Introduction to Fungi, Third Edition

7
Zygomycota
7.1 Introduction
The phylum Zygomycota comprises the first
group of fungi considered in this book which
lacks any motile stage. Asexual reproduction is
by spores which are called aplanospores because
they are non-motile, and sporangiospores
because they are typically contained within
sporangia. They are dispersed passively by wind,
insects and rain splash, although violent liberation
of entire sporangia (e.g. Pilobolus) or individual
spores (e.g. Basidiobolus, Entomophthora) can
also occur. Sexual reproduction is by gametangial
copulation which is typically isogamous and
results in the formation of a zygospore. The
mycelial organization is coenocytic, and the
cell wall contains chitin and its deacetylated
derivative, chitosan (Bartnicki-Garcia, 1968, 1987;
see Fig. 1.5). As in the Chytridio-, Asco- and
Basidiomycota, the mitochondria possess lamellate
cristae, and the Golgi system is reduced to
single cisternae. Lysine is synthesized by the
a-aminoadipic acid (AAA) route, as it appears to
be in all Eumycota.
General accounts of the Zygomycota have
been given by Benjamin (1979), Benny (2001) and
Benny et al. (2001). Molecular evidence indicates
that the group may have diverged from the
Chytridiomycota early in the history of terrestrial
life. The Zygomycota, in turn, probably
gave rise to the Asco- and Basidiomycota, i.e.
the ‘higher fungi’ (Jensen et al., 1998; Schüssler
et al., 2001). Two classes are included in the
Zygomycota, namely Zygomycetes comprising
870 species in 10 orders, and Trichomycetes
with 218 species in 3 orders (Kirk et al., 2001).
The most prominent orders of the Zygomycetes
are the Mucorales, Entomophthorales and
Glomales. Mucorales are ubiquitous in soil and
dung mostly as saprotrophs, although a few are
parasitic on plants and animals. Entomophthorales
include a number of insect parasites, but
some saprotrophic forms also exist. Glomales
are mutualistic symbionts associated with
almost all kinds of terrestrial plants as arbuscular
and vesicular arbuscular mycorrhiza. Trichomycetes
are mostly commensal in the guts of
arthropods, e.g. millipedes and the larvae of
aquatic insects.
The Zygomycetes are almost certainly polyphyletic,
but the precise evolutionary relationships
within this class are still controversial
(O’Donnell et al., 2001; Schüssler et al., 2001;
Tanabe et al., 2004, 2005), and comparisons of
numerous representative organisms with several
different DNA sequences, e.g. genes encoding
ribosomal RNA, cytochrome oxidase or cytoskeletal
proteins, will be required before a satisfactory
natural arrangement can be found. A recent
phylogenetic scheme is presented in Fig. 7.1.
7.2 Zygomycetes: Mucorales
In most members of the Mucorales, numerous
spores are contained in globose sporangia borne
at the tips of aerial sporangiophores (Fig. 7.2).
Within the sporangium the spores may surround