Phylum Zygomycota The Coenocytic True Fungi

Although black bread molds are probably the best-known members of Phylum Zygomycota, they are not the only fungi that grow on bread. So many organisms can, in fact, contribute to bread spoilage that nearly all commercially baked goods have, in the past, had chemicals such as calcium propionate added to the dough to prevent or retard the growth of such organisms. There is now a trend toward eliminating preservatives from bread, pies, and other bakery items since the chemicals that have made the goods a less-suitable medium for the growth of fungi apparently are unhealthy for humans after prolonged use. Alternative ways of retarding fungal growth and spoilage are being sought.

Rhizopus (Fig. 19.3), a well-known representative black bread mold, has spores that are exceedingly common everywhere. They have been found in the air above the North Pole, over jungles, on the inside and outside of buildings, in soils, clothing, automobiles, and hundreds of kilometers out to sea, easily carried there by prevailing winds and breezes.

Asexual Reproduction

When a spore lands in a suitable growing area, it germinates and soon produces hyphae that may become an extensive mycelium. Like the mycelium of water molds, it is coenocytic (not partitioned into individual cells) and contains numerous haploid nuclei. After the mycelium has developed, certain hyphae called sporangiophores grow upright and produce globe-shaped sporangia at their tips (Fig. 19.4). Numerous black spores are formed within each sporangium. When these spores are released through the breakdown of the sporangium wall, they may blow away and repeat the cycle.

Sexual Reproduction

Black bread molds reproduce sexually by conjugation. Although there is no visible difference in form, black bread mold mycelia occur in two different mating strains. When a hypha of one strain encounters a hypha of the other, the chemicals they produce create an attraction, and swellings called progametangia develop opposite each other on the hyphae. The progametangia grow toward each other until they touch. A crosswall is formed a short distance behind each tip, and the two gametangia merge, becoming a single, large multinucle-ate coenozygote in which the nuclei of the two strains fuse in pairs. A thick, ornamented wall then develops around this coenozygote or zygospore with its numerous diploid nuclei. This structure, called a zygosporangium, is the characteristic sexual spore of members of this division. A zygosporangium may lie dormant for months, but eventually, it may crack open, and one or more sporangiophores with sporangia at their tips grow out. Meiosis apparently takes place just before thick-walled zygosporangium fertilization spore germinate;

mycelium

\ sporangium germination ^with spores thick-walled zygosporangium fertilization

Images For Bread Mold

\ sporangium germination ^with spores spore on bread germinates

Figure 1Q.3 Life cycles of the black bread mold Rhizopus.

spore on bread germinates

Figure 1Q.3 Life cycles of the black bread mold Rhizopus.

this occurs, and thousands of black spores are produced in the sporangia. In some species, the spores are produced externally on hyphae instead of being formed in sporangia.

One dung-inhabiting genus of fungi in this division has the scientific name of Pilobolus, derived from two Greek words meaning "cap thrower." The name is quite appropriate, as the mature sporangia are catapulted a distance of up to 8 meters (26 feet), where they adhere to grass or other vegetation (Fig. 19.5). When the vegetation is ingested by animals, the spores germinate in the digestive tract and are already growing in the dung when it is released.

The sporangia of Pilobolus fungi are forcibly released precisely in the direction of light, to which the sporangio-phores are very sensitive. This action can be demonstrated by placing some horse dung (preferably at least two days old) in a glass dish that has a lid and then covering the dish with black paper. An opening of any shape is cut in the paper; the dish should then be set where it will receive adequate light for a few days. Any Pilobolus sporangia that have been produced and forcibly discharged will form a black pattern on the glass closely corresponding to the cut-out area.

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Responses

  • keir
    What is the phylum of black bread mold?
    8 years ago
  • eden
    How to grow mold on bread?
    7 years ago
  • stella sandy
    Why zygomycota hyphae are called coenocytic?
    7 years ago
  • Noah
    Which phylum of fungi is coenocytic?
    7 years ago
  • jacqueline
    What is common reprensentaive of mold found in phylum zygomycota?
    7 years ago
  • Valdemar
    How large is a coenozygote?
    3 years ago
  • Kody Kennedy
    What Is Coenozygot How Large Is It?
    2 years ago

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