Introduction to Fungi, Third Edition

200 ZYGOMYCOTA
Fig 7.32 Mortierella umbellata, stages in zygospore
development (traced from Degawa & Tokumasu,1998).
(a) Developing progametangia with
micro-progametangium on the left. (b) Swelling of
progametangia. (c) Septum formation in
micro-progametangium, arrowed. (d) Septa have formed
in both progametangia to delimit the terminal gametangia
from sub-terminal suspensors. (e) The macrogametangium
and macrosuspensor have enlarged. (f) Mature
zygosporangium containing a thick-walled zygospore.
becomes converted into a subspherical zygosporangium
with a dimpled wall. The narrower progametangium
does not enlarge appreciably. It is
not divided by a septum and remains as a lateral
attachment to the zygosporangium (Ansell &
Young, 1983, 1988). Delimitation of the zygosporangium
by means of a single wall in only one of
the fusing progametangia occurs in several other
species of Mortierella (Kuhlman, 1972).
Mortierella capitata shows an unusual mode of
zygospore development (Degawa & Tokumasu,
1997). It is heterothallic and heterogametangic,
with two mating types designated A and B. When
compatible vegetative hyphae meet in culture,
their tips swell to form progametangia. The
hyphal tips from strain B are always larger
than those from A and are designated as macroprogametangia.
The narrower hyphae from
strain A (microprogametangia) coil around the
macroprogametangia, branch dichotomously
and become septate, resulting in the formation
of microsuspensors and microgametangia. A
septum divides the terminal macrogametangium
from its macrosuspensor. The macrogametangium
becomes the zygosporangium and eventually
contains a thick-walled hyaline zygospore.
The macrosuspensor elongates and persists so
that the mature zygosporangium appears at the
end of a long stalk surrounded at its base by the
coiled microsuspensors. Apart from the other
unusual features of this developmental process,
M. capitata is distinctive in that the morphology
of its gametangia is linked to mating type, i.e.
the formation of macrogametangia occurs only
in the B strain and microgametangia in the A
strain. This condition, termed morphological
heterothallism, is comparatively rare in
Zygomycota and in fungi generally.
Complete investment of the zygospore by
branching hyphae is a feature of M. rostafinskii
(see Figs. 7.31b,c) and M. ericetorum (Kuhlman,
1972). The zygospores proper in Mortierella are
hyaline with thick smooth walls, sometimes
showing coarse, undulating folds (see Fig.
7.31d). Little is known about the germination of
zygospores.
7.4 Zoopagales
The order Zoopagales contains soil- and dunginhabiting
parasites of fungi and small terrestrial
animals such as protozoa and nematodes.
Reproduction is by conidia, merosporangia
and zygospores. Benny et al. (2001) recognized