Mushroom Life Cycle

Ustilago zeae (U. maydis) is an example of a smut. It infects maize, and its life cycle is much simpler than that of rusts. There are no spermatia, no

Monokaryotic Mycelium

Ustilago Scitaminea Teliospore Sugarcane


Dikaryotic Mycelium

Figure 20-7 Ustilago maydis, corn smut. Basidiospores infect the cells of the corn plant, producing a monokaryotic mycelium, which later becomes dikaryotic. The dikaryotic cells produce teliospores. The nuclei fuse, producing a diploid spore, which produces a basidium by meiosis.


Monokaryotic Mycelium

Dikaryotic Mycelium

Figure 20-7 Ustilago maydis, corn smut. Basidiospores infect the cells of the corn plant, producing a monokaryotic mycelium, which later becomes dikaryotic. The dikaryotic cells produce teliospores. The nuclei fuse, producing a diploid spore, which produces a basidium by meiosis.

aeciospores, no uredospores, and no alternation of hosts. Corn smut is a # Notes % costly infection that commonly causes an annual loss of nearly fifty-five million bushels of grain in the United States.

Basidiospores invade the floral organs and the adjacent tissues of the corn and produce a mycelium, which is at first mononucleated. Later and as a result of nuclear division without cell division the mycelium becomes bi-nucleated. The mycelium spreads and, in maturity, releases binucleated teliospores. The nuclei of the teliospores then fuse to create a diploid condition. The 2n spores can then germinate, forming a basidium and, by meiosis, four basidiospores. The basidiospores are budded from the sides of the basidium rather than being borne on the tips of sterigmata (as in the rusts).

A description of a typical basidium was given in the introductory comments of this chapter; but neither the rusts nor the smuts generate the typical form of basidium. Rusts and smuts belong to a subclass called Heterobasidiomycetidae, in which basidia commonly arise from thick-walled resting spores and divide into cells. The subclass Homobasidiomycetidae includes mushrooms, puffballs, and bracket fungi, which produce club-shaped, nonseptate basidia. Coprinus, a common gill fungus, is representative of this subclass. It has two kinds of spores: plus and minus. A spore resting on an appropriate soil will produce an extensive, monokaryotic mycelium. When this mycelium makes contact with another monokaryotic mycelium of a mating type (for example, a plus mycelium makes contact with a minus mycelium), a union will occur that results in a dikaryotic condition. The plus and minus nuclei are associated in the same cell but are not fused at this time.

The mycelium produces fruiting bodies called mushrooms, or toadstools, entirely composed of compacted hyphae. Figure 20-9 illustrates mushroom development. In the earliest stage, the mushroom takes the form of a button.

Mushrooms Illustration

Figure 20-8 Coprinus. At maturity, the piieus breaks down into an inky mass. (Illustration by Laurette Richin)

% Notes # As the button expands, the covering, called the universal veil, breaks, revealing an elongated stem, the stipe, and, at the tip, the pileus. On the underside of the pileus, gills develop. They are made of compacted parallel hyphae that form a hymenium. The basidia grow out perpendicularly from the hymenium. In the early stages of basidium development, the two nuclei of the dikaryotic cell fuse to create a 2n zygotic nucleus. This nucleus then undergoes meiosis, yielding four haploid basidiospores, which come to lie at

Life Cycle The Ustilago Scitaminea
Figure 20-9 Mushroom development, (a) and (b) Button stage, (c) Development further advanced, with the universal veil beginning to break, (d) The pileus. (e) The universal veil broken, (f) The gills in the pileus. (g) A basidium, which grows from the surface of the gill, (h) Paraphysis.

the tips of projections (sterigmata). Paraphyses (sterile filaments) also grow # Notes &

out from the hymenium and under microscopic examination can be seen resting among the basidia. Coprinus being a heterothallic form, the union of hyphae take place between mycelia of mating types. Some smut forms are homothallic and thus allow a union of cells from the same mycelium.


Puffballs are another form of Basidiomycetes. They grow to varying sizes. While they often measure approximately one inch across, the diameter can sometimes be measured in feet. A very large puffball is estimated to produce trillions of spores, a somewhat smaller one, billions of spores. If two of a number of billions of spores should successfully grow new puffballs, the population of them would double, and one would conclude from this that the chances of their reproduction is extremely slight. Yet, they are common. They are also edible.

Remember that of the two nuclei in a dikaryotic cell, one is plus and the other minus. An elegant mechanism called a clamp connection ensures that when cells divide, the same arrangement of nuclei is maintained (see figure 20-10). A small lateral branch forms on the side of a terminal cell. This branch curves back to the hypha, makes contact with it, and constructs a passageway through which a nucleus can migrate. A new cell wall then forms. Nuclear migration ensures that daughter cells continue to have the plus and minus arrangement of nuclei. The formation of the side branch is suggestive of crozier formation in ascomycetes.

Clamp Connection Fungi

Figure 20-10 Clamp connections, a mechanism that maintains the arrangement of plus and minus nuclei in each cell.

# Notes * Mycorrhiza

Many plant diseases are caused by fungi, too many, in fact, to catalog here. Silkworm disease, brown rot of plums, early blight of potatoes, Fusarium wilt of cotton, a smut disease of sugar cane are but a few. Certain other fungi, however, are worthy of praise.

Fungi assist in the germination of seeds. The mycelia of various basidiomycetes and ascomycetes are associated with the roots of flowering plants in symbiotic relationships. The name for such an association is mycorrhiza (myco meaning "fungus," rhiza meaning "root"). The fungi involved in such relationships are present in the soil. The fungal hyphae connect to the younger portions of roots and form a sheath of compact mycelium around the roots. Pines, heaths, and orchids lack root hairs and can grow successfully only when they associate in this way with fungal hyphae that furnish the absorbing mechanism. When pines are planted in an area where they have not grown before, the forester mixes the proper fungi with the soil to encourage successful growth.

Two kinds of union are recognized: an ectotrophic association, wherein the hyphae of the fungus grow between the cortical cells of the root but do not, or only rarely, penetrate the root cells; and an endotrophic association characterized by the absence of the fungal sheath and wherein the hyphae penetrate the cortex of the root and enter the cells. In endotrophic mycorrhizae, the hyphae that penetrate the cells disintegrate over time and are then digested by the host cells. This liberates certain products that can be absorbed by the roots. Perhaps one hundred species of fungi are capable of forming mycorrhizae with forest trees.

The mycorrhizal fungi may have special nutritional requirements, such as carbohydrates and vitamins, and it is presumed that they derive these benefits from tree roots. The reverse condition also occurs. A conifer grown in conditions wherein it is denied association with fungal hyphae often fails to develop normally but can sometimes recover when inoculated with a suitable fungus.

The mycorrhiza of orchids is endotrophic, the mycelium being widely distributed through the cortex or, if the orchid is a rootless form, penetrating the chlorophyll-free tissues of absorbing organs. The minute seeds of orchids do not germinate unless they are infected with the appropriate fungus, and until they have emerged aboveground, they are totally dependent on the fungal infection.

Consider the following scenario. A crayfish living in the darkness of a cave loses the capacity to form eyes. A fungus that ensnares worms (refer to chapter 18, figure 18-5) cannot produce the mechanism of entrapment in the absence of the worms. A tree having mycorrhiza loses the capacity to produce root hairs. In chapter 5 on Mendelian genetics, some discussion centered on Lamarck's concept of the inheritance of acquired characteristics, wherein organisms achieve traits to fulfill needs. Would the preceding examples, then, illustrate the disinheritance of unneeded characteristics? It sounds t like reverse Lamarckism. What, then, causes the disappearance of genes for # Notes # unused traits?

Insects are involved in some instances. For example, certain scale insects may press closely against the smooth bark of a tree, where they both suck the juices of the plant and become infected with a fungus. The fungus may derive benefits from the plant indirectly by absorbing plant nutrients from the insects.

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