Overview of the Zygomycetes

Zygomycetes are fungi characterized by the formation of sexual spores (zygospores), and vegetative mycelium that lack septa except to delimit old or injured hyphae or reproductive structures in Mucorales and Zoopagales but plugged septa are regularly formed in Dimargaritales and Kickxellales (Benny et al. 2001). Asexual reproduction occurs most commonly by the formation of nonmotile, unicelled sporangiospores in uni- or multispored sporangia or merosporangia. In addition, arthrospores, chlamydospores, and yeast cells can be formed by some species. The mature asexual spores can be released either dry or wet (in a spore droplet) and dispersal occurs as a result of air currents, water droplets, or foraging by small animals (Ingold 1978). Spores can be dispersed singly or several may be released in an intact sporangium (Zoberi 1985).

Benjamin (1979) transferred several of the families of Mucorales recognized by Hesseltine and Ellis (1973) to orders of their own. Three orders were validated (Endogonales, Kickxellales, Zoopagales), one was newly described (Dimargaritales), and three (Entomophthorales, Harpellales, Mucorales) were extensively discussed by Benjamin (1979). Harpellales have been treated as members of the Trichomycetes and not the Zygomycetes by all other mycologists.

Early in 2001 the Zygomycetes, was thought to include ten orders (Kirk et al. 2001). Late last year, however, a new fungal phylum, the Glomeromycota (Schüssler et al. 2001), was described that now contains fungi from two of the orders, Geosiphonales and Glomerales (Morton and Benny 1990; Cavalier-Smith 1998) formerly in the Zygomycota. Two orders, Basidiobolales and Mortierellales, were described recently by Cavalier-Smith (1998). Fungi in all eight of the orders of the Zygomycetes have been discussed and illustrated in review articles (Benjamin 1959, 1966, 1979; Benny 1982; Humber 1989; Benny et al. 2001). During the last decade a phylogeny of the Zygomycetes, based particularly on the comparison of SSU rRNA, but also using LSU rRNA, other genes (elongation factor 1-alpha [EF-1a]) or morphology, has revealed the relationships of the eight orders currently accepted in the Zygomycetes (Nagahama et al. 1995; O’Donnell et al. 1998, 2001; Jensen et al. 1999; Tanabe et al. 1999, 2000; Voigt et al. 1999 Benny et al. 2001).

Described species of the Zygomycetes.

The most recent edition of the Dictionary of the Fungi (Kirk et al., 2001) described the Zygomycetes as consisting of ten orders, 32 families, 124 genera, and 870 species. They also stated that the trichomycete orders, Asellariales and Harpellales, might also belong in the Zygomycetes. Dr. Robert Lichtwardt is collecting Trichomycetes (Lichtwardt 1986; Benny 2001)and his students will probably solve the problems of the placement of these orders by sequencing 18S rRNA from representative taxa in Asellariales and Harpellales. Other workers have the expertise and are studying several of the nonculturable orders of the Zygomycetes. These orders are: Endogonales, Entomophthorales, Glomales, and two families of the Zoopagales (Cochlonemataceae, Zoopagaceae). Benny and coauthors (Benny, 1982; Benny et al., 2001) published reviews of zygomycete taxonomy; the Trichomycetes were also reviewed (Benny, 2001).

Phylogeny of Zygomycetes.

The phylogeny of the Zygomycetes, or portions of the class, have been presented by various authors, the most recent being Benjamin (1959) and Hesseltine and Ellis (1973). These treatments, however, are phenetic discussions of the relationships of these fungi. A phylogenetic analysis of selected species of the Harpellales (Trichomycetes) and Kickxellales (Zygomycetes) using combined molecular (18S rDNA) and morphological data was recently published (O’Donnell et al., 1998). Other molecular studies using 18S rDNA have been done on Entomophthorales (Nagahama et al., 1995) and the Glomales (Schüssler et al., 2001) but these included only a few species from each order. Comparative analysis of 18S rDNA sequences (O’Donnell et al., 2001) representing all orders, supported the seven orders of the Zygomycetes (Benny et al., 2001) proposed by Benjamin (1979) and Morton and Benny (1990). At least two additional orders, Basidiobolales (based on Basidiobolaceae, Entomophthorales) and Mortierellales (based on Mortierellaceae, Mucorales), recently were proposed by Cavalier-Smith (1998). Basidiobolales and Mortierellales are supported by 18S sequence data.

Updated Jan 15, 2005