A generalized life cycle of these fungi is shown in
Fig. 2.8. After a phase of rapid somatic (vegetative)
growth, they produce erect aerial branches (sporan-
giophores) behind the colony margin, then a thin-
walled sporangium is formed at the tip and the
sporangial contents are cleaved around the individual
nuclei to produce many uninucleate spores (sporan-
giospores). These spores are darkly pigmented, with
melanin in their walls. They are released and usually
wind-dispersed by breakdown of the thin sporangial
wall, but in some species they are water-dispersed.
The tip of the sporangiophore projects into the
sporangium as a columella, which remains after the
spores have been shed (Fig. 2.8). Sporangiospores can
germinate rapidly in suitable conditions, repeating the
asexual cycle.
Within the Zygomycota there are several variations
in the ways that sporangia and sporangiospores are
produced. Several species produce sporangia at the
tips of long, unbranched sporangiophores (Fig. 2.9a)
but the thermophilic species often produce branched
sporangiophores (Fig. 2.10b). Thamnidium elegansis a
psychrophilic (cold-loving) species which can grow on
meat in cold-storage, and at one stage was patented for
tenderizing steak. It produces a large sporangium at thetip of a long sporangiophore, but the sporangiophore
also produces complex branching structures that ter-
minate in many few-spored sporangioles (Fig. 2.9b).
Other variations are found in different Orders and
Families of the Zygomycota (Fig. 2.11). In this respect
it is important to recognize that the spore-bearing
structures of fungi have functional significance – their
role is to maximize the chances of dispersing spores to
sites where the fungus can establish new colonies. We
cover this topic in detail in Chapter 10.
Many members of the Mucorales also have a sexual
stage. In appropriate culture conditions aerial branches,
termed zygophores, grow towards one another and
swell at their tips to produce progametangia(Fig. 2.8).
Then the progametangia develop a complete septum,
which cuts off each gametangiumfrom the subtend-
ing hypha. The gametangia then fuse to form a single
cell that develops into a zygospore– a thick-walled,
ornamented resting spore. Coupled with this, the ter-
minal parts of the zygophores can swell, to produce
bulbous suspensors (Figs. 2.8; 2.12). Zygospores exhibit
a period of constitutive dormancy, but eventually
germinate to produce a sporangium, releasing haploid
spores. Many species in the Mucorales are heterothallic
(outcrossing) and develop zygospores only when24 CHAPTER 2
Fig. 2.8General life cycle of Mucorales. Asexual reproduction occurs by the production of sporangia which release
haploid spores. Sexual reproduction occurs by the fusion of gametangia at the tips of zygophores, leading to a diploid
zygospore that undergoes meiosis to release haploid spores.FB4eC02 04/20/2005 02:52PM Page 24