Chapter 3b Zygomycota

The second eumycotan phylum is the Zygomycota. This phylum contains two classes, class Zygomycetes and class Trichomycetes. Since most Trichomycetes are parasites or commensals inside the guts of living arthropods, they are only a footnote, albeit a fascinating one, to this chapter.

Basic features Although the class Zygomycetes contains only about 1% of the known species of fungi, its members are distinctive, and some of them are common, successful, fast-growing, primary colonizers of substrates containing accessible carbon sources like sugar or starch. Others are specialized parasites.

Zygosporangia The name of the class is derived from the way in which they reproduce sexually by the physical blending - fusion or conjugation - of morphologically similar gametangia to form a zygosporangium (the teleomorphic phase). 'Zygos' is Greek for a yoke or joining. The gametangia arise from hyphae of a single mycelium in homothallic species, or from different but sexually compatible mycelia in heterothallic species. Zygosporangia usually develop thick walls, and act as resting spores.

Below, left, is a photograph of a petri dish containing nutritive agar medium. Two sexually compatible strains of Phycomyces blakesleeanus were inoculated on opposite sides of the plate (large dark spots). The mycelia spread across the agar surface. Where they met, a line of zygosporangia with spiny appendages formed.

The right-hand picture shows what these developing zygosporangia look like under the dissecting microscope.

The four diagrams below show how a zygosporangium develops.

When compatible mycelia of Phycomyces blakesleeanus meet, individual hyphae establish intimate contact, developing finger-like outgrowths and seeming to grapple with one another. This lets them exchange chemical signals which establish that they are indeed sexually compatible. Then the two hyphae grow apart again, only to loop back, swelling as they approach each other, and finally meeting head-on. They have become gametangia, which fuse when their tips touch. Note that there isn't any sexual differentiation in size or shape here: since we can't call them male and female, we simply label the mycelia '+' and '-'

After the walls between the two gametangial tips have broken down and their multinucleate contents have mixed, the mixture is quickly isolated by two septa, and the paired-off nuclei fuse. The structure is now called a zygosporangium, and it develops a thick and often ornamented wall, even while still supported on either side by the former gametangia, which are now called suspensors. Although the two suspensors are now just empty appendages, they make it easy to recognize a zygosporangium when you see one. Here's one from Rhizopus.

Anamorphs You won't often see zygosporangia in field collections, though I sometimes find a homothallic Syzygites producing them profusely on wild mushrooms. But asexual or anamorphic phases of zygomycetes are easy to find on mouldy bread or peaches, or on horse dung. A number of examples are illustrated below.

Collect some fresh horse dung, keep it in a damp chamber, and look at it through a dissecting microscope, or even a hand lens, every day. You should be able to follow a sequence of specialized coprophilous fungi - and the first to develop will probably be the spectacular anamorph of Pilobolus, which is discussed below and in Chapter 11. The asexual mitospores are usually formed inside mitosporangia borne at the tips of specialized sporangiophores. Zygomycete cell walls are mainly of chitin and the nuclei in their vegetative hyphae are haploid. Now for a taxonomic survey of the phylum and its two classes.

I will introduce you to four of the orders: the Mucorales - Entomophthorales - Zoopagales - Kickxellales

1) Order Mucorales 13 families, 56 genera, 300 species. This order includes all the common saprobic zygomycetes. Here belong the ubiquitous bread mould, Rhizopus stolonifer (below), and the equally common genus Mucor.

The classification outlined here has been built on studies of morphology, development and ecology. But now molecular biology is telling us that some of our assumptions are incorrect. For example, the Mucoraceae is now believed to be polyphyletic, as are the Thamnidiaceae, Chaetocladiaceae and Radiomycetaceae. Not only that, but some of the genera, such as Mucor, Absidia and Backusella are also polyphyletic, with their species distributed among different clades. But since the new classification has not yet appeared in a complete form, I will present a version of the extant classification, with the warning that it will eventually undergo major revision.

The globose mitosporangia of the Mucoraceae are produced at the ends of tall, stout, simple or branched hyphae called sporangiophores (below). The sporangia often look dark when mature (below, right). A common name for this group is 'pin moulds.'

Each sporangium contains hundreds of non-motile, asexual spores (SEM - left) within a delicate outer membrane called a peridium.

The trade-mark of the family Mucoraceae, as recognized until very recently, is a swollen extension of the sporangiophore called a columella, which protrudes like a balloon into the sporangium (left).

The columella often persists after the delicate outer skin or peridium of the sporangium has disappeared and the sporangiospores have been dispersed.

