The Wonderful World of Fungi, Part V
Division Zygomycota

By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter

Fungal taxonomy follows very specific rules regarding how a given fungus is named and how it is related to other fungi. All true fungi are placed in the Kingdom Fungi. The fungi are then subdivided into smaller and smaller taxa based upon presumed relationships among the members. The largest subgroup is called a division, which is then subdivided into classes, classes are subdivided into orders, then into families, into genera, and finally into species. The fungi grouped into a division have some characteristic(s) that all members of that division share, but which are not found in other fungi. (As an aside, Division is an equivalent term to Phylum). A fungus that produces a sexual reproductive spore called a zygospore or zygosporangium is placed in the Division Zygomycota. This division has two classes, Zygomycetes and Trichomycetes. The Class Zygomycetes contains six orders, 29 families, 120 genera, and about 800 species. The Class Trichomycetes contains four orders, seven families, 52 genera, and about 210 species. Please note that all fungal divisions end in -mycota; all fungal classes end in
-mycetes; all fungal orders end in -ales; and all fungal families end in -aceae. These taxonomic endings are the same as those used for organisms that are placed in the plant kingdom.

The zygomycetes (generic term I use for all members of Zygomycota, not just the class Zygomycetes) are mostly terrestrial fungi that live in soil as saprophytes and feed on decaying plant or animal material. Some members of this class are parasites of plants, insects, and small animals, while some exist symbiotically with plants. The fungal body or thallus of zygomycetes is composed of hyphae (long, branching thread-like filaments that are the vegetative portion of the thallus) which collectively form a mycelium. The hyphae of zygomycetes are coenocytic; that is, they lack septa or cross-walls except where gametes are formed or to wall off dead hyphae.

Perhaps the best-known member of the Zygomycetes is Rhizopus stolonifera, the black bread mold. It is the species that is commonly used in general biology classes to show students what zygospores (the sexual reproductive structure in the Division Zygomycota) look like. This species has two mating-types, a plus strain and a minus strain. Hyphae from the two mating-types must fuse to permit haploid nuclei to fuse inside of the gametangia to form diploid nuclei. These diploid nuclei can then undergo meiosis to produce the meiospores inside of a zygosporangium. The spores that are released from the zygospore at maturity are haploid and will be either a plus or a minus mating-type. Upon germination, each spore can potentially produce a fungal mycelium that grows and reproduces both asexually and sexually, thus repeating the life cycle. (Note: not all species of zygomycetes have different mating-types; some species are self-fertile and produce meiosporangia after forming a diploid nucleus in a developing zygosporangium.)

Rhizopus, like other members of this class also produce asexual spores called mitospores (sporangiospores), in specialized structures, called mitosporangia or just sporangia. Sporangia may contain only a few spores, or may contain thousands of haploid spores depending on the species. The sporangia are borne at the tip of specialized hyphae called sporangiophores that are negatively geotropic and positively phototropic. This helps ensure that following release from the sporangia the spores can be easily dispersed and disseminated in air currents. Since the spores are very light they readily become part of the aerospora and can travel great distances from the sporangia that produced them. Also, the walls of the sporangia are very thin and easily ruptured at maturity by passing animals, wind, or raindrops, which permits easy release of the spores into the air.

In my next blog, we will explore the fascinating world of some of the members of the Zygomycota that have unique and interesting life cycles and modes of nutrition; fungi that are dung lovers as well as parasites of fungi, amoebae, and nematodes.

Good online reference for additional information on Zygomycetes:
http://www.mycolog.com/CHAP3b.htm