Introduction to Fungi, Third Edition

178 ZYGOMYCOTA
Fig 7.11 Development of the zygospore wall in Rhizopus sexualis (diagrammatic, after Hawker & Beckett,1971). (a) Primary wall
before inflation of zygospore, showing thin electron-dense outer layer, thicker less electron-dense inner one, and scattered
lomasome-like bodies. (b) Blocks of secondary material (wart initials) developing locally on inner surface of primary wall. (c) Wart
initials growing by deposition of secondary material at the rims to give saucer-shapedpigmentedmasses. (d) Warts becoming flower
pot shaped by further growth at rims, inner layer of primary wall becoming gelatinous and swollen. Note pockets of cytoplasm
between warts. (e) Rims of warts nearly touching, inner layer of primary wall showing stress lines, pockets of cytoplasm between
warts much reduced. (f) Edges of warts touching, warts lined with tertiary smoothing layer, outer layer of primary wall torn.
(g) Thick stratified impermeable layer of quaternary material laid down inside smoothing layer, inner gelatinous layer of primary
wall has collapsed as a horny skin enveloping the warts.
and forked, whilst in Absidia they are hyaline
and coiled or curved inwards (see Figs. 7.17, 7.18).
The function of such appendages is unknown;
possibly they assist in attaching zygospores to
passing animals. The forked appendage tips
of Phycomyces bear a drop of liquid, and they
have been interpreted as hydathodes (i.e. watersecreting
structures). In the homothallic species
A. spinosa the appendages arise on only one
suspensor.
Mating behaviour
Analysis of the results of crosses involving several
genes suggest that there is a single mating type
locus with two alternative alleles, (þ) and ( ),
which segregate at meiosis. However, no DNA
sequences of this locus have as yet been
published, and there are also a number of
anomalous results for which a full cytological
explanation is still awaited.
Hybridization experiments have been
conducted between different species and genera
of Mucorales, and in some cases imperfect
zygospores are formed. Attempted copulation
has also been observed between homothallic and
heterothallic strains. An unusual type of mating
behaviour has been discovered in Mucor pusillus
which is predominantly heterothallic but in
which homothallic strains are known. It has
been possible to induce a (þ) strain to mutate
to a ( ) strain, and also to a homothallic strain
by g-irradiation (Nielsen, 1978).
7.2.6 Zygospore germination
After a resting period the zygospore may germinate
by developing a germ sporangium which
resembles an ordinary sporangium and contains
sporangiospores of the normal type. In some
cases vegetative mycelium develops from the
germinating zygospore. The conditions for zygospore
germination are, in many cases, imperfectly
known, but a protocol for germination
has been established for Phycomyces blakesleeanus
(Eslava & Alvarez, 1987). Mature zygospores