untitled

2 hyphae that typicallylack cross walls, so all the

nuclei are contained within a common cytoplasm (a

coenocytic mycelium);

3 the production of a thick-walled resting spore – the

zygospore– which if formed by a sexual process

involving the fusion of two gametangia;

4 the production of asexual sporesby cytoplasmic

cleavage within a sporangium;

5 a haploid genome.

Ecology and significance

The Zygomycota contains a diverse range of species,

with different types of behaviour, but for convenience

we start with the most familiar examples in the

Order Mucorales. This includes several common

species of the genera Mucor, Rhizopus, Phycomycesand

Thermomucor(a thermophilic fungus that can grow up

to about 60°C). All these fungi grow mainly as sapro-

trophs in soil, on animal dung, on composts in the case

of thermophilic species, or on various other substrates

such as over-ripe fruits. They grow rapidly, often cover-

ing an agar plate in 24 –36 hours, and they are among

the commonest fungi found on soil dilution plates. They

have been termed “sugar fungi”, to denote the fact that

they often depend on simple, soluble carbon nutrients.

However, some fruit-rotting species such as Rhizopus

stoloniferproduce pectic enzymes that can rapidly

liquefy apples and soft fruits (see Fig. 14.6). In addi-

tion, several species of Mortierelladegrade chitin by

producing chitinases (as also do several chytrids).

Chitin is a common and important polymer, because

it is a major component of fungal walls and of the

exoskeletons of insects and other invertebrates.

DIVERSITY OF FUNGI 23

Hypomyces

Neurospora

Ophiostoma

Blumeria

Eremascus

Eurotium

Morchella

Saccharomyces

Coprinus

Russula

Tremella

Tilletia

Gigaspora margarita

Scutellospora

Acaulospora (A)

Entrophospora

Glomus mosseae

Glomus intraradices

Glomus versiforme

Glomus etunicatum

Geosiphon pyriforme

A. gerdemannii

A. trappei

Glomus occultum

Mortierella

Endogone

Diphanoeca

Ichthyophanus

Dermocystis

400

90

290

240

400

460

0.01 substitutions/site

Zygomycota

and

outgroups

Glomeromycota

Basidio-mycota

Ascomycota

d e c f b a

Fig. 2.7A phylogenetic tree based on small subunit ribosomal RNA gene sequences. Triangle a indicates fossil spores

of the Ordovician (460 mya). Triangle b indicates fossil spores of presumed arbuscular mycorrhizal (AM) fungi from the

Rhynie chert (400 mya). Triangle c indicates a fossil clamp connection (Chapter 4) – the earliest evidence of

Basidiomycota (290 mya). Triangle d indicates Ascomycota from the Rhynie chert (400 mya). Triangle e indicates a gilled

mushroom in amber (90 mya). Triangle f indicates AM fungi of the Gigasporatype in the Triassic (about 240 mya). As

reference points, the “outgroups” are Diphanoeca(a choanoflagellate from which fungi are thought to have arisen),

and two early divergent forms of animals, Ichthyophanusand Dermocystis. (Courtesy of Redecker et al. 2000.)

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