Introduction to Fungi, Third Edition

202 ZYGOMYCOTA
of germ tubes towards host hyphae (Fig. 7.33d),
with preferential growth towards the hyphal
tips. On agar the chemotropic stimulus can be
detected over distances as great as 5 mm (Evans
& Cooke, 1982). Fimbriae extending outwards for
up to 25 mm from the cell walls of potential host
fungi may play a role in directing the growth of
Piptocephalis germ tubes towards the host hyphae
(Rghei et al., 1992). At the point of contact an
appressorium develops, but in some combinations
the parasite hyphae may coil around those
of their host and several appressoria form. In
successful host parasite combinations, the host
wall is penetrated beneath the appressorium by
mechanical and possibly also enzymatic means.
An infection peg penetrates the host wall.
Enclosed by the plasmalemma of the host cell,
the tip of the penetration peg expands to form a
haustorium which may branch inside the host
hypha. The haustoria of Piptocephalis have close
similarity to those of biotrophic haustorial parasites
of plants (Manocha & Lee, 1971; Jeffries &
Young, 1976). Nutrients taken up by the haustorium
are translocated to the germinating spore
and its germ tubes may then grow out to form
a mycelium which extends over the host hypha,
producing further haustoria. The distinctive
biochemical features of the Mucorales which
are correlated with their ability to support the
growth of these mycoparasites are that their
walls contain chitosan and that their cytoplasm
is rich in the polyunsaturated fatty acid
g-linolenic acid which is essential for growth
of the mycoparasite (Manocha, 1975, 1981;
Manocha & Deven, 1975).
Recognition between the mycoparasite and
its hosts operates on at least two levels, the cell
wall and the protoplast surface (Manocha et al.,
1990). There are qualitative and quantitative
differences in the carbohydrates present at the
hyphal surface of host and non-host species.
Attachment is favoured by the presence of two
distinctive glycoproteins in the wall of susceptible
host hyphae. These two glycoproteins act as
subunits of an agglutinin which may serve as
receptor to a complementary protein in the
mycoparasite (Manocha et al., 1997).
Piptocephalis virginiana readily infects young
but not old cultures of Choanephora cucurbitarum.
This is correlated with the fact that the wall of
young hyphae of C. cucurbitarum is single-layered
whilst that of older hyphae is double-layered.
Although appressoria and penetration pegs
develop on older hyphae, penetration of the
inner layer of the cell wall is rarely successful.
The inner wall layer develops a papilla opposite
the point of attempted penetration (Manocha,
1981). Similar findings were made when
P. virginiana failed to penetrate the resistant
species P. articulosus (Manocha & Golesorkhi,
1981). Where successful penetration of a susceptible
host occurs, the mycoparasite P. virginiana
can suppress wall synthesis by the host in the
vicinity of infection points, so overcoming one
of its defence reactions (Manocha & McCullough,
1985; Manocha & Zhonghua, 1997).
The effects of Piptocephalis spp. on the growth
of their hosts are very variable (Curtis et al., 1978).
In some combinations the rate of growth of
dual cultures was not significantly different
from that of uninfected hosts, in others it was
reduced, whilst in yet others it was enhanced.
These effects are temperature-dependent. Growth
and sporulation of the coprophilous fungus
Pilaira anomala were reduced in culture when
infected by P. fimbriata or P. freseniana (Wood &
Cooke, 1986). A curious effect was found in
culture when P. fimbriata challenged its normally
susceptible host Mycotypha microspora. In the
presence of P. fimbriata the host grew in a yeastlike
state which was not infected. In contrast,
the mycelial state of this fungus is readily
infected (Evans et al., 1978).
Most species of Piptocephalis are homothallic
(Leadbeater & Mercer, 1957). In culture zygospores
are usually formed within the agar. The
mature zygospore is a spherical dark brown
sculptured globose cell held between two tongshaped
suspensors.
7.5 Entomophthorales
Many Entomophthorales are parasites of insects
and other animals, whilst some parasitize
desmids, nematodes or fern prothalli, or grow
saprotrophically in plant litter, dung or soil.