It persists in a collapsed condition in
Rhizopus (right).

Spinellus is a parasitic member of the Mucoraceae which attacks agarics, especially species of Mycena.

Other families of the Mucorales often have fewer spores per sporangium, and their sporangia often have no columella.

Thamnidium elegans (Thamnidiaceae) (left), seems to compromise. Its tall sporangiophores have one large, terminal, columellate sporangium, but lower on the stalk there are branches which fork repeatedly in a dichotomous manner, the final branchlets ending in tiny mitosporangia (sporangioles) which contain only a few spores.

The reductionist tendency is also evident in Helicostylum (Thamnidiaceae) (left), which has 10-20 spores per sporangium, and in Blakeslea trispora (Choanephoraceae) (right), which has only three spores per sporangium.

This trend reaches its logical conclusion in Cunninghamella (Cunninghamellaceae) (left), which has only one spore per mitosporangium, and in which the walls of spore and sporangium appear to have fused. Now the whole mitosporangium becomes detached and acts as a dispersal unit.

Although zygomycetes can go through cycle after cycle - spore, mycelium, sporangium, spore - producing only the anamorph, they sometimes form sexual zygosporangia (the teleomorph), perhaps as a survival mechanism, perhaps for the benefits of genetic recombination, or perhaps just because compatible strains have met. The photo (right) shows both teleomorph and anamorph of Phycomyces.

The anamorph-teleomorph alternation is diagrammed below for one of the commonest and most successful members of the Mucorales, Rhizopus stolonifer. Note that when the zygosporangium germinates, it produces a mitosporangium.

Zygosporangia vary in minor ways from one genus to another, and among families and orders, but they are generally rather similar: if they are present, they are the easiest way to tell if a fungus is a zygomycete.

By contrast, the anamorphs of zygomycetes -- mitosporangia and the structures on which they are borne -- have evolved some amazing and bizarre adaptations. This contrast between teleomorphic constancy and anamorphic diversity is presumably the result of differing evolutionary pressures. Long-term survival, one of the main objectives of the teleomorph, is presumably best ensured by structures with minimal surface area and thick, protective walls. Dispersal, the main purpose of anamorphs, can be achieved in many ways. Let's look at three of the more specialized zygomycetous anamorphs.

Pilobolus crystallinus (Pilobolaceae, below) is an atypical but fascinating coprophilous (dung-inhabiting) member of the order Mucorales. It grows very rapidly, and is one of the first fungi to fruit in the extended succession that occurs on dung (see Chapter 11). Its unbranched sporangiophores are 2-4 cm tall, and have a unique explosive dispersal mechanism.

Beneath the black apical mitosporangium is a lens-like subsporangial vesicle, with a light-sensitive `retina' at its base that controls the growth of the sporangiophore very precisely (above), aiming it accurately toward any light source. In a word, it is phototropic. Osmotically active compounds cause pressure in the sporangiophore and the subsporangial vesicle to build up until it is more than 100 pounds per square inch (7 kilograms per square centimetre). This eventually causes the vesicle to explode, hurling the black sporangium away to a distance of up to 2 metres, directly toward the light. The mucilaginous contents of the subsporangial vesicle go with the sporangium, and glue it to whatever it lands on. Can you explain why Pilobolus needs such a specialized mechanism for spore dispersal: such a powerful cannon, so carefully aimed? You can find the answer in Chapter 11.

Note that the originality of Pilobolus extends only to the behaviour of its anamorph - the teleomorph (the zygosporangium, shown here), is fairly conventional.

Entomophthora muscae infects, and eventually kills, houseflies. Dying flies, their bodies riddled by the fungus, usually crawl into exposed situations -- I find them on windows, and on the growing tips of shrubs in my garden -- where the fungal infection bursts through the insects' exoskeleton and produces tightly-packed masses of sporangiophores (left and below).

Each sporangiophore bears one unicellular, sticky mitosporangium that is shot away at maturity. When the fly dies on a window, this barrage produces a whitish halo of mitosporangia on the glass. These sporangia can infect other unsuspecting flies that come to pay their last respects. As you may already have guessed, species of Entomophthora are being investigated for their potential in biological control of insect pests (see Chapter 14).

Note again that the zygosporangia of Entomophthora, though developing in an unusual way, by the fusion of hyphal bodies inside the fly, are still recognizable as zygosporangia.

3) Order Zoopagales parasites of fungi, nematodes, amoebae, etc.
Many taxa produce merosporangia (sporangia that break up at maturity, looking rather like the thallic-arthric conidia of some ascomycetes and basidiomycetes).

