Phylum ZygomycotaThe zygomycetes (phylum Zygomycota)lack septa in their hyphae except when theyform sporangia or gametangia. Zy-gomycetes are by far the smallest of thethree phyla of fungi, with only about 1050named species. Included among them aresome of the more common bread molds(figure 36.8), as well as a variety of othermicroscopic fungi found on decaying or-ganic material. The group is named after acharacteristic feature of the life cycle of itsmembers, the production of temporarilydormant structures called zygosporangia.

In the life cycle of the zygomycetes(figure 36.8b), sexual reproduction occursby the fusion of gametangia, which contain numerous nu-clei. The gametangia are cut off from the hyphae by com-plete septa. These gametangia may be formed on hyphae ofdifferent mating types or on a single hypha. If both + and – mating strains are present in a colony, they may growtogether and their nuclei may fuse. Once the haploid nucleihave fused, forming diploid zygote nuclei, the area wherethe fusion has taken place develops into an often massiveand elaborate zygosporangium. A zygosporangium maycontain one or more diploid nuclei and acquires a thick

coat. The zygosporangium helps thespecies survive conditions not favorable forgrowth. Meiosis occurs during the germi-nation of the zygosporangium. Normal,haploid hyphae grow from the haploidspores that result from this process. Exceptfor the zygote nuclei, all nuclei of the zy-gomycetes are haploid.

Asexual reproduction occurs much morefrequently than sexual reproduction in thezygomycetes. During asexual reproduction,hyphae grow over the surface of the breador other material on which the fungusfeeds and produce clumps of erect stalks,called sporangiophores. The tips of thesporangiophores form sporangia, whichare separated by septa. Thin-walled hap-

loid spores are produced within the sporangia. Their sporesare thus shed above the substrate, in a position where theymay be picked up by the wind and dispersed to a new foodsource.

Zygomycetes form characteristic resting structures,called zygosporangia, which contain one or morezygotic nuclei. The hyphae of zygomycetes aremultinucleate, with septa only where gametangia orsporangia are separated.

Chapter 36 Fungi 725

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FIGURE 36.8Rhizopus, a zygomycete that grows onmoist bread and other similarsubstrates. (a) The dark, spherical,spore-producing sporangia are onhyphae about a centimeter tall. Therootlike hyphae anchor the sporangia. (b) Life cycle of Rhizopus. This phylumis named for its characteristiczygosporangia.

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