untitled

membrane (Fig. 12.10). The parasite draws nutrients

from the host hyphae, and uses these nutrients to

produce sporulating structures on the host colony.

Often this type of parasitism causes little damage, as

long as the host fungus has an adequate food supply.

Biotrophic mycoparasitism similar to that in Fig. 12.10

is found in several Zygomycota (Piptocephalis,Dispira,

and Dimargalisspp.) that have elongated, few-spored

sporangia (merosporangia; see Fig. 2.11). With only few

exceptions, these fungi parasitize other Zygomycota such

asMucorandPilairaon dung or in soil. Most of these

biotrophic mycoparasites can be grown in laboratory

media containing extracts of host or nonhost hyphae.

The need for hyphal extracts can be replaced by

relatively high concentrations of vitamins (especially

thiamine) and amino acids, and by providing glycerol

instead of glucose as the carbon source. These nutrients

could be expected to occur in the host hyphae.

The haustorial biotrophs seem to depend entirely on

their fungal hosts in nature. Their spores are triggered

to germinate near host hyphae, and the germ-tubes

show pronounced tropism towards the host. Then the

germ-tube tip produces an appressorium on the host

surface and a penetration peg enters the host to form

a haustorium. The mycoparasite Piptocephalis virginiana

shows evidence of specific recognition in the infection

process (Manocha & Chen 1990). It infects only some

members of the order Mucorales, and the walls of

these hosts were found to have two surface-located

glycoproteins. Removal of these glycoproteins by

treating hyphae with NaOH or proteinase led to

impaired attachment and appressorium formation. The

treated hyphae (with their glycoproteins removed)

were found to bind lectins that recognize fucose,

N-acetylgalactosamine, and galactose, and this binding

pattern of the treated hyphae was identical to that of

nonhost hyphae. However the untreatedhost hyphae

did not bind these lectins, so it is suggested that the

glycoproteins that occur normally on host hyphae

cover the fucose,N-acetylgalactosamine, and galactose

residues (which somehow interfere with infection), and

enable the parasite to attach and infect. In support

FUNGAL INTERACTIONS 245

Fig. 12.10Electron micrograph of an
appressorium (ap) and a branched
haustorium of the mycoparasite
Piptocephalis unispora(Zygomycota) in
a fungal host, Cokeromyces recurvatus.
hw =host wall; ol and il =outer layer
and inner layer of the Piptocephalis
wall. The haustorium is surrounded
by a continuous membrane (the extra-
haustorial membrane, labelled e).
(Courtesy of P. Jeffries; from Jeffries &
Young 1976.)