Introduction to Fungi, Third Edition

224 ZYGOMYCOTA
Fig 7.47 Harpella melusinae. (a) Thallus
attached by a holdfast to the peritrophic
membrane from the mid-gut of a larva of a
blackfly, Simulium sp.The thallus is divided by
septa into three generative cells, each of which
is producing a curved trichospore at its upper
end.The apical spore body at the upper end of
the trichospore contains material extruded to
cement the holdfast to the peritrophic
membrane. (b) Detached trichospore bearing
four filamentous appendages at its base.
(c) Developing trichospore showing the coiled
filamentous appendages wrapped around inside
the wall of the generative cell.
(d) Chlamydospore which has germinated to
form two generative cells, each of which is
developing a trichospore. (e) Zygospore
development.Two adjacent thalli have
conjugated and from one of the conjugating
cells a zygosporophore has been produced,
terminating in a biconical zygospore. Bars: (a,b)
¼ 10 mm, (c,e) ¼ 5 mm, (d) ¼ 15 mm. (d) after
Moss and Descals (1986); (e) after Lichtwardt
(1967).
appendages cause the trichospores to be
entangled with faeces and other particulate
material. Further development of trichospores,
i.e. the extrusion of a sporangiospore from its
sporangium and the formation of a holdfast,
only occurs after ingestion and is stimulated by
conditions in the larval gut.
Smittium culisetae (Harpellales) inhabits the
midgut and hindgut of larval mosquitoes and
can be grown in culture. Horn (1989a,b, 1990) has
investigated in vitro the conditions which trigger
sporangiospore extrusion in this and related
species of Smittium. The trigger for extrusion in
S. culisetae is a two-stage process. Phase 1, which
simulates mid-gut conditions, involves exposure
to 20 mM KCl at pH 10 followed by phase 2, in
which the pH is reduced to 7, simulating hindgut
conditions. Following this sequence of treatments
sporangiospores (¼ trichospores sensu
stricto) are rapidly extruded, a process in which
they increase in size by the uptake of water
and by vacuolation, generating turgor pressure
which aids extrusion. Spore germination quickly
follows with the secretion of holdfast material
from the apical spore body through canals in the
distal wall of the sporangiospore (Horn, 1989a).
A second asexual, free-living stage in the
life cycle of H. melusinae is the chlamydospore
(sometimes termed ovarian cyst or cystospore),
masses of which are deposited by adult female