Piptocephalis (right and below) is a parasite of Mucorales, and occurs commonly on dung.

Syncephalis (right) is another parasite of Mucorales. The photomicrograph shows a crown of young merosporangia arising from a vesicle at the apex of a tall sporangiophore.

4) Order Kickxellales (Named after a mycologist called Kickx). Members of this order are atypical of the Zygomycetes in that they often have regularly septate hyphae. Their teleomorphs are unremarkable, but they develop some of the most complex anamorphs known. I will show you one or two of these.

I have found Coemansia (left) on bat dung from a cave. Its tall sporangiophore bears many fertile side branches called sporocladia. Each of these produces a row of lateral cells called pseudophialides (true phialides are discussed in Chapter 4). Finally, from the apex of each pseudophialide arises an elongate, one-spored mitosporangium (a sporangiole).

That's complex enough, but it looks simple beside Spirodactylon (right).
This, surely the most elaborate of all zygomycetous anamorphs, grows on the dung of rats in Death Valley, California, and produces a tall, dichotomously branched sporangiophore (a,b) that is repeatedly thrown into tight coils (c). Within these coils arise the sporocladia (d), which bear pseudophialides (d), that in turn bear one-spored sporangioles (d).

It is hard to imagine why this strange configuration might have evolved, until one learns that the fungus grows on mouse and rat dung. Coprophilous fungi have various highly evolved strategies for getting back inside the gut of the animals that produce their preferred substrate. This isn't too difficult for genera like Pilobolus, that grow on herbivore dung, since all they have to do is get their spores onto the animal's food, which is all around. But rats and mice are not herbivores, and it is essentially impossible for the fungus to ensure that its spores will be present on their food. The only alternative (as I see it) is to attach spores to the animal itself, in the hope that they will be ingested during grooming activities. Rats and mice are creatures of habit, using well-trodden paths each day. Along these trails they deposit dung, and there, later, the coils of Spirodactylon become entangled in their hair. Only the zygosporangia of the Kickxellales convince us that these strange fungi are indeed zygomycetes.

Class Trichomycetes. 4 orders, 7 families, 52 genera, about 210 species.

Here is the footnote, as promised. This eccentric group of fungi live almost exclusively attached to the lining of the guts of living arthropods, which is why you won't run into them very often. But they are examples of the opportunism displayed by fungi, and the determination shown by mycologists in winkling out fungi wherever they are to be found. Bob Lichtwardt's 1986 book gives a fine account of this offbeat group, though 13 new genera and 79 new species of Trichomycetes have been described since it was published, bringing the total to 52 genera and 210 species.

Trichomycetes are common in herbivorous or detritivorous arthropods, but rare in predaceous species. A single species of trichomycete can usually inhabit several related host species. They often seem to be commensals, doing little or no harm to their hosts: sometimes they may even be associated with faster growth of their hosts. But at least one species, Smittium morbosum, causes the death of mosquito larvae.

The Class contains four Orders. Three of these, the Asellariales, Eccrinales and Harpellales, are closely related, and seem to have common ancestry. The fourth, the Amoebidiales, is an outlier.

These fungi are found attached to the gut lining of insects, crustacea and millipedes. M.M. White, a Canadian doing graduate studies at Kansas, has recently published the first report of a new genus of Harpellales in isopod Crustacea, of Trichomycetes in Trichoptera (Caddisflies), and of Eccrinales in Plecoptera (Stoneflies). These discoveries indicate that there is much to be learned about the taxonomy and host range of this group.

This plate of phase-contrast photomicrographs by Richard Benjamin (from his chapter in 'The Whole Fungus' -- see reference below) shows the characteristic structures of some Trichomycetes. Top left (A) are developing trichospores of Smittium. Top right (B) are trichospores of Stachylina showing the hair-like appendages that give them their name. Bottom left (C) is a trichospore of Smittium. Bottom centre (D) is a developing zygosporangium of Trichozygospora, and bottom right (E) a released, mature zygosporangium with a collar and a bunch of hair-like appendages below it. Only this last structure (look at the conical "suspensors," top and bottom) places the Trichomycetes in phylum Zygomycota, since otherwise the group does not closely resemble the zygomycetes.

Here are some illustrations of Orphella catalaunica, a Trichomycete recently described from the hindgut of a stonefly nymph (Plecoptera, Insecta) in Spain.

1 mature thallus
2, 3 sporulating heads with trichospores
4,5 holdfast cells with branches
6,7 sporulating heads

from Santamaria, S. and J. Girbal (1998) Mycol. Res. 102: 174-178.

