Introduction to Fungi, Third Edition

166 ZYGOMYCOTA
Fig 7.1 Recent phylogenetic scheme of
the Zygomycota based on partial
sequences of the gene encoding a subunit
of RNA polymerase II.Orders discussed in
detail in this book include Mucorales
(Sections 7.2 7.3), Zo opagales
(Section 7.4), Entomophthorales
(Section 7.5) in the Zygomycetes, and
Harpellales (Section 7.7) in the
Trichomycetes.The Glomales (Section 7.6),
only distantly related to other
Zygomycota, were not included in this
analysis. Redrawn and modified from
Tanabe et al. (2004), with permission
from Elsevier.
a central core or columella, although in some
species (e.g. Mortierella spp.) the columella is
greatly reduced. Some species possess fewspored
sporangia, termed sporangiola, which
are often dispersed as a unit, and in some groups
the spores are arranged as a single row inside a
cylindrical sac termed a merosporangium. Yet
other Mucorales reproduce by means of unicellular
propagules which are sometimes termed
conidia, but Benjamin (1979), in his consideration
of asexual propagules formed in the
Zygomycota, has recommended the use of the
term ‘sporangiolum’ instead of ‘conidium’ in
this context. It is believed that ‘conidia’ may have
evolved several times within different groups of
Mucorales from forms with monosporous sporangiola.
A distinction between sporangiospores
and conidia is that germinating sporangiospores
lay down a new wall, continuous with the germ
tube, within their original spore wall, whilst
within germinating conidia there is no new wall
layer formed.
The Mucorales are mostly saprotrophic and
are abundant in soil, on dung and on other
organic matter in contact with the soil. They may
play an important role in the early colonization
of substrata. Sometimes, however, they can
behave as weak pathogens of soft plant tissues,
e.g. Rhizopus stolonifer can cause a rot of sweet
potatoes or fruits such as apples, tomatoes and
strawberries (Plate 3d). Such infections may
cause spoilage of food (Samson et al., 2002).
Some species are parasitic on other fungi, a
common example being Spinellus fusiger which
forms a tuft of sporangiophores on the caps of
moribund fruit bodies of Mycena spp. (Plate 3e).
Others cause diseases of animals including
man, especially patients suffering from diabetes,
leukaemia and cancer. Lesions may be localized
in the brain, lungs or other organs, or may be
disseminated, e.g. at various points in the
vascular system (Kwon-Chung & Bennett, 1992).
Species of Rhizopus and Mucor are reported from
human lesions, and these genera together with