Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
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collected from 6-week-old cultures and placed on
moist filter paper at 22°C under alternating
light/dark illumination will germinate after
about 8 days, reaching maximum germination
after a further 8 10 days. In Mucor piriformis, the
germination rate is highest in fresh zygospores
(Guo & Michailides, 1998), a vigorous germ tube
emerging through one of the suspensors or
through a crack in the zygospore wall. The
germ tube may continue development as mycelium
or grow into the air and form a germ
sporangium at the tips of single or branched
sporangiophores.
Mating-types represented in germ sporangia
The distribution of mating types amongst the
germ spores which are present in germ sporangia
falls into three categories.
1. Pure germinations in which all the spores
are homothallic, e.g. in Mucor genevensis,
Zygorhynchus dangeardi and Syzygites megalocarpus.
2. Pure germinations in which all sporangiospores
are of one mating type, i.e. all (þ)
or all ( ). Mucor mucedo, M. hiemalis and
P. blakesleeanus generally behave in this way. In
P. blakesleeanus, the analysis of progeny from
crosses involving up to four unlinked factors
which included mating type were best explained
on the basis of the survival of a single diploid
nucleus from the thousands which are present
in the young zygospore (Cerdá-Olmedo, 1975;
Eslava et al., 1975a,b). This single diploid nucleus
undergoes meiosis and one or more of the
resultant nuclei divide mitotically to provide
nuclei for the germ sporangium (Fig. 7.12).
Occasionally two or three diploid nuclei may
survive and undergo meiosis. In some germ
sporangia heterokaryotic spores are present. If
these are heterokaryotic for mating type, the
mycelium which develops from them may be
abnormal and ‘neuter’, i.e. it is unable to mate
with (þ) as well as ( ) strains.
3. Mixed germinations. In Phycomyces nitens,
the same germ sporangium sometimes contains
(þ), ( ) and homothallic (i.e. self-fertile) spores.
The finding that diploid nuclei enter the
germ sporangia may be the explanation for the
presence of homothallic spores which should
properly be described as secondarily
homothallic. Mixed germinations have also
been reported by Gauger (1961) for Rhizopus
stolonifer in which both (þ) and ( ) spores were
present in some germ sporangia, whereas others
contained spores of either mating type. For this
type of mixed germination to occur, it would
be necessary only to postulate the survival of
more than one meiotic product so that both
mating types are represented in the sporangium.
‘Neuter’ spores were found in some germ
sporangia. Thus, in Choanephora cucurbitarum
mixed germinations have been reported in
which the majority (usually all) of the germ
sporangia contained only either (þ) or( ) spores,
but a low proportion gave heterokaryotic spores
of mating-type (þ/ ). A characteristic feature of
C. cucurbitarum cultures derived from heterokaryotic
(þ/ ) germ spores or fusion of (þ) with ( )
protoplasts is that they produce azygospores
(Yu & Ko, 1996, 1999; see below).
7.2.7 Azygospores
In some Mucorales, if gametangial copulation
fails to take place normally, one or both
gametangia may give rise parthenogenetically
to a structure morphologically similar to the
zygospore, termed an azygospore (azygosporangium).
Azygospores therefore usually appear as
warty spherical structures borne on a single
suspensor-like cell, or occasionally on a sporangiophore.
They are formed regularly in cultures
of Mucor bainieri and M. azygospora (Fig. 7.13), both
of which are obligately azygosporic and do not
form true zygospores (Benjamin & Mehrotra,
1963), and they have also been reported in
Rhizopus azygosporus (Yuan & Yong, 1984). The
development of azygospores of M. azygospora
resembles that of normal zygospores in other
Mucorales (O’Donnell et al., 1977b; Ginman &
Young, 1989). Azygospore formation may occur
in intergeneric and interspecific crosses, for
example in crosses between a (þ) strain of
Gilbertella persicaria and a ( ) strain of Rhizopus
stolonifer (O’Donnell et al., 1977c) and between
different species of Rhizopus (Schipper, 1987).
Azygospore development has also been seen
in intraspecific crosses, e.g. in certain isolates
of M. hiemalis (Gauger, 1966, 1975). These azygosporic
isolates of M. hiemalis were derived from