Class Glomeromycetes

The third eumycotan phylum is the very recently erected Glomeromycota. This new phylum has relatively few members (fewer than 200 species have been described) but it is of enormous importance in the biosphere. There is a single class, the Glomeromycetes.

These soil-inhabiting fungi were placed in the Zygomycota until very recently, albeit rather tentatively, since they do not reproduce sexually. Nevertheless, they are extremely important, because their hyphae enter the living root cells of perhaps 90% of all higher plants and establish with them obligate mutualistic symbioses called arbuscular mycorrhizae (AM) or endomycorrhizae. These are discussed in detail in Chapter 17.

The large, thick-walled spores of the five currently recognized endomycorrhizal genera are illustrated below (courtesy of Steve Bentivenga). Most mycologists now prefer to merge a sixth genus, Sclerocystis, (which was segregated on the basis of its many-spored fruiting structures) with Glomus.

Here are some thick-walled, lipid-filled spores of Glomus that have been extracted from the soil by repeated sieving.

AM fungi won't grow in axenic culture: they must be associated with a plant root. Their generally very large and thick-walled resting spores are common in most soils, and are stimulated to germinate by the proximity of plant roots (almost any plant will do, because these fungi have such wide host-ranges).

Their usually non-septate hyphae spread through the soil and enter living roots, where they develop structures that are diagnostic of the order: intracellular, finely branched, tree-like arbuscules (left) which are the interface across which the fungus exchanges mineral nutrients, especially phosphorus, for photosynthates (sugars, etc.) provided by the plant.

Many of the Glomeromycetes produce both arbuscules and lipid-filled structures called vesicles or intramatrical spores inside plant roots, as this photomicrograph of a root squash shows..

The soil-inhabiting mycelium is very efficient at mobilizing insoluble phosphorus and translocating (moving) it to the plant. Since phosphorus is often the limiting nutrient for plant growth, AM fungi help plants to thrive in poor soils. These fungi are therefore vital in many natural habitats, and of great potential value in agriculture. Again, for details consult Chapter 17.

Further Reading about Zygomycetes and Glomeromycetes

(N.B. As yet, there don't appear to be any good web sites I can point you to that deal exclusively with zygomycetes.)

Benjamin, R.K. (1959) The merosporangiferous Mucorales. Aliso 4:321-453.

Benjamin, R.K. (1979) Zygomycetes and their spores. pp. 573-621 (in) The Whole Fungus. Vol. 2. (Ed.) B. Kendrick. National Museums of Canada, Ottawa.

Bentivenga, S. P. (1998) Ecology and evolution of arbuscular mycorrhizal fungi. McIlvainea 13: 30-39.

Brundrett, M., L. Melville and L. Peterson (Eds.) (1994) Practical Methods in Mycorrhiza Research. Mycologue Publ., 8727 Lochside Dr.,Sidney, BC V8L 1M8, Canada.

Cerda-Olmedo, E. and E.D. Lepson (Eds.) (1987) Phycomyces. Cold Spring Harbor, N.Y.

Fuller, M.S. (Ed.) (1978) Lower Fungi in the Laboratory. Dept. of Botany, Univ. of Georgia, Athens.

Ingold, C.T. (1978) The Biology of Mucor and its Allies. Edward Arnold, London.

Kendrick, B. and S.M. Berch (1985) Mycorrhizae: applications in agriculture and forestry. pp. 109-152 (in) Comprehensive Biotechnology. Vol. 3. (Ed.) C. Robinson. Pergamon, Oxford.

Lichtwardt, R.W. (1986) The Trichomycetes. Springer-Verlag, New York.

Morton, J.B. and G.L. Benny (1990) Revised classification of arbuscular mycorrhizal fungi (Zygomycetes): a new order, Glomales, two new suborders, Glomineae and Gigasporineae, and two new families, Acaulosporaceae and Gigasporaceae, with an emendation of Glomaceae. Mycotaxon 37: 471-491.

O'Donnell, K.L. (1979) Zygomycetes in Culture. Department of Botany, University of Georgia, Athens

Schuessler, A., D. Schwarzott and C. Walker (2001) A new fungal phylum, the Glomeromycota: phylogeny and evolution. Mycol. Res. 105: 1413-1421.

White, M.M. (1999) Legerioides, a new genus of Harpellales in isopods and other Trichomycetes from New England, U.S.A. Mycologia 91: 1021-1030

Zycha, H., R. Siepmann and G. Linnemann (1969) Mucorales. Eine Beschreibung aller Gattungen und Arten dieser Pilzgruppe. Cramer, Lehre.