What fungal group produces flagellated spores?

We’ve just looked at characteristics shared by many species of fungi. Now let’s examine their distinctive phylogenetic groups. Most biologists now recognize four phyla within the kingdom Fungi. Species in each phylum have distinguishing characteristics in their reproductive life cycles.

Chytrids are small organisms once classified as protists. They’re the only fungi with flagellated cells. Zygote fungi, or zygomycetes, are characterized by the formation of a durable structure called a zygosporangium as part of their sexual life cycle. Sac fungi, or ascomycetes, develop distinctive saclike structures, called asci, in which their spores are produced. The phylum of club fungi, or basidiomycetes, includes species that produce conspicuous fruiting bodies, such as mushrooms. They’re named for their tiny club-shaped spore-producing structures, called basidia.

Since the classification of fungi largely depends on characterizing their sexual reproduction, it has been difficult to classify some species in which only asexual reproduction has been observed. These species have been kept in a “holding area,” the category called imperfect fungi, or deuteromycetes. Molecular systematics is helping to assign species from this category to their proper phylum.

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Many chytrids are saprobes that live in aquatic and terrestrial habitats. Others are parasites. The reproductive cycles of chytrids are diverse, but many have flagellated spores called zoospores that swim to a new location before germinating. Some chytrids also have flagellated gametes.

Molecular analysis of chytrids suggests that they are the phylum most similar to the common ancestor of fungi. It’s possible that the first fungi possessed flagella, and that this trait was lost by the other three phyla over the course of evolution.

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Hundreds of species of zygote fungi have been identified. One of the best known is the black bread mold of the genus Rhizopus. The life cycle of Rhizopus exemplifies important aspects of the reproduction of this group. The mycelium of the mold grows over its food. The organism reproduces asexually by producing sporangia from its mycelium. Haploid spores, genetically identical to the parent, are dispersed through the air.

If two individuals of different mating type encounter each other, specialized hyphae may come into contact, develop gametangia, and fuse. The zygote that results develops into a zygosporangium, which may remain dormant for weeks or months. The zygosporangium is resistant to adverse conditions, such as freezing and drying. When conditions are favorable, the diploid cells of the zygosporangium undergo meiosis, and the structure develops a sporangium that releases genetically diverse haploid spores.

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Biologists have identified tens of thousands of species of sac fungi. Their habitats and lifestyles are diverse. Some are single-celled yeasts. Others develop elaborate and often beautiful fruiting structures in the shape of a cup. The fruiting bodies called morels and truffles are human delicacies. Many sac fungi are the fungal constituents of lichens.

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The distinguishing feature of sac fungi, from which they derive their name, is the saclike structure called an ascus. Spores are produced in the ascus at the last stage of sexual reproduction. Like other fungi, sac fungi can reproduce asexually. Naked asexual spores are produced at the ends of hyphae in chains or clusters.

In sexual reproduction, mating structures form and fuse, leading to the development of dikaryotic hyphae. The terminal cells of these hyphae elongate, and their nuclei fuse to form a diploid zygote. Meiosis and mitosis follow, leaving 8 haploid spores within the walls of the original zygote, which has become the ascus.

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The most familiar club fungi are those that produce fruiting bodies called mushrooms. Tens of thousands of species of club fungi are known. They include many other fungi with conspicuous fruiting bodies, such as puffballs and shelf fungi.

Club fungi are important decomposers of wood, and many also form symbiotic relationships with trees and other plants. Some species of club fungi cause important fungal crop diseases known as smuts and rusts. A variety of mushrooms are used for human food, but many others are extremely poisonous and often difficult to distinguish from the edible varieties. Mushroom gathering is best left to experts.

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Unlike zygote fungi and sac fungi, club fungi don’t often reproduce asexually, although they can produce naked spores as sac fungi do. The mycelium produced from the fusion of haploid mycelia of compatible mating types remains dikaryotic for a long time, and is the major growth stage of the club fungus.

At times, in response to environmental conditions, the mycelium develops elaborate fruiting bodies such as mushrooms. The fruiting body is a tightly packed mass of hyphae. A club-shaped dikaryotic cell called a basidium forms at the tips of hyphae in the fruiting body. Basidium is Latin for “little pedestal,” which the structure is also thought to resemble.

Fusion of the nuclei is followed by meiosis, resulting in four haploid nuclei. The basidium develops four small projections into which the nuclei migrate and develop into spores. The spores are released from the fruiting body to the wind. Millions of spores may be released from a single fruiting body such as a mushroom.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education

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