Introduction to Fungi, Third Edition

EXAMPLES OF MUCORALES
197
of the ungerminated sporangiolum consisted of
at least two layers, the outermost layer relatively
thin, enclosing a distinct thicker inner layer. On
germination only the inner layer extends as a
germ tube.
The zygospores of Cunninghamella resemble
those of Mucor.
7.3.8 Mortierellaceae
The distinctive feature of this family is that the
sporangiophore produces only a rudimentary
columella or lacks it altogether. In the most
frequently encountered genus, Mortierella, zygospores
are often heterogametangic and may be
naked or enclosed in a weft of mycelium. The
family, which includes about seven genera, has
been monographed by Zycha et al. (1969). DNA
sequence analyses indicate relationships to
certain genera usually placed in Mucoraceae
(O’Donnell et al., 2001; Tanabe et al., 2004;
see Fig. 7.1), although many authors regard
Mortierella and its allies as a separate order,
Mortierellales.
Mortierella
About 90 species of Mortierella are known mainly
from soil, the rhizosphere, and plant or animal
remains in contact with soil (Gams, 1977;
Domsch et al., 1980). These fungi can be isolated
readily on nutrient-poor media which prevent
the growth of more vigorous moulds. Many
species are psychrophilic and may comprise the
bulk of fungal isolates from soil if the isolation
media are incubated near 0°C (Carreiro & Koske,
1992). Mortierella wolfii is associated with mycotic
abortion in cattle and can be isolated from the
placenta and foetal stomach contents and from
liver. In nature it grows in warm soils, overheated
silage and rotten hay and can grow well
at 40 42°C (Austwick, 1976; Domsch et al., 1980).
Certain species of Mortierella, e.g. M. alpina, have
been used in fermentations as catalysts of
biotransformations in the production of pharmaceuticals
(Kieslich, 1997). Another focus
of biotechnological interest is their accumulation
of lipid, notably polyunsaturated fatty
acids (PUFAs) which are of nutritional value
(Dyal & Narine, 2005). These are also produced
by thraustochytrids (see p. 73). The genus
Mortierella is polyphyletic, and many of the
best-known species, including M. isabellina,
M. ramanniana and M. vinacea, are now placed
in other genera such as Micromucor or Umbelopsis
(Meyer & Gams, 2003).
The mycelium of most species of Mortierella
is fine and, in agar culture, often shows a
characteristic series of fan-like zones. Cultures
frequently have a garlic-like odour. The sporangia
are borne on branched or unbranched tapering
sporangiophores (Fig. 7.30a). The sporangium wall
is delicate and may collapse around the spores.
There is no protruding columella (Fig. 7.30b).
Frequently the entire sporangium is detached. In
a number of species, and also dependent upon
environmental conditions, there may be only
one or a few spores per sporangium (Figs. 7.30c,d).
Asexual reproduction may also include the
formation of sessile, intercalary chlamydospores
which are not dispersed but remain in
the soil when their subtending mycelium
breaks down (Fig. 7.30e). Stylospores are also
produced; unfortunately this term has been used
for two different, non-homologous structures. In
its original application by van Tieghem, it
referred to aerial chlamydospores, i.e. relatively
thick-walled, stalked spores as seen, for example,
in M. polycephala (Domsch et al., 1980). In other
species, classified in the section Stylospora, e.g. M.
humilis and M. zonata (Gams, 1977), single-spored
sporangiola (Fig. 7.31a) have been termed stylospores.
On detachment of the sporangiolum, the
remnants of the sporangiolum wall can often be
seen at the tip of the sporangiophore (Domsch
et al., 1980). In some species, e.g. M. stylospora and
M. zonata, only sporangiola are present and true
sporangia are lacking. Mortierella chlamydospora
also lacks true sporangia, reproducing asexually
by intercalary smooth or stalked echinulate
chlamydospores and sexually by zygospores
(Ansell & Young, 1982).
The zygospores of Mortierella spp. may be
naked or surrounded by a partial or complete
investment of sterile hyphae (see Figs. 7.31b,c).
Of the 90 species, 26 are known to form zygospores,
half of which are homothallic (Watanabe
et al., 2001). It is likely that the majority of the
remaining species will prove to be heterothallic.