Introduction to Fungi, Third Edition

TRICHOMYCETES
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group have a worldwide distribution and are
especially common in the guts of larvae of
aquatic insects. A few species belonging to one
order (Harpellales) have been grown in culture
and appear to have no unusual nutritional
requirements. The term trichomycete (Gr. ‘hairy
fungus’) refers to the fuzzy appearance of heavily
infested gut linings. Branched or unbranched
thalli are attached by a holdfast to the
hindgut cuticle or to the peritrophic membrane,
a transparent membranous sleeve which
surrounds digested food material in the mid-gut
of certain insects. Asexual reproduction is by
various types of spore, including trichospores,
chlamydospores, arthrospores or sporangiospores.
Sexual reproduction by the formation
of zygospores is known in the Harpellales.
The occurrence of zygospores, the presence of
chitin in the walls of Smittium culisetae (Sangar &
Dugan, 1973) and molecular studies (O’Donnell
et al., 1998; Gottlieb & Lichtwardt, 2001) all
provide evidence linking Trichomycetes with
the Zygomycota. It is possible that the class
Trichomycetes is polyphyletic, and it is therefore
preferable to refer to the gut fungi as a biological
group, trichomycetes with a lower-case ‘t’
(Lichtwardt, 1986).
Three orders have been distinguished, namely
the Harpellales, Asellariales and Eccrinales, of
which we shall consider only the first. The
Amoebidiales, previously included, are now
classified with the protozoa.
7.7.1 Harpellales
Harpella melusinae is one of the most common and
abundant trichomycetes with a worldwide distribution
in temperate regions. It is found in larval
blackflies (Simulium spp.) which live attached to
stones, twigs and aquatic vegetation submerged
in rapidly flowing streams. The dissection of
larval guts reveals the peritrophic membrane, to
the inner wall of which unbranched cylindrical
thalli are attached. Developing thalli receive
nutrients from the material passing through
the gut. The peritrophic membrane is continuously
secreted by endothelial cells lining the
upper part of the mid-gut, i.e. new membrane
material is added at the upper end. Young thalli
are present here and progressively older thalli
further down. Attachment is by a simple holdfast
(Fig. 7.47a). The holdfast consists of a chamber at
the base of the thallus from which numerous
finger-like projections protrude, cemented to but
not penetrating the peritrophic membrane
(Reichle & Lichtwardt, 1972). The cylindrical
part of the thallus is divided by septa into 2 12
or more uninucleate segments called generative
cells. The septal ultrastructure consists of a
flared pore associated with a plug, somewhat
resembling the bordered pit of a conifer xylem
tracheid. This feature is characteristic of trichomycetes
(Moss, 1975).
Asexual reproduction
The entire contents of the thallus are converted
to reproductive cells. Reproduction begins at the
terminal generative cell and progresses basipetally
(towards the holdfast) by production of
trichospores (Figs. 7.47a c). Trichospores are
really monosporous sporangia. They have been
defined as ‘exogenous, deciduous sporangia
containing a single uninucleate sporangiospore
and normally having one to several basally
attached filamentous appendages’ (Lichtwardt,
1986). The trichospores, which are usually coiled
but sometimes straight, develop at the upper end
of a generative cell (Fig. 7.47a). The nucleus of the
generative cell divides mitotically, one daughter
nucleus remaining in the generative cell, the
other entering the developing trichospore. In
H. melusinae there are four basal appendages
which, before trichospore release, are spirally
coiled inside the upper part of the generative cell
(Figs. 7.47b,c; Reichle & Lichtwardt, 1972). At the
distal end of the trichospore within the cytoplasm
is an elongated apical spore body
(Fig. 7.47a). This contains holdfast material
which is released after extrusion of the sporangiospore
on its germination within the insect
gut, cementing the holdfast to the gut wall (Moss
& Lichtwardt, 1976; Horn, 1989a). Trichospores
are separated from the generative cell by a
septum and are released by breakdown of the
wall beneath it. After release, the appendages
uncoil and extend up to 10 times their original
length. The released trichospores are passed out
from the larval gut with faecal material and the