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The Zygomycota contains approximately 1% of the described species of true Fungi (~900 described species; Kirk et al. 2001). The most familiar representatives include the fast-growing molds that we encounter on spoiled strawberries (Figure 1) and other fruits high in sugar content. Although these fungi are common in terrestrial and aquatic ecosystems, they are rarely noticed by humans because they are of microscopic size. Colonial growth and the taxonomically informative asexual reproductive structures Zygomycota produce are typically studied after culturing on various agar media. Direct microscopic observation of suitable substrates is required for those species that either have not or cannot be cultured. Fewer than half of the species have been cultured and the majority of these are members of the Mucorales, a group that includes some of the fastest growing fungi.
Figure 1. Moldy strawberries covered with Rhizopus mycelium. Photo K. O'Donnell.
Zygomycota are defined and distinguished from all other fungi by sexual reproduction via zygospores following gametangial fusion (Figure 2A,B) and asexual reproduction by uni-to-multispored sporangia (Figure 3A,B) within which nonmotile, single-celled sporangiospores are produced. The phylum comprises at least seven phylogenetically diverse orders. Monophyly of the phylum and interrelationships among orders are currently under intensive investigation using multilocus DNA sequence data. This introduction to the Zygomycota does not include the order comprising the arbuscular mycorrhizal (AM) fungi, the Glomales, because it has been elevated to the rank of phylum, the Glomeromycota (Schüßler et al. 2001). One other group, the Microsporidia, were previously considered protozoa, however, DNA, biochemistry, and morphology suggest these highly reduced, obligate, intracellular parasites may have evolved from a zygomycete-like ancestor (Keeling 2003).
Figure 3. Asexual reproduction. (A) Scanning electron micrograph of unispored sporangia of Benjaminiella poitrasii and (B) dehisced multispored sporangium of Gilbertella persicaria releasing sporangiospores. (From O'Donnell 1979).
Zygomycota are arguably the most ecologically diverse group of fungi, functioning as saprophytes on substrates such as fruit, soil, and dung (Mucorales), as harmless inhabitants of arthropod guts (Harpellales), as plant mutualists forming ectomycorrhizae (Endogonales), and as pathogens of animals, plants, amoebae, and especially other fungi (all Dimargaritales and some Zoopagales are mycoparasites). A number of species are used in Asian food fermentations, such as Rhizopus oligosporus in the Indonesian staple tempeh, and Actinomucor elegans in Chinese cheese or sufu (Hesseltine 1991).
Conversely, some species have a negative economic impact on human affairs by causing storage rots of fruits (particularly strawberries by Rhizopus stolonifer [Figure 1]), as agents of plant disease (e.g., Choanephora cucurbitarum flower rot of curcurbits), while other species can cause life-threatening opportunistic infections of diabetic, immuno-suppressed, and immuno-compromised patients (de Hoog et al. 2000). In addition to some Mucorales that attack immuno-suppressed humans, several species of microsporidia cause serious human infections. Some zygomycetes are regularly isolated by veterinarians from domesticated animals in tropical and subtropical regions of the world, including the US gulf states.