Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
173
The sporangial wall is sometimes colourless
or yellow, but it often darkens and develops a
spiny surface due to the formation of crystals of
calcium oxalate dihydrate (weddellite) beneath
the surface layer (Fig. 7.14c; Jones et al., 1976;
Urbanus et al., 1978; Whitney & Arnott, 1986). In
some species similar crystals may develop on the
sporangiophore. A possible function ascribed to
these structures is that they form a barrier
against grazing arthropods.
Despite the apparent similarity in sporangial
structure across members of the Mucorales,
spore liberation may be brought about by two
different mechanisms (Ingold & Zoberi, 1963;
Zoberi, 1985). In many of the commonest species
of Mucor (e.g. M. hiemalis), the sporangium wall
dissolves and the sporangium becomes converted
at maturity into a ‘sporangial drop’ adhering to
the columella. Sporangial walls which dissolve in
this way are said to be diffluent. In large
sporangia, for example of M. plasmaticus, M.
mucedo and Phycomyces, the spores are embedded
in mucilage. The sporangial wall does not break
open spontaneously, but the slimy contents
exude when the wall is touched. Such sticky
spore masses are distributed by insects or rain
splash, or by wind after drying. In the second
spore liberation mechanism, the sporangial wall
breaks into pieces, and here air currents or
mechanical agitation readily liberate spores. An
example of this is Mucor plumbeus (Figs. 7.14c,d).
In M. plumbeus the columella terminates in
one or more finger-like or spiny projections
(Fig. 7.14d), and in some Absidia spp. the
columella may also bear a single nipple-like
projection (Fig. 7.17b). In Rhizopus stolonifer the
columella is large, and as the sporangium dries
the columella collapses so that it appears like a
basin balanced at the end of the sporangiophore
(Fig. 7.16d). Associated with these changes in
columella shape, the sporangium wall breaks up
into many fragments and the dry spores can
escape in air currents.
7.2.5 Sexual reproduction
The Mucorales reproduce sexually by a process of
gametangial conjugation resulting in the formation
of zygospores. By strict definition, what we
describe as a zygospore is actually a zygosporangium,
the dark warty ornamentation representing
its outer wall (Benny et al., 2001).
According to this definition the zygosporangium
contains a single globose zygospore, sometimes
referred to as the zygospore proper. For convenience,
we continue to use the term ‘zygospore’
in a wide sense.
Some species are homothallic, zygospores
being formed in cultures derived from a
single sporangiospore (e.g. Rhizopus sexualis,
Syzygites megalocarpus, Zygorhynchus moelleri and
Absidia spinosa). However, the majority of species
are heterothallic and only form zygospores
when compatible strains are mated together.
It is believed that homothallic species are derived
from heterothallic ancestors (O’Donnell et al.,
2001). There is, in reality, no absolute distinction
between the homothallic and heterothallic
conditions because some species normally
homothallic or heterothallic are ambivalent, i.e.
they can change their mating behaviour under
certain conditions (Schipper & Stalpers, 1980).
Zygospore formation is affected by environmental
conditions, being generally favoured by
darkness (Hesseltine & Rogers, 1987; Schipper,
1987). The effects of temperature are variable. In
Mucor piriformis lower temperatures (0 15°C,
optimum 10°C) favour zygospore formation,
whilst for Choanephora cucurbitarum the optimum
is 20°C (Michailides et al., 1997).
In heterothallic species, if the appropriate
strains are inoculated at opposite sides of a Petri
dish, the mycelia grow out and a line of
zygospores develops where they meet (Fig. 7.8).
The two compatible strains rarely differ from
each other in any obvious morphological or
physiological features, although there may be
slight differences in growth rate and carotenoid
content. Because it was not possible to designate
one strain as male and the other as female,
Blakeslee (1906) labelled them (þ) and ( ).
The two compatible strains are said to differ in
mating type. The morphological events preceding
zygospore formation are sufficiently similar
to allow a general description of the process.
When two compatible strains approach each
other, three reactions can be distinguished.