3.6 Zygomycota

The members of this group generally have a multinucleate mycelium with little or no septation (= coenocytic) that produces asexual spores in a mitosporangium (= a sporangium in which spores are produced by mitosis). But the characteristic feature of the Zygomycota (informally, zygomycetes) is that they form sexual spores in a zygosporangium.

The name of the phylum is derived from the way in which they reproduce sexually. Zygos is Greek for 'joining' or 'a yoke' (the wooden crosspiece bound to the necks of a pair of oxen or horses that is then connected to the load they are pulling). So Zygomycota reproduce through the fusion or conjugation of two hyphal branches (=  gametangia) to form a zygote which eventually gives rise to the zygosporangium when it produces spores.

The gametangia arise from hyphae of a single mycelium in homothallic species, or from different but sexually compatible mycelia in heterothallic species. Zygosporangia usually have thickened walls and act as resting spores; they are notoriously difficult to germinate in the laboratory. The zygospore wall contains sporopollenin, which is a complex, highly cross-linked polymer also found in the exine (outer wall layer) of pollen grains. Its presence, together with melanin, helps to explain the long-lived nature of zygospores. Long-lived resistant spores spread the organism in time, to survive periodic adverse environmental conditions; while non-dormant spores (dispersal spores) spread the organism in space, to colonise new territory.

These fungi are ecologically very diverse, very widely distributed and very common, though frequently overlooked. CLICK HERE to see a page of illustrations.

Clearly, Zygomycota is a primitive and early diverging lineage of Kingdom Fungi; primitive features being that they have coenocytic aseptate hyphae for all or part of their life cycle and they are unable to make tissues or complex fruiting structures. They may be primitive, but they are certainly successful and you are never far away from a member of this group. In the traditional classification some organisms were included that are more properly placed with the slime moulds (e.g. Amoebidiales and Eccrinales) protists that separated from the true fungi when animals and fungi diverged from the opisthokont lineage. The problem with the traditional ‘Phylum Zygomycota’ is that it is polyphyletic and cannot retain rank as a natural phylum. In the 2007 classification (Hibbett et al., 2007) the name remains undefined leaving the following subphyla in an uncertain position:

  • Mucoromycotina (containing the Orders Mucorales, Endogonales and Mortierellales);
  • Entomophthoromycotina (Order Entomophthorales);
  • Zoopagomycotina (Order Zoopagales);
  • Kickxellomycotina (Orders Kickxellales, Dimargaritales, Harpellales, and Asellariales).

When relationships among the constituent fungal lineages are resolved the name Zygomycota could be resurrected and validated, perhaps including subphylum Mucoromycotina, which is the core group of the traditional Zygomycota.

Most species are saprotrophic, in soil and dung (Mucorales, Mortierellales and Kickxellales). Some (Mucorales) are extremely fast growing in culture and can often be found on mouldy bread or fruit. The Zoopagales and Entomophthorales are parasites of small animals, including arthropods, rotifers and even amoebae. The Dimargaritales are obligate mycoparasites, and members of the Zoopagales and Mucorales may also parasitise other fungi (including mushrooms and other zygomycetes).

The Harpellales and Asellariales (together called Trichomycetes) are specialised for attachment the gut wall of arthropods and are obligate endosymbionts, so widespread that just about every arthropod that crawls past you will carry these fungi within. The Endogonales are either ectomycorrhizal on plant roots (remember the Glomeromycota are endomycorrhizal) or are saprotrophic. CLICK HERE to see a page of illustrations.

Asexual spores in the Zygomycota are single-celled and include mitosporic sporangiospores (made within the sporangium by internal cleavage of sporangial cytoplasm (as described above for Blastocladiella) and true conidia (which are borne on specialised hyphae called conidiophores). Sporangiospores are dispersed by wind or animals after rupture of the sporangium wall. Conidia are formed by the Entomophthorales and are forcibly discharged.

Names to look out for include Mucor, a typical filamentous mould that occurs as a saprotroph in soil, and on decaying fruits and vegetables. It is found everywhere in nature. Some Mucor spp. cause diseases (called zygomycosis) in man, frogs, amphibians, cattle, and pigs. Strains isolated from human infections are unusually thermotolerant, most other strains being unable to grow at 37°C. Species names to look out for are Mucor hiemalis, M. amphibiorum, M. circinelloides, and M. racemosus.

The genus Rhizopus is characterised by very fast growing and coarse hyphae and colonies spread by means of creeping aerial hyphae known as stolons that branch into pigmented root-like rhizoids when they contact the substratum. Sporangiophores, singly or in groups, arise from nodes directly above the rhizoids. The fungus is common on decaying fruits, and in soil and house dust, and can be a pest in microbiology laboratories because their rapid growth and dry, easily air-distributed spores enable them to take over culture media in a few days.

Rhizopus oligosporous is used to transform soy bean paste into the fermented food called tempeh in some Asian countries. However, it also produces the alkaloid agroclavine which is toxic to humans. R. oryzae (= R. arrhizus) is the most common causative agent of zygomycosis, accounting for some 60% of cases; it is a serious (and often fatal) opportunistic infection in immunosuppressed patients (in an opportunistic infection a vigorous saprotrophic organism becomes a disease organism by taking advantage of a weakened host).

Rhizopus rot is a soft rot of harvested or over-ripe stone fruits, such as peaches, nectarines, sweet cherries, and plums; it can be caused by several species including  R. nigricans and R. stolonifer.

Phycomyces blakesleeanus is another filamentous fungus in the order Mucorales; this one gets its fame from the amount of basic research done with it over the years. Phycomyces produces numerous tall sporangiophores that are very sensitive to light. An enormous amount of research has been directed to investigate phototropism, and then other environmental sensitivities like gravitropism. Spore germination, carotene biosynthesis and sexual development have all been studied in depth and genetic analysis carried out. It may be worth mentioning that the non-septate, and therefore unicellular, sporangiophores of P. blakesleeanus are more than 100 μm wide (= 0.1 mm) and can grow to a height of 60 cm or more. Another remarkable fungus!

Basidiobolus is usually considered to be a member of the Entomophthorales (although some phylogenetic studies have grouped Basidiobolus with Chytridiomycota), but in the AFTOL study Basidiobolus spp. emerged in a novel position, separate from the majority of chytrids and Entomophthorales (White et al., 2006). Basidiobolus is isolated from the dung of amphibians, reptiles, and insectivorous bats, as well as wood lice, plant debris and soil. It is cosmopolitan, but human infections due to Basidiobolus are reported mostly from Africa, South America, and tropical Asia. Isolates pathogenic to humans belong to a unique species, Basidiobolus ranarum. The real claims to fame, though, come from the size of the mycelial cells - several hundred µm long (the apical compartment can be 300 to 400 µm long - and the fact that the cells have a single large nucleus which may be more than 25 µm in length (a yeast cell is only about 5 µm long). The nucleus undergoes a mitosis that is readily visible by conventional light microscopy and this feature has been exploited to study how inhibitors of mitosis work. This is a good example of using a very unusual fungus to study a particular biological problem. Importantly, there are many unusual fungi around for such research!

In addition, Basidiobolus has remarkable conidia which are shot explosively from the mycelium using what’s been described as a ‘rocket mechanism’ – the conidium remains intact and attached to the upper part of the conidiophore. Basidiobolus appears ideal for studies of the cell cycle, mitosis, and explosive spore dispersal.

The genus Cunninghamella is also commonly found in soil and plant material, particularly in Mediterranean and subtropical zones. It has also been recovered from animal material, cheese, and Brazil nuts. Cunninghamella bertholletiae, C. elegans, and C. echinulata are the most common species, C. bertholletiae is the only known human and animal pathogen - another opportunistic fungus that may cause infections in immunocompromised hosts.

Mortierella species are also common soil fungi and the genus contains over 70 recognised species, however, M. wolfii is probably the only (again, opportunistic) pathogen of man and other animals.

The words ‘entomo + phthora’ mean ‘insect destroyer’ and Entomophthora is the generic name for fungi that attack and kill flies and many other insects with two wings (= Diptera). Entomophthora muscae, attacks houseflies. The fungus grows inside the body of the insect. The mycelium of the fungus grows into the region of the fly’s brain that controls behaviour, inducing the fly to land on a nearby surface and crawl up as high as possible. Eventually the hyphae of the fungus grow throughout the body of the fly and it dies. The mycelium then proliferates to the point where it pushes the abdominal segments apart and bursts through to give the fly a banded appearance - the bands being tight columns of cells that produce and shoot off the spores that eventually form a white halo encircling the body of the dead fly. Waiting to infect the next victim.

Updated December 16, 2016