Division (Phylum): Ascomycota − Rakyat Biologi

The Ascomycota comprise about 60,000 described species, including a number of familiar and economically important fungi. Most of the blue-green, red, and brown molds that cause food spoilage are Ascomycota. This includes the salmon-colored bread mold Neurospora sp., which has played an important role in the development of modern genetics. Many Ascomycota are the cause of serious plant diseases, including powdery mildews that attack fruits, chestnut blight, and Dutch elm disease (caused by Ceratocytis ulmi, a fungus native to certain European countries). Yeasts, the edible morels and truffles are also Ascomycota. This group of fungi, as a whole, is relatively poorly known, and thousands of additional species await scientific description.

Ascomycota, with the exception of the unicellular yeasts, are hyphal. The hyphae are septate, or divided by cross walls. The hyphal cells of the vegetative mycelium may be either uninucleate or multinucleate. Some species of Ascomycota can self-fertilize and produce sexual structures from a single genetic strain; others require a combination of + and - strains.

1) Asexual reproduction in the majority of the Ascomycota occurs by the formation of specialized spores, known as "conidia" (a Greek word for "fine dust"), which are cut off from tips of modified hyphae called "conidiophores"("conidia bearers"). The conidiophores partition conidia in longitudinal chains. Each conidium contains one or more nuclei. Conidia form on the surface of conidiophores in contrast to spores that form within sporangia in Rhizopus. When mature, conidia are released in large numbers and germinate to produce new organisms. Pencillium sp., which you will examine in this laboratory, is a common example of a fungus that forms conidia.

2) Sexual reproduction in Ascomycota always involves the formation of an "ascus" (pl. asci), a sac-like structure that is characteristic of this division and distinguishes the Ascomycota from all other fungi. Ascus formation is usually within a complex structure composed of tightly interwoven hyphae - the "ascocarp". Many ascocarps are macroscopic, and are the only part of these fungi that most people ever see. An ascocarp may be open and more or less cup-shaped (called an "apothecium"), closed and spherical in shape (called a "cleistothecium"), or flask shaped, with a small pore through which the ascospores escape (called a "perithecium"). The layer of asci is called the hymenium", or

Figure 13: Some typical "ascocarps" found in the division Ascomycota.

a) Sexual Life Cycle of Ascomycota

Figure 14 illustrates a typical life cycle of Ascomycota. When examining the slides and preserved materials in this lab, try to place it into this generalized life cycle. The mycelium is initiated with the germination of an ascospore, or conidium. Many "crops" of conidia are produced during the growing season, and it is the conidia that are responsible for propagation of the fungus.

Asci formation occurs on the same mycelia that produce conidia. They are preceded by the formation of multinucleate gametangia called "antheridia" and "ascogonia". The male nuclei of the antheridium pass into the ascogonium via a tubular outgrowth of the ascogonium known as the trichogyne. "Plasmogamy", or the fusion of the two cytoplasms, has now taken place. The male nuclei then pair with the genetically different female nuclei within the common cytoplasm but do not fuse. Hyphae now begin to grow out of the ascogonium. As the hyphae develop, pairs of nuclei migrate into them and simultaneous mitotic divisions occur in the hyphae and ascogonium. Cell division in the developing hyphae occurs in such ways that the resulting cells are "dikaryotic" (i.e. containing two haploid nuclei, one from each strain). The dikaryotic hyphae grow together to form a reproductive structure called an "ascocarp".

The ascus first forms at the tip of the developing dikaryotic hypha. The two nuclei in the terminal cell (ascus) of the dikaryotic hyphae then fuse into a single diploid nucleus ("karyogamy"). This is the only zygote. The ascus then elongates and the diploid nucleus divides by meiosis, forming 4 haploid nuclei. Each haploid nucleus usually divides again by mitosis, resulting in a total of 8 haploid nuclei. These haploid nuclei are then cut off in segments of the cytoplasm to form "ascospores". In most Ascomycota, the ascus becomes turgid at maturity and finally bursts, sending its ascospores explosively into the air.

Figure 14: Life Cycle of a Typical Ascomycota.

3) Examples of Ascomycota

a) Yeast

Yeast are somewhat atypical of most Ascomycota. They are predominantly unicellular and reproduce asexually by fission or by budding (pinching-off of small buds), rather than by spore or conidia formation. Sexual reproduction in yeasts occurs when either two cells or two ascospores unite and from a diploid zygote. The zygote may produce asexual buds, or may undergo meiosis to produce four haploid nuclei. In some species there may also be a subsequent mitotic division producing eight haploid nuclei. The single cell in these unicellular yeasts is acting as an ascus, and therefore the whole organism becomes the reproductive structure. Within the ascus/zygote wall, walls are laid down around the nuclei so that eight ascospores are formed. These are liberated when the ascus wall breaks down. The ascospores either bud asexually or fuse with another cell to repeat the sexual process.

Two genera of yeasts, Saccharomyces sp. and Schizosaccharomyces sp. are commonly used in the baking and brewing industry. In both these genera the asci are formed by the fusion of two haploid cells.

Note the presence of 4 ascospores in each ascus. Also note that in these unicellular yeasts, there is no "ascocarp". Figure 15 shows yeast cells in various stages of asexual and sexual reproduction.

Figure 15: Yeast cells budding and sporulating.

b) Penicillium sp. (blue - green molds)

Penicillium sp. has become celebrated in connection with antibiotics. Penicillin, a by-product of Penicillium notatum when liberated into the culture medium, inhibits the growth of gram-positive bacteria. Penicillin was discovered by Sir Alexander Fleming in 1929, but was not exploited until World War II. The great importance of this substance is that it represses bacterial growth without being toxic to animal tissues. Interest in penicillin led to an intensive search for other antibiotics, and molds suddenly became of considerable economic interest. However, Penicillium sp. is of economic importance in other respects. For example certain species give some types of cheese the flavor, odor, and character so highly prized by gourmets. One such mold, P. roquefortii, was first found in caves near the French village of Roquefort. Legend has it that a peasant boy left his lunch, a fresh piece of mild cheese, in one of these caves and on returning several weeks later found it marbled, tart, and fragrant. Only cheeses from the area around these particular caves are permitted to bear the name of Roquefort. Another species of this genus, P. camembertii, give Camembert cheese its special qualities.

Penicillium sp. reproduces asexually by forming spores called conidia.

Penicillium sp. conidiophores and conidia.

c) Cup Fungi

The cup fungi are the most advanced group of Ascomycota. They produce an ascocarp called an "apothecium", with the asci arranged in an exposed layer. Although many apothecia are disc or cup-shaped, other forms also exist.

Peziza sp.

The apothecia of Peziza sp. often exceed 10 cm in diameter. These apothecia are usually bowl-shaped when young but will become flattened and distorted with age. At maturity, the thousands of asci on the surface of the cup develop hydrostatic pressure. If the cup is disturbed, the asci rupture releasing thousands of ascospores in a visible "puff". Wind currents then transport the spores to a new environment.

Figure 17: Diagram of the apothecium of Peziza sp. (top) and microscopic view of asci (bottom).

Morchella sp. (morels)

These are some of the most highly prized edible fungi. The apothecium of Morchella sp. has a stalk, or stipe, and a fertile portion called the "pileus" (Figure 18). The pileus is essentially discoid, but it is folded over the stipe apex and is highly contorted. These distortions greatly increase the surface area of the pileus. The asci line the large pits, which are separated by sterile ridges. Each "pit" is similar to the cup of Peziza sp. In other words, each pit is an apothecium producing ascospores.