Introduction to Fungi, Third Edition

EXAMPLES OF MUCORALES
195
nodding sporangia is enhanced in culture by
growth at a temperature of 30°C and that of
sporangiola by a temperature of 26°C, with
mixed sporulation at 28°C (Tereshina &
Feofilova, 1995). The number of spores within
sporangiola is affected by nutrition, and when
grown on media with limiting nutrient content
the sporangiola may contain only a single spore,
thus resembling Choanephora. InB. unispora the
sporangiola generally contain only one spore,
rarely two. The sporangiola of B. trispora are
readily detached by wind and break open in
water like the two halves of a bivalve shell to
release the spores which are carried by insects
from one plant to another (Fig. 7.27b). Blakeslea
trispora is heterothallic and has brown striate
zygospores resembling those of Choanephora
(Mistry, 1977).
The Choanephoraceae have been the subject
of physiological investigations. An interesting
phenomenon observed in intra- and inter-specific
crosses is that the production of b-carotene is
markedly enhanced when (þ) and ( ) strains are
mated on liquid media, as compared with
production from either strain grown singly.
Commercial production of b-carotene and lycopene
from fermentations of mixed cultures of
(þ) and ( ) strains of B. trispora is possible (Mehta
et al., 2003). The discovery that b-carotene
production can be stimulated by an acid fraction
of culture filtrates from mixed cultures of
B. trispora led to the discovery of trisporic acid
as the sex hormone of Mucorales (see p. 173).
Syncephalastrum
Syncephalastrum racemosum (Fig. 7.28) can be
isolated from soil and dung in tropical and
subtropical areas (Domsch et al., 1980). It grows
rapidly in culture over a wide range of temperatures
(7 40°C) and is mainly saprotrophic, but
has been implicated in mucormycosis in human
and animal hosts. It has also been isolated from
foodstuff, cereal grains, other seeds and spices.
In culture it forms aerial branches terminating
in club-shaped or spherical vesicles. The vesicles
are multinucleate and bud out all over their
surface to form cylindrical outgrowths, the merosporangial
primordia. Into these outgrowths
one or perhaps several nuclei pass, and nuclear
division continues. The cytoplasm in the merosporangium
cleaves into a single row of 5 10
7.3.6 Syncephalastraceae
A characteristic feature of this family is that
asexual reproduction occurs by means of cylindrical
sporangia containing typically a single row
of sporangiospores. Such sporangia are termed
merosporangia and are formed in groups on
inflated vesicles (Benjamin, 1966). Merosporangia
appear to have evolved independently in the
Piptocephalidaceae (Zoopagales) (see p. 201).
There is only a single genus, Syncephalastrum, in
the Syncephalastraceae. DNA sequence analysis
indicates close relationships with certain genera
traditionally classified in Mucoraceae and
Thamnidiaceae (O’Donnell et al., 2001).
Fig 7.28 Syncephalastrum racemosum. (a) Sporangiophore
bearing a vesicle and numerous merosporangia.
(b) Merosporangia and merospores.