Introduction to Fungi, Third Edition

176 ZYGOMYCOTA
Phycomyces, after arrest of the growth of vegetative
hyphae, certain submerged hyphal tips
develop short branches called ‘knobbly knots’
(Fig. 7.18) which break through the surface of the
agar and become progametangia (O’Donnell
et al., 1976; Sutter, 1987). The progametangia
become tightly appressed and their close contact
is enhanced by the formation of extracellular
fimbriae whose presence appears to be essential
for further development in Phycomyces (Yamakazi
& Ootaki, 1996) as well as in other groups of
fungi (see p. 652). Fimbriae may be the lectinbearing
structures. In other Mucorales the
zygophores are aerial and club-shaped. The tip
of each progametangium becomes cut off by a
septum to separate a distal multinucleate gametangium
from the subterminal suspensor (see
Fig. 7.10).
The walls separating the two gametangia
break down so that the numerous nuclei from
each cell become surrounded by a common
cytoplasm. The fusion cell, or zygote, swells and
develops a dark warty outer layer to become the
zygospore.
Cytology of zygospore formation
There have been numerous accounts of the
cytology of zygospore formation. Meiosis usually
occurs before zygospore germination, so that the
zygospore can be regarded as a meiosporangium.
Four main types of nuclear behaviour can be
distinguished.
(1) In Mucor hiemalis, Absidia spinosa and some
other species, all the nuclei fuse in pairs within a
few days, then quickly undergo meiosis so that
the mature zygospore contains only haploid
nuclei.
(2) In Rhizopus stolonifer and Absidia glauca,
some of the nuclei entering the zygospore do not
pair, but degenerate. The remainder fuse in
pairs, but meiosis is delayed until germination
of the zygospore.
(3) In Phycomyces blakesleeanus, the haploid
nuclei continue to divide mitotically in the
young zygospore and then become associated in
groups, with occasional single nuclei also
present. Before germination some of the nuclei
pair up, and in the germ sporangium diploid
nuclei and also haploid nuclei are found; some of
these may be products of meiosis, others may
represent the scattered solitary nuclei which
failed to pair up.
(4) In Syzygites megalocarpus mitotic nuclear
divisions continue in the young zygospore, but
nuclear fusion and meiosis apparently do not
occur. This fungus can therefore be described as
amictic (Burnett, 1965).
The fine structure of zygospore development
has been studied in the homothallic Rhizopus
sexualis (Hawker & Beckett, 1971; Ho & Chen,
1998). Following contact of the tips of the two
zygophores, their walls adhere to each other and
become flattened (Figs. 7.10a c) to form the
fusion septum. On either side of the fusion
septum, each cell becomes distended to form
a progametangium. In each progametangium,
an oblique septum, concave to the developing
zygospore, develops by gradual inward extension
mediated by the coalescence of vesicles. When
the septum is complete, it separates the terminal
gametangium from the progametangial base
now called the suspensor. However, cytoplasmic
continuity between the suspensor and the
gametangium persists through a series of pores
which probably enable nutrients to flow into the
developing zygospore from the surrounding
mycelium. Numerous nuclei congregate on
either side of the fusion septum. It has been
estimated that there may be over 150 nuclei in
a pair of progametangia, but the number
may rise to over 300 in a pair of completely
delimited gametangia, reflecting further nuclear
divisions.
The breakdown of the fusion septum is
associated with an accumulation of vesicles in
the vicinity of the dissolving wall. These may
contain wall-degrading enzymes. The fusion
septum is completely dissolved, and once the
cytoplasmic contents of the two gametangia are
continuous, the nuclei become arranged in the
periphery of the cytoplasm. In R. sexualis, itis
probable that most of the gametangial nuclei
fuse in pairs immediately, and that the fusion
nuclei then quickly divide.
Even before dissolution of the fusion wall is
complete, the primary outer wall of the zygote
thickens, and beneath this original wall, the
warts (which will eventually ornament the wall