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Division Zygomycota

Asexual Spores

Reproduce asexually by sporangiospores in most orders of the Zygomycetes, conidia in the Entomophthorales and arthrospores in some Trichomycetes. Sporangia may contain from 50 to 100,000 spores. Sporangiola are smaller and contain from one to as many as 30 spores. Sporangiola have considerable morphological variation. When spores are found in a single row, they are known as uniseriate merosporangia. Single spored sporangiola found in some Mucorales have been referred to as conidia, but as they form within a sporangial wall, the basal septum is not part of the wall of the spore and they are sporangiospores.

Classes

Class: Zygomycetes

Characterised by well developed mycelium, which may be immersed in substrate or host tissue. Please note that the taxonomy of this group is undergoing massive change. We will incorporate these changes as we come to understand their implications.

Orders: Dimargaritales, Endogonales, Kickxellales, Mucorales, Zoopagales.

Note that the Entomophthorales are now placed in a new division which remains to be formally descibed

Class: Trichomycetes

Characterised by a much reduced thallus, with a holdfast that attaches to the host exoskeleton. The mycelium is usually simple though may be branched.

Orders: Amoebidiales, Asellariales, Eccrinales, Harpellales.

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Cultural Conditions

The division contains a variety of fungi. The members of the Mucorales are commonly referred to as sugar fungi, meaning that saprophytes respond rapidly to and grow rapidly on simple sugars in artificial media. Often they are the fungi found to be fruiting first in natural substrates such as dung, presumably because of their rapid response to readily available organic nutrients. Culture of Trichomycetes appears complicated because of the obligate association with arthropod hosts. The order Endogonales contains culturable fungi, some are saprophytes and others form ectomycorrhiza.

Yeast forms have been induced in some species by the use of media containing high concentrations of nutrients and anaerobic conditions.

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Ecology

Zygomycetes are commonly saprotrophs, symbionts or parasites in terrestrial habitats. Members of the Mucorales are easily isolated from soil, humus and dung where they grow saprobically.

Trichomycetes are obligate associates of arthropods, including insects, millipedes and crustaceans. The host may be an adult or larva, in terrestrial or aquatic habitats. The fungi are usually found attached to the chitinous gut lining of the host by a holdfast. The precise relationship is difficult to determine in most cases. However, one pathogen is known and in another example, the fungus has been found to provide its host with sterols and vitamins. The fungus is assumed to gain nutrients from the gut of the host.

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Hyphal Characteristics

Mycelia are characteristically aseptate with septa only being found to delimit sporophores or dying portions of hyphae. The hyphae rarely anastomose, so do not form interconnected mycelia common in the higher fungi. The walls of hyphae contain chitin, chitosan and polyglucuronic acid.

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Physiology

In general, members of the Zygomycetes do not produce enzymes that enable them to utilise complex polysaccharides. While some fungi possess cellulases and pectinases, these appear to enable the fungi to colonise plants rather than digest their component parts. Only a few plant pathogens are found in this group, and many of these penetrate the plant through existing openings such as wounds, nectaries and stomates.

Some members of the Mucorales are commonly used to ferment foods and produce important industrial products such as lactic acid, amylases, rennin and organic acids. The fermentation of foods is nutritionally important because the fungi contribute substantially to the nutritional value of the foods.

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Pores

While the fungi are known to be aseptate in the vegetative mycelium, secondary septa form during separation of sporophores from the mycelium. At least four different types of septa have been described. Multiperforate septa are found in members of the Mucorales and Endogonales. Another common type of septum remains open with the edges of the pores retaining a lenticular cavity containing a plug. The lenticular pore is found in members of the Kickxellales, Dimargaritales, Harpellales and Asellariales.Septa of the Zoopagales remain to be examined, though some are known to be perforate.

Note that the Entomophthorales have a complete, bilayered septum lacking perforations.

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Sexual Spores

The division is characterised by formation of zygospores. The formation of zygospores has been followed in several species of Mucorales and these details follow. In heterothallic fungi, presence of hyphae of each of the two mating types induces the formation of specialised sites called gametangia.

Different pheromones are produced by each mating type. The gametangia grow towards one another and the tips fuse (plasmogamy). Specialised cells at the tips, called progametangia, are walled off and the fusion zone dissolves. The resulting gametangium becomes the zygosporangium and the attached hyphae, the suspensors. Haploid nuclei in the zygosporangium fuse (karyogamy) and a wall forms around the now diploid zygospore inside the zygosporangium. The zygosporangium wall thickens and often becomes ornamented and melanised. Meiosis takes place prior to germination inside the zygospore. The germ tube is always haploid.

Homothallic fungi are also known in the Mucorales. In these fungi, both mating types are found in one mycelium and the isolates are self-mating.

Germination of zygospores is nearly always preceded by a period of dormancy. It is assumed that the sexual spore is a structure that enables long-term survival of the fungi.

Sexual reproduction has not yet been demonstrated in the Trichomycetes, though several genera are known the form thick-walled bi-conical spores following conjugation.

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Further Reading

Alexopoulos, Mimms & Blackwell 1996 Introductory Mycology (4th edition), Ch 5/6.

Carlile & Watkinson, 1994 The Fungi , pp 34 - 39.

Ingold & Hudson 1993 The Biology of Fungi pp 25 - 40.

Kendrick B. 1992 The Fifth Kingdom (2nd edition) Ch 3.

Walker, 1996 Fungi of Australia , Vol 1a, pp 1 - 170.

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