipment - in fact almost any conceivable substance. Even in temperate regions, they are the
scourge of food producers and sellers alike.
They can grow on bread, fresh fruits, vegetables, meats, and other
products. Fungi reduce the nutritional
value, as well as the palatability, of such foodstuffs. Some also produce extremely poisonous toxins,
some of which - the aflotoxins - are extremely carcinogenic and show their
effects at concentrations as low as a few parts per billion. Many fungi
are pathogenic, and attack living organisms, rather than dead ones. They
are the most important single cause of plant diseases. Well over 5000
species of fungi attack economically valuable crop and garden plants, as well
as many wild plants. Other fungi are the cause of serious diseases in humans
and other animals.
Many
fungi are commercially valuable. For
example the yeasts, are useful because of their abilities to produce substances
such as ethanol and carbon dioxide (which plays a central roll in baking). Others are of interest as sources of
antibiotics, including penicillin, the first antibiotic to be widely used, and
as potential sources of proteins.
Associations
between fungi and other organisms are extremely diverse. For example, about four-fifths of all land
plants form associations between their roots and fungi called mycorrhizae. Mycorrhizae increase the absorptive surfaces of the plant roots and aid
in mineral exchange between the soil and the plant. These
associations play a critical role in plant nutrition and distribution.
Basic Structure
and Nutrition
Some fungi exist as
single cells and are known as yeasts. However,
most species are multicellular. The
basic structure of a fungus is the hypha
(pl., hyphae)—a slender filament of cytoplasm and nuclei enclosed by a
cell wall (Figure 9). A mass of these
hyphae make up an individual organism, and is collectively called a mycelium. A mycelium can
permeate soil, water, or living tissue; fungi certainly seem to grow
everywhere. In all cases the hyphae of a fungus secrete enzymes for
extracellular digestion of the organic substrate. Then the mycelium and
its hyphae absorb the digested nutrients. For this reason, fungi are
called absorptive heterotrophs. Heterotrophs obtain their energy from
organic molecules made by other organisms.
Fungi feed on many types of substrates. Most fungi obtain food from dead
organic matter and are called saprophytes.
Other fungi feed on living organisms and are parasites. Many of the parasitic fungi have modified hyphae
called haustoria (Figure 9), which are thin extensions of the
hyphae that penetrate living cells and absorb nutrients.
Hyphae of some species of fungi have crosswalls called septa that separate cytoplasm and nuclei into cells. Hyphae
of other species have incomplete or no septa (i.e., are aseptate) and therefore
are coenocytic (multinucleate).
The cell walls of fungi are made of chitin,
the same polysaccharide that comprises the exoskeleton of insects and
crustaceans.
Figure
9: Basic structure of fungus.
Reproduction
in Kingdom Fungi
Reproduction in
fungi can be sexual or asexual. Many, but not all, fungi reproduce both
sexually and asexually. Some reproduce
only sexually, others only asexually. All
divisions, however, share similar patterns of reproduction and
morphology.
a) Asexual Reproduction
Fungi commonly
reproduce asexually by mitotic production of haploid vegetative cells called spores in sporangia, and conidia
on conidiophores. Spores
are microscopic and surrounded by a covering well suited for the rigors of
distribution into the environment.
Budding and fragmentation are two other methods
of asexual reproduction. Budding is mitosis with an uneven distribution
of cytoplasm and is common in yeasts. After budding, the cell with the
lesser amount of cytoplasm eventually detaches and matures into a new
organism. Fragmentation is the breaking of an organism into one or more
pieces, each of which can develop into a new individual.
b) Sexual Reproduction
The sexual cycle
of fungi includes the familiar events of vegetative growth, genetic
recombination, meiosis, and fertilization. However, the timing of these
events is unique in fungi. Fungi reproduce sexually when hyphae of two
genetically different individuals of the same species encounter each
other. Following are the four important features of the sexual cycle of
fungi:
- Nuclei of a fungal mycelium are haploid during most of the life cycle.
- Gametes are produced by mitosis and differentiation of haploid cells rather than directly from meiosis of diploid cells.
- Meiosis quickly follows formation of the zygote, the only diploid stage.
- Haploid cells produced by meiosis are not gametes; rather they are spores that grow into a mature haploid organism. Recall that asexual reproduction produces spores by mitosis. In both cases, haploid spores grow into mature mycelia.
- The union of the cytoplasm of two parent mycelia is known as plasmogamy.
- The union of two haploid nuclei contributed by two parents is known as karyogamy.
Classification of Fungi
The taxonomy of fungi is currently the subject of much research. Although there is considerable discrepancy in the classification of fungi, we will treat the kingdom as having 3 divisions (phyla): the Zygomycota, Ascomycota, and Basidiomycota. Note, that the terms phylum and division can be used interchangeably. Where the Ascomycota and Basidiomycota are considered the "higher fungi", the other division is known as the "lower fungi", because they have retained many primitive characteristics. The separation of the 3 divisions in our system is based on reproductive structures. The Zygomycota (or zygote fungi) produce "zygospores" (heavy walled zygotes not associated with an oogonium), as their sexual reproductive spores. The Ascomycota (or sac fungi) produce "ascospores", within a special structure termed an ascus. The Basidiomycota (or club fungi) produce "basidiospores" in a basidium. As you examine members of these major groups, carefully note variations on the fundamental structure of vegetative mycelia and specialized structures associated with sexual and asexual reproduction.
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