Introduction to Fungi, Third Edition

174 ZYGOMYCOTA
Fig 7.8 Phycomyces blakesleeanus.Maltagarplate5days
after inoculation with a (þ)anda( )strain.Alineof
black zygospores has been produced where the two
mycelia have met.
(1) A ‘telemorphotic reaction’ which involves
the formation of aerial (or occasionally
submerged) swollen hyphal tips. These are
called zygophores or, when they have made
contact with each other, progametangia. They
are often coloured yellow due to a high b-
carotene content. (2) A ‘zygotropic reaction’, in
which directed growth of zygophores of (þ) and
( ) mating partners towards each other is
observed. (3) A ‘thigmotropic reaction’, i.e.
a touch response involving the events which
occur after contact of the respective zygophores,
such as gametangial fusion and septation of
the progametangia to form gametangia and
suspensors.
Hormonal control of sexual reproduction
The mating process is under the control of
mating hormones (sex hormones, gamones,
pheromones), and the hormones involved are
effective in all members of the Mucorales studied
(see Gooday, 1994; Gooday & Carlile, 1997). Early
evidence of the involvement of pheromones
was the demonstration that in Mucor mucedo the
mating process can be initiated between mycelia
of different mating types separated by a collodion
membrane. The effect of the mating
hormone is to switch the vegetative mycelium
from asexual to sexual development. Other
effects are the accumulation of carotenoids in
cultures containing both mating types and, in
Phycomyces, of a marked reduction in the growth
rate of the vegetative mycelia as they approach
each other (Drinkard et al., 1982). The mating
hormones have been identified as trisporic acid,
actually a family of structurally related molecules,
and its precursors. Trisporic acid was so
named after Blakeslea trispora (see Fig. 7.27) from
which this substance was first isolated (Austin
et al., 1969; Sutter, 1987). Liquid media inoculated
with a mixture of (þ) and ( ) spores of
B. trispora developed more intense yellow pigmentation
than unmated cultures due to a
massive stimulation of b-carotene synthesis.
Trisporic acid itself is derived from b-carotene
(see Fig. 7.9) and is synthesized by collaborative
metabolism of the two different mating type
strains. Each strain has an incomplete enzyme
pathway for the synthesis of trisporic acid so that
intermediates accumulate which can only be
metabolized further by mycelium of the opposite
mating type. As shown in Fig. 7.9, the enzymatic
steps in the conversion of b-carotene (I) to
4-dihydrotrisporol (III) via retinal (II) are
common to both mating types. The (þ) strain
can convert 4-dihydrotrisporol to methyl-4-dihydrosporate
(IV), whereas the ( ) strain converts
4-dihydrotrisporol to trisporol (V). Thus IV and V
function as two complementary prohormones,
each of which is inactive in its own mycelium