Fig. 22.6. Fungi Reproduction

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 multinucleate 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 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 phylum 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. 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 relea sed precisely in the direction of light, to which the sporangiophores are very sensitive. This action can be demonstrated by placing some horse dung (preferably at least 2 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. The sporangia are thrown far from the dung, frequently sticking to blades of grass, where they will be ingested again and ultimately end up in a new dung pile.

Table 22.1

Date: 2016-01-03; view: 2135