INTRODUCTION TO THE MUCOROMYCOTA

The core conjugating fungi comprise a diverse group of different molds. Most are common saprobes that can be an opportunistic pathogens. A few taxa are ectomycorrhia.. Some are pathogenic. They include the common bread molds (Mucor and Rhizopus). Their synapomorphies include asexual sporangiospores (text with tooltip) Asexual spores formed with the sporangium. and the formation of zygospores that develop from the fusion of similar gametangia (Figures 1 and 2, respectively).

Pilobolus (Figure 1), Mucor, and Rhizopus (Figure 2) are the common pin molds. they produce mycelia of branched hyphae that are not septate (i.e. siphonaceous). Gametangia are formed laterally and fuse. The fusion product is a thick-walled resting spore called a zygospore. Meiosis occurs within the zygote and a sporangium bearing haploid sporangiospores rises from the zygospore. In most of these taxa, the sporangiospore, not the zygospore, functions as the means for dispersal. Pilobolus (Figure 1), a name that literally means hat tosser, flings the entire sporangium up to 7 meters away by explosive hydrostatic pressure at the end of the stalk that supports the sporangium (note the swollen stalk end in Figure 1). The sporangium is covered with a sticky mass that adheres to grass where it is eaten by a herbivore and then deposited with the dung. At that point spores in the sporangium begin to grow and feed on the dung. The ability to move the spores away from the dung heap is important for taxa that get into the dung by being eaten because herbivorous mammals typically will not feed immediately around a dung heap. Foos (2001) reported observing a dung eating nematode with the same requirements as Pilobolus, climbing the sporangial stalk and riding the sporangium when it was launched.

Rhizopus (Figure 2) is a common bread mold with typical zygospore formation, but, unlike Pilobolus, its sporangiospores are carried on the air. Most Rhizopus species are common soil fungi, but they do cause numerous plant diseases including strawberry rot. Rhizopus also can be an opportunistic pathogen of the lungs of vertebrates, including our species.

Endogone produces typical zygospores that are formed within sporocarps (Figure 5). Though common in the Ascomycota and Basidiomycota, sporocarps are rare in the other fungi. The sporocarps of Endogone form small pea-sized truffles that are edible. Endogone is a common soil fungus that makes mycorrhizal associations with vascular plants (See the Glomeromycota for an explanation of this). Daft and Okusanya (1973) provided evidence that mycorrhizal infections of tomatoes, petunias, strawberries, and maize enhanced the growth of all four plants.

Mortierella is a soil fungus that is common in alpine soils, and some of them are known as ‘snow molds’ (Schmidt et al. 2011). They are associates with tree roots and many of them are very efficient as chitin-degrading fungi.

FIGURE 1. The sporangium of Pilobolus. Image from The Systematic Biology Biodiversity Collectionl	FIGURE 2. Suspensors and zygospores of Rhizopus. Image from http://www.biology.iastate.edu/Courses/201L/Fungi/FungINDX.htm

	FIGURE 3. STAGES IN THE LIFE HISTORY OF RHIZOPUS. A. Hyphae with sporangiophores B. Diagram of sporangium C. Release of sporangiospores D. Details of sporangiospores E-H. Stages in the fusion of gametangia in the formation of a zygospore. Figure from Scagel et al. (1982)
	FIGURE 4. DIAGRAM OF THE MUCOROMYCOTA LIFE HISTORY. Most of the life history is haploid with very short stages that have paired nuclei or are diploid. Meiosis occurs in the zygote, from which a sporangium emerges. Images from Scagel et al. (1982)

FIGURE 5. Sporocarps of Endogone. Image from Scagel et al. (1982).

SYSTEMATICS OF THE MUCOROMYCOTA

The zygomycotes are taxa that superficially resemble some of the conjugating algae. They clearly are fungi in which the the dikaryotic phase does not go beyond the bounds of the zygospore. The vegetative filaments rarely have septa and are haploid. The typical means of asexual reproduction is through the release of sporangiospores (a kind of aplanospore). Bruns et al. (1992) in their examination of 18S rRNA nucleotide sequences, confirmed that the chytrids and the Zygomycota were basal groups within the clade of the Kingdom Fungi. Mainly, the Zygomycota and the rest of the fungi never produce flagella at any stage in their life history. More recently, further molecular evidence (see Tudge, 2000 and Patterson, 1999 for a synopsis) has confirmed their position near the root of the fungi [see also Lang, The Fungal Mitochondrial Genome Project], which is part of a larger clade called the opisthokonts, a group that includes the choanoflagellates and the metazoans (Patterson, 1999). Margulis and Schwartz (1998) still maintain that the fungi are a kingdom of conjugating taxa, and, therefore, include the zygomycota.

Molecular work confirmed that the Zygomycota is not monophyletic. Schussler et al. (2001) removed the arbuscular mycorrhizal (AM) fungi from the Zygomycota and defined them as a separate phylum, the Glomeromycota. Lutzoni et al. (2004) in a SSU and LSU r-RNA study of the Kingdom Fungi showed that the orders of the Zygomycota occupied at least 4 different monophyletic lines. Also, White et al. (2006), in an analysis of the r-RNA operon, confirmed the paraphyletic nature of the traditional groups of Zygomycota. In their analysis, the zygomycotes could be divided into three independent clades, Thus, the group could be fragmented into at least three (White et al. 2006) or four (Lutzoni et al. 2004) more phyla.

This phylum has one class and 3-4 orders as a modification of Hibbett et al. (2007). It includes the core Zygomycota (all taxa within the Mucoromycetes = Zygomycetes). This is a necessary change in the taxonomy of the conjugating fungi (formerly the Zygomycota) that had been shown to be polyphyletic (Lutzoni et al. 2004, White et al. 2006, and James et al. 2006b) and better described as separate phyla: Glomeromycota, Mucoromycota, Entomophthoromycota, and Kickxiellomycota. Formerly, the Zygomycota had two classes and up to seven orders (see Alexopoulos et al. 1996, Adl et al. 2005 and 2012, Hudson 1984, Bold et al. 1987, and Alexopoulos and Mims 1979). Margulis and Schwartz (1998) defined the group as having 7 higher taxa, which they describe as being classes. Currently, we have interpreted the analyses of Voigt and Wostemeyer (2001), Lutzoni et al. (2004), Tanabe et al. (2004 and 2005), White et al. (2006), James et al. (2006), Hibbett et al. (2007), and Adl et al. (2012) to have a single phylum with two classes, Mucoromycetes and Mortierellomycetes.