Phylum Ascomycota The Ascomycetes Sac Fungi

Should you happen one summer to be touring or visiting in the south of France, you might be startled to see men and women with coils of rope slung around their shoulders pushing pigs in wheelbarrows. They happen to be wheeling the pigs into the woods to help them find truffles.

Truffles are gourmet "mushrooms," which grow mostly between 2.5 and 15 centimeters (1 and 6 inches) beneath the surface of the ground, usually near oak trees. They are somewhat prunelike in appearance and may be more than 10 centimeters (4 inches) in diameter, although most are smaller. They give off a tantalizing aroma that has been shown to contain pig sex pheromones (chemicals that produce specific responses). Pigs can detect truffles a meter (3 feet) below the surface and more than 15 meters (50 feet) away. The animals, in an attempt to get to them, strain at the ropes that are tied around their necks. The owners dig up the truffles, which were selling in the United States in 2001 for about $300 a pound, and reward the pigs with acorns or

Pilobolus Mycelium

sporangiophore light-sensitive sporangial base sporangiophore light-sensitive sporangial base

Figure 1Q.5 The dung fungus Pilobolus. The spores are released with force toward a light source.

other less interesting food. Trained dogs are now often substituted for pigs in truffle hunting, as they are less likely to eat the truffles.

Truffles are the reproductive bodies of representatives of a large and varied phylum of true fungi called ascomycetes (sac fungi). More than 30,000 species, including yeasts, powdery mildews, brown fruit rots, ergot, morels, microscopic parasites of insects, fungi associated with canned fruit, and many others, have thus far been described. Most produce mycelia superficially similar to those of the water molds and coenocytic true fungi, but the hyphae are partitioned into individual cylindrical cells. Each crosswall has one or more pores through which the single to many tiny nuclei can pass. Little bodies that serve as plugs are located near the pores. The "plugs" seal off individual cells if they become damaged.

Asexual Reproduction

Asexual reproduction is by means of conidia (singular: conidium). Conidia are spores that are produced externally—outside of a sporangium—either singly or in chains at the tips of hyphae called conidiophores (see Fig. 19.34). Asexual reproduction in yeasts is by budding (Fig. 19.6). As a yeast cell buds, the nucleus divides, and a small protuberance appears to balloon out slowly from the cell. One daughter nucleus moves into the bud, which becomes pinched off as it grows to full size.

Sexual Reproduction

Sexual reproduction involves the formation of tiny fingerlike sacs called asci (singular: ascus). When hyphae of two different "sexes" become closely associated in the more complex sac fungi, male antheridia may be formed on one

Budding Sac Fungi
Figure 1Q.6 How a yeast cell reproduces asexually by budding.

and female ascogonia on the other, although in many species, antheridia and ascogonia may be produced on the same mycelium. Hyphae grow and connect an antheridium and an ascogonium to each other; male nuclei then migrate into the ascogonium. There, the male nuclei pair with the female nuclei present but do not unite. New hyphae (ascoge-nous hyphae), whose cells each contain one male and one female nucleus, grow from the ascogonium, the cells dividing in a unique way so that each cell has one of each kind of nucleus. This special cell division begins with the cell at the tip of an ascogenous hypha forming a hook called a crozier. The two nuclei in the crozier divide simultaneously, and crosswalls then form so that there are now three cells, the middle cell containing one male and one female nucleus. This middle cell becomes an ascus in which the two haploid (n) nuclei unite and become a diploid (2n) zygote nucleus. The fingerlike, tubular asci develop in a layer (referred to as the hymenium) at the surface of a structure called an ascoma (previously called an ascocarp). The zygote nucleus undergoes meiosis, and the resulting four haploid nuclei usually divide once more by mitosis so that there is a row of eight nuclei in each ascus. These nuclei become ascospores as they are walled off from one another with a little cytoplasm (Fig. 19.7).

Thousands of asci may be packed together in an ascoma, which often is cup shaped (apothecium—Fig. 19.8) but also may be completely enclosed (cleistothecium—Fig. 19.9) or flask shaped with a little opening at the top (perithecium). Truffles actually are enclosed ascomata (apothecia). Cup-shaped ascomata may be several centimeters (2 to 3 inches) in diameter and may be brilliantly colored on the inside. When ascospores are mature, they are often released with force from the asci. If an open ascoma is jarred at maturity, it may appear to belch fine puffs of smoke consisting of thousands of ascospores that are dispersed to new locations by air currents.

Kingdom Fungi 361

When an ascospore lands in a suitable area, it may germinate, producing a new mycelium, and then repeat the process. In many instances, however, a number of asexual generations involving conidia are produced between the sexual cycles.

Sexual reproduction in yeasts is somewhat streamlined in that individual yeast cells function as ascogonia and antheridia. When two haploid cells unite, they become a diploid fusion cell that may serve as an ascus in which meiosis takes place and ascospores are formed—all within the original cell wall. In some yeasts, however (including the common baking and brewing yeast Saccharomyces cerevisiae), the fusion cell may multiply asexually into a diploid colony, with all the cells eventually undergoing meiosis